Taking Back Visual Astronomy II: Resolving Binary Stars with Newtonian Reflectors

Octavius the Progressive.

Octavius the Progressive.















 De omnibus dubitandum

The Newtonian reflector has a long and distinguished history among dedicated observational astronomers. With the advent of generous aperture, silver-on-glass mirrors in the late 19th century, many more amateurs could enter the field and make valuable contributions to the study of the Moon and planets. What’s more, their comparatively enormous light gathering power compared with traditional refractors made it possible to see new morphological details of hitherto elusive deep sky objects, thereby aiding in their classification.

The traditional instrument of choice in double star astronomy has been the classical refractor. With their long, native focal lengths and excellent thermal stability, they are especially adept at separating point sources at very high magnifications, at or near the theoretical limit imposed by their aperture. Refractors don’t scale well though and become impractically cumbersome and expensive in apertures above 6 inches (and if you really want to do sub arc second work you’ll need something larger anyway). I have demonstrated in earlier work that more economical telescope designs – the Maksutov Cassegrain in particular- can be excellent double star instruments. Having used a large, 17cm f/16 Maksutov continuously for a year, this author debunked a long standing assumption about these telescopes that prevented many from exploring their considerable charms. Specifically, some prominent amateurs, perhaps in some desperation to justify the purchase of much more expensive refractors, cultivated the idea that large Maksutovs (and, by implication, other catadioptrics) would not acclimate. This assertion was found to be largely unsubstantiated, after extensive field testing showed that these instruments can and do work well, even in winter.

In more recent times, this author has begun to explore anew the many attributes of the Newtonian reflector. As described in an earlier review lasting about six months, a closed-tube 8” f/6 Newtonian reflector was found to cool quickly (typically 40 minutes for a temperature differential of 20C) – significantly faster than even a 5 inch refractor. What is more, no cooling fan was deemed necessary and the telescope offered up excellent, high resolution images of planets like Jupiter. What was most surprising however, was its ability to split tricky double stars when contemporary wisdom said otherwise. This led to further investigation by examining the historical literature in order to establish whether Newtonians were ever used for double star astronomy and, if so, how efficacious they were in this capacity.

Having explored the life and work of the Reverend T.W. Webb (1806-1885), it came to my attention that the celebrated 19th century observer had indeed used a large 9.25 inch f/8 silver-on-glass reflector made by George With to resolve very tight pairs at or close to the limit imposed by its aperture. As a follow up, double star observer, John Nanson, alerted me to the work of an obscure British 19th century observer – Kenneth J. Tarrant – who employed a 10.25 inch Calver reflector (probably a f/7 or f/8 relative aperture) during the 1880s and 1890s to not only observe double stars, but to measure them also!

I would invite you to examine the documents presented here, noting the dates and seasons when the measures were made, thereby providing information on the frequency and likely conditions (like English summer temperature swings) under which observations were conducted – as well as the measures themselves, some of which show that the mirror was indeed capable of resolving pairs at or near the theoretical resolution of the telescope. I canvassed the opinion of the double star expert, Bob Argyle, based at the Institute of Astronomy, Cambridge, for his take on Tarrant’s data. Specifically, I asked Argyle whether there was anything in the Victorian amateur’s data that would stretch credulity, calling his attention to Tarrant’s measures of 25 Canum Venaticorum.

“As far as I can see, looking at Tarrant’s results, these are what I would expect from a good Calver telescope – in fact he did not seem to stretch the telescope very often. Specifically 25 CVn looks very plausible – the current WDS mags are 5.0 and 7.0 so it’s somewhat brighter than the values Tarrant gives (and currently at 1″.7).”
Tarrant’s measures demonstrate three things;

1. The British climate allowed him to frequently work to very high standards, which included sub arc second pairs.
2. The Calver reflector must have produced images stable enough for mensurative purposes.
3. Tight pairs with very significant brightness differences (up to two or three stellar magnitude differences) were also resolved.

Not much else is known about Tarrant however. “I don’t know of any other references to Tarrant’s work, “ said Arygle, “but he seemed to hold the BAA Double Star Section together before WWI finished it, and probably deserves a paper from one of the historical groups.”

In more recent times, a number of other observers using Newtonian reflectors have come to the fore. This author has already brought to your attention some of the ongoing work of Christopher Taylor, who employs an open-tubed 12.5 inch F/7 Calver reflector to watch a number of sub-arc second pairs moving rapidly in only a few years. You can see a few images of his telescope here. In addition, I am mindful of the work of the French double star observer, Jean-Francois Courtot, who has resolved pairs down to 0.66” using his homemade, 8-inch Newtonian since 1993.

It would also be worthwhile considering the portfolio of the well known astronomical artist, Jeremy Perez, who has sketched many double stars using both a 6″ f/8 and a 8″ f/6 Newtonian reflector, as well as the observations of Mircea Pteancu, who has used a 8″ f/6 reflector to successfully resolve sub-arc second pairs.

Thus, not only is there a historical precedent for the use of the Newtonian reflector in doing the kind of work traditionally associated with the classical refractor, but the notion that the former instruments would only be capable of such work in tropical or temperate climates is not supported by the evidence.

That said, not all Newtonians are equally well favoured to carry out such work!

To see why, we need to explore aspects of the physics of the Newtonian telescope.

Modern parabolic mirrors of decent quality are (or should be) essentially devoid of spherical aberration. The main optical defects in the Newtonian are due to other Seidel aberrations, particularly coma and astigmatism. Let C represent coma and A represent astigmatism.

Mathematically, the angular expansion (theta) of the image due to coma is given;

C = 3theta/(16F^2) where F is the focal ratio (relative aperture) of the telescope.

Astigmatism is given by:

A = ( D/2f) tan^2(theta), where f is the focal length of the telescope.

Since D/f = 1/F and if we consider small angles, where tan (theta) expressed in degrees ~ theta radians, the formula for astigmatism simplifies to;

A = (theta)^2/2F.

We can see from the formula for both C and A that coma (C) scales proportionately with theta while A scales as (theta)^2, so that for very small angles ( << 1 radian) it follows that coma will always overwhelm astigmatism in any properly executed mirror.

Let us now set the resolution of the telescope to the Dawes limit (in arc seconds) given by 4.56”/D
To convert this formula to radians, we need to do some more arithmetic.

1 degree = 60 x 60 = 3600”

Also 1 angular degree = 1/57.3 radians =0.017 radians

Thus if 0.017 radians = 3600” then 4.56” = (0,017/3600) x 4.56 radians = 2.21 x 10^-5 radians

So the Dawes formula expressed in radians is:

(2.21 x 10^-5)/ D where D is in inches.

For critical work at maximum resolution we may equate the expressions for coma and astigmatism with the Dawes limit;


A + C = (2.21 x 10^-5)/D

But since A << C for any small angles (which is appropriate here), we may simplify this to just:

C = (2.21 x 10^-5)/D

Thus, since we have C = 3theta/(16F^2)

We get: (2.21 X 10^-5)/D = 3 theta/(16F^2).

Cross multiplying and rearranging, we obtain:

Theta = (16F^2 x 2.21 x 10^-5)/3D

Simplifying gives theta (in radians) = (1.18 x 10^-4 x F^2)/D

For convenience, we can now convert this formula to arc minutes;

1 arc minute = 1/60 degree = (1/60) /57.3 = 2.9 x 10^-4 radians

So, 1.18 x 10^-4 = (1.18 x 10^-4)/ 2.9 x 10 ^-4 = 0.407

Thus our final result is that

Theta (arc minutes) = (0.407F^2)/D.

We are now in a position to analyse what happens when we use various different numbers for the focal ratio (F). The formula predicts that for a constant aperture D, the maximum available field (theta) over which the image contains no appreciable aberrations scales as F^2.

This means that the faster the F ratio, the smaller the true field over which aberrations are minimized.

For example, a 8 inch f/6 mirror would have an optically corrected radius of (0.406 x 6^2)/8 = 1.83 arc minutes or 3.66 arc minutes in angular diameter. Doing the same math for F=5 and F=4 yields diameters of 2.54 and 1.62 arc minutes, respectively.

To see how this impacts work at the eyepiece, consider my own telescope, a 8” f/6 Newtonian. In order to get adequate image scale for sub-arc second pairs, I like to use a magnification of 548x (3.5mm Baader zoom and 1.6x Barlow). Since my eyepiece has an apparent field of 72 degrees, the true field available at this magnification will be 7.88 arc minutes [ that is (72/548) x 60]. Thus, the percentage (linear) of the field that gives perfect definition will be (3.66/7.88) x 100 ~ 50 per cent. When we get to an F/5 system, the percentage falls to just 30 per cent, and at F/4, a pesky 20 per cent!

One can see that at F/5 or faster, positioning the image of the double stars will become problematical, but that’s not the end of the story!

As anyone familiar with the operation of a Newtonian will tell you, the lower the F ratio, the harder it is to collimate the optics accurately. Indeed, the sensitivity to mis-collimation (a quantity called primary mirror axial error) in millimetres is given by the 0.022 x F^3. It follows that the wiggle room for a F/6 Newtonian will be a comfortable 4.8mm but just 2.8mm at F/5 and only 1.4mm at F/4!

What does all this mean?

In a nutshell, the faster the F ratio of the primary mirror, the smaller the true field at any given magnification that is truly free of aberrations and the greater the likelihood of mis-collimation. I was being kind when I described the result linearly; but when you recognise the relevant field area (which scales with r^2), you suddenly realise you’re in deep water. X marks the spot! LOLl

These are the principle reasons why an F/5  or faster Newtonian will be less likely to resolve to the Dawes limit. F/6 is about good enough – thank goodness for small mercies! – and anything slower is a bonus!***

This also agrees with my own experience, having never satisfactorily resolved sub arc second pairs with an F/5 or F/4 Newtonian. It also agrees with the aforementioned historical curiosities!

Look again at Tarrant’s measures of 25 CVn conducted in the summer of 1885.

Octavius; a ‘scope to believe in!

***Note added in proof: The above calculations do not preclude the possibility that a precisely aligned, fast Newtonians (f/5 or slower) can’t do this type of work  but rather serve to illustrate that the difficulty of achieving these high resolution results becomes more difficult as the F ratio falls. Investing more money in precision focusers and more exotic collimating devices can increase the odds of success, as could the possibility of introducing optical accoutrements like coma correctors (now being made by various manufacturers) into the optical train.


Bell, L The Telescope, Dover (1971)

R.W. Argyle (Ed.) Observing and Measuring Visual Double Stars, Springer (2012).

Results so far: In the last six months or so, I have had the privilege of using this fine SkyWatcher 8-inch f/6 Newtonian reflector. As explained in an earlier review, I modified the instrument by purchasing a smaller secondary mirror (22 per cent by diameter) made by Orion Optics, Newcastle Under Lyme, England. I could have reduced this further but I wanted the telescope to be an excellent all-rounder rather than just a one trick pony. Both the primary and the new secondary were treated to enhanced Hilux coatings, which significantly increased its light grasp, reduced scattered light around images and has a longevity that is guaranteed for at least 25 years. Such an instrument provides breath-taking views of the Moon and planets and serves up a 2.25 degree true field for stunning deep sky vistas.

Even before I had these modifications done, I was very impressed by its ability to resolve some tricky doubles and triple systems. On the best nights, stars present as tiny Airy disks, round as buttons, even at very high powers ( > 500x). The spherical correction of the mirror is excellent and displays no on-axis astigmatism, which is a definite show stopper for this kind of work. My best images yet came just a few nights ago, where on the mild evening of Friday, June 26 at 22:20 UT, I beheld the most striking image of Epsilon Bootis (340x) I have seen in just about any telescope! The components – a soft yellow primary and a royal blue secondary – were magnificently rendered with acres of dark sky separating them. The same was true when I examined Delta and Mu Cygni, as well as Pi Aquilae (1.5″); text book perfect renderings if ever I have seen them!

At twenty minutes past midnight on the morning of June 9 last, I managed to glimpse the elusive companion to Lambda Cygni (my best yet at this location, 0.9” and 1.6 stellar magnitude differential), convincing me that I could go still further.

My methodology is fairly straightforward and is based on the recommendations of Christopher Taylor, who I mentioned earlier.

• The telescope is checked for accurate alignment using an inexpensive laser collimator before the commencement of each vigil and backed up by careful star testing.

• Only stars above a certain minimum altitude are examined, not less than 35 degrees

• I use a Baader Neodymium Moon and Sky Glow filter, which darkens the twilit sky at my location, reduces glare from very bright stars, and retains a neutral colour balance.

• After charging the telescope with the appropriate optical power, the stellar image is swung to the east of the field and left to drift slowly into the centre, where it is critically examined by my eye. The above is repeated again and again until I am satisfied that what I am seeing is not a diffraction artifact or some such.

• The time, date and conditions, magnification etc are always recorded. And if at first you don’t succeed……. try try again Lol!

In my correspondence with Bob Argyle, he was kind enough to suggest two stellar systems which are especially ripe for study with the 8-inch speculum; 78 UMa, now conveniently located near the bright star Alioth in the Plough Handle (components have magnitudes 5.02 and 7.88, with a current separation of ~0.8”) and Tau Cygni (magnitudes 3.38 and 6.57 with an angular separation of 0.9”).

I will begin with 78 UMa, as it should be fairly easy to find near Alioth in the twilight.  I shall leave Tau Cygni to later in the season.

I will report back on my progress in due course.

If you have a similar ‘scope at home, why not give it a try too?

If these stars are not suitably located for you, seek out others of similar difficulty by looking up the WDS catalog.

This project will certainly tax your powers of observation.

It would be great to hear about your experiences!

 July 1, 2015

NB: Taylor used a ‘routine’ magnification of 825x with his 12.5 inch f/7 Calver to achieve separations of 0.35 -0.40″ pairs. May attempt slightly higher powers on my own (smaller, 8 inch) telescope, perhaps 600x plus?

Nae luck as yet. A heat wave has settled in over the UK. While southern Britain basks in sunshine, conditions have remained stubbornly sultry with lots of cloud hampering any attempts to track down UMa 78.

Attempted a brief vigil late in the evening of Friday, June 26. Although my ‘easier’ test systems mentioned above all looked excellent, cloud prevented me from locating  my target near Alioth. I did however ‘uncover’ a delightful new binary system about half a finder field away from Alioth; STF 1662 ( RA  12h 36 min, Dec: 56 34, magnitudes 7.83 an 9.75, separation 19.3″).

Just received word that my article on modifying the SkyWatcher Skyliner 200P will be featured in the August 2015 issue of Astronomy Now………hallelujah!

July 2, 2015

Time 22:50h UT

Ambient: Clear, good transparency, 14C, slight SW wind, strong twilight, seeing not so hot (Ant III-IV), midge flies legion.

Four ‘warm up’ systems  observed @ 340x

Epsilon 1&2 Lyrae: well resolved.

Epsilon Bootis: resolved with some distortion.

Delta Cygni: Companion seen periodically, but with some considerable distortion.

Pi Aql: Resolved fairly well but only occasionally.

A 1.5″ night. Little point in continuing. Packed up early.

 July 4, 2015

Happy Holidays to all my viewers in the United States!


Semper eadem.

Weather still rather unsettled, very humid with lots of heavy down pours, so little else to report from my own observations.

Investigo: I love data and admire diligence. Though I don’t know him from Adam, the American amateur astronomer, Mr. Tom Bryant, gave me both in bucket loads!

Mr. Bryant has been very busy testing the performance of his C8 on hundreds of double stars from all across the heavens.

You can see the fruits of his considerable labours here.

Go on; have a good, long look at that huge list. Dates (all year round!!!), times, instruments, are recorded, and, crucially, the location of those observations.

Input! Input! Input!


And I see he’s constantly updating (see the latest dates listed).

Way to go!

He’s done remarkably well on many sub-arc second pairs don’t you think?

0.7″ doesn’t seem too much of a stretch for him and he’s elongated pairs down to 0.5″!

Here’s a recent review of a modern C8.

This instrument has a central obstruction of ~ 35 per cent and takes a while to acclimate…. apparently.

Here’s  the climate data for Bethesda, MD, which is quite near Silver Spring, MD, where Mr. Byrant uses his C8 inside his cosy, wee observatory, Little Tycho.

Typing in the months, one by one, we see diurnal swings of about 10C throughout the year, and which is a little larger than those encountered at my location.

My 8″ f/6 Newtonian, with a 22 per cent central obstruction, ought to do just as well – if not better – would you not think?

Only the seeing and my laziness can limit its performance.


 July 5, 2015

Some thoughts on a lazy, Sunday afternoon:

The diligence of Tom Bryant and Carlos has delivered treasures to them. Work pays.

God endowed King Solomon with wisdom because he desired it ahead of wealth and power.Still, because of his faith, the Lord gave Solomon all three, and in great abundance.

Yet, he was better at dispensing that wisdom to others than applying it to himself.

In the proverbs of that ancient King, we learn of the traps laziness sets for us;

No matter how much a lazy person may want something, he will never get it. A hard worker will get everything he wants. 

Proverbs 13:4

A lazy person is as bad as someone who is destructive.

Proverbs 18: 9

Why don’t lazy people ever get out of the house? What are they afraid of? Lions?

Proverbs: 26:13

Nuff said, eh?

20:30 UT

At last, another opportunity will likely present itself later this evening to visit 78 UMa.

With a bit of luck, I’ll have more to report back on soon enough.

But let’s not confuse ourselves. There is one telescope forum in particular that harbours a few lazy liars I’m in the processing of flushing out.

Folk who masquerade as being ‘experienced’ but ostensibly reveal very little of that quality. Nor do they show any real insight except that which they borrow from others.

They neither understand their observing environment, nor the kinds of instruments that would best work there. e.g. using a large, fast reflector to split low-altitude double stars in a desert?!

How dumb is that? Lol!

But this is just ignorance, and I’m willing to overlook that.
That said, there’s a more insidious side to all this, which I am not willing to overlook.

Lies, lies, porky pies.

You see, some individuals spend their time cultivating untruths about what can and can’t be done with certain telescopes, without ever testing these claims in a scientific way.

Worst still, they persist in maintaining these myths, despite the mounting counter-evidence presented to them.

I suppose it’s a form of blindness.

Why shouldn’t a Newtonian deliver the readies?

If you know, tell me; I’m all ears!.

iustitia! iustitia! iustitia!

July 6, 2015

00:20h BST.

Ambient: Mostly clear, tranquil, cool (10C), twilit.

Seeing: II-III

A better night tonight. Seeing fairly good.

All warm up systems beautifully resolved at 340x

0.9″ companion to Lambda Cygni well glimpsed at 548x during moments of better seeing

78 UMa: diffraction pattern examined on and off for 20 minutes at 548x. Higher powers found to be unhelpful. Companion unseen.

Heavy dew this evening.

Good, productive night, all in all.


Teeming down with rain tonight.

Thus far, it’s not the kind of Summer we enjoyed last year.

Still, when are two ever the same? lol


Semper eadem.

It occurred to me that I’ve already achieved what I set out to demonstrate; that a decently executed Newtonian can be used to explore the dynamic realm of sub-arc second binary star astronomy; I mean, I’ve already bagged (a few times now) a 0.9″ with a sizable brightness differential (1.7), so anything beyond that just reaffirms my premise.

But I don’t think I’m being overly ambitious to work for something better. Do you?

I will continue to work with 78UMa until the skies get darker.

July 8, 2015

00:30h BST

Test everything; hold fast to what is good.

                                                                   1 Thessalonians 5:21

Ambient; mostly cloudy, 13.5C, a few patchy sucker holes opening and closing. Breezy (7mph westerlies).

Seeing: II, certainly a notch up on last night.

Only three test stars examined tonight; all images at 340x were clean and crisp but shaky in the wind.

Spent a few minutes on and off examining 78UMa at 340x and 544x. Complex diffraction image, no elongation observed at 544x, so the companion must be ‘disembodied’ from the primary (Airy disk round as a button). Wind and cloud making detailed observations very difficult. Companion unseen.

I have noticed, going back through my notes, and again tonight, that on windier evenings, the images through the Newtonian can look especially fine. I have thought about why this might be. Perhaps the breeze circulates the air inside the tube more efficiently and might be ‘brushing off’ any boundary layer that might be on the mirror?

I think there is something in this.

Mother Nature lending a helping hand, just as she must have done with other observers using their specula over the decades and centuries.

Thank goodness for the wind!

09:50h BST

Last night was most interesting. Not much in the way of systems observed but the quality of the images in the modest wind was duly noted.

It was such a simple revelation to me that I cannot help but think it is universally true.

My previous observing records with refractors and a large Maksutov have shown that good to excellent seeing can accompany windy weather. I look back fondly at the wonderful skies of last Summer, where I got superb results with a 17cm Maksutov. I note especially my observations made on the evening of July 16, 2014, where the Maksutov cleanly resolved Lambda Cygni  during a windy (9mph) spell.

In the case of the Newtonian, I think windy conditions can have additional benefits in improving image quality, independent of the seeing.

Open air observing with Newtonians appears to be a good thing and I shall continue with this custom.

Might a fan be beneficial?

Maybees aye, maybees naw.

Would I consider installing one?


I get enough breezy evenings in a year to continue as I am.

Besides, I am willing to bet that the foolishness of the wind is smarter than the ingenuity of any man-made fan.

A curious aside: Our Victorian friend, Kenneth J. Tarrant, observed 25 CVn with his Calver reflector on the 189th day of the year. Curiously this was July 8, 1885 – almost exactly 130 years ago today!


I found some old British archives for the general weather for that month here.

I note that in this meteorological document, for the dates July 7-11, there were ‘favorable South-westerly winds in most places’.

Might  Mr. Tarrant have enjoyed a few breezy evenings when he made these measures?

I wonder!

July 9, 2015

00:20h BST

Ambient: Clear, cloudless sky, very beautiful twilight, no ground wind, unseasonably cold (6.5C), seeing III-IV. Cool Arctic air flow tonight; bright stars scintillating strongly.

Test systems all resolved, but the more difficult ones not so cleanly. U78Ma examined at 340x an 544x but too turbulent to study.

Vigil aborted.

11:20h BST

I have been thinking about the wind again and how best to use it. When Mr. Tarrant observed 25 CVn, his telescope would have pointed westward, towards Canes Venatici, and if there were a southwesterly breeze during the time he observed the system, some part of it would have blown over his Calver primary mirror.

This immediately presented a simple activity that I could use profitably during breezy evenings. When first placed outside, I could remove the cap that covers the front of the instrument and point the telescope directly into the prevailing winds. That way, the air would be blown over the mirror and it would help expel any ‘stagnant’ air inside the tube.

When observing an object in a part of the sky away from the natural direction of the wind for any prolonged period of time, I could swing the instrument back into the natural air flow  periodically, for a minute or two perhaps, before resuming my work.

I did some searching this morning to ascertain if anyone had recommended this procedure, either in printed texts or online. To my astonishment, I came up with nothing.

Maybe you know better?

In addition, I have been looking at images of those silver-on-glass reflectors of old (existing before the era of the electric fan) and noticed that many of the tubes have little hinged  ‘windows’ at the side, near the primary mirror, so as to assist (presumably) the circulation of air in the optical train. I may consider something along these lines myself; perhaps drilling a coupe of small holes on opposite sides of the tube and fitting a fine wire gauze over them to enable air to flow through but not particulates.

I can make the wind work harder for me.

Something to think about anyways.

To my chagrin, more unsettled weather is forecast for the weekend ahead.

Mair anon..

July 13, 2015

23:45h BST

Ambient: almost entirely clear, tranquil skies, seeing excellent (I-II), 10C, humidity high.


Started on Delta Cygni (340x) and was rewarded with a beautiful calm image! Companion resolved from its primary by a veritable country mile.

Pi Aql: Very cleanly resolved (340x) even at less than optimal altitude.

78UMa: Companion seen fairly well, roughly due east of the primary and inside first Fraunhofer diffraction ring. Glimpsed at 22:50h but better seen at 23:30h.  Checked the WDS data on the system Der Admiral sent me the other week. Its estimated position angle of ~118 degrees agrees fairly well with my observation.

No’ bad ken.

Where next Columbus? LOL

Anyone following me?

Vigil ended owing to heavy dew.

July 14, 2015

Bastille Day, New Horizons hurtles past Pluto, ken.


Consummatum est.

No more to prove. No more work to be done. No one left to fight.

A 8 inch f/6 reflector can indeed be used to resolve sub arc second pairs. You don’t need an expensive telescope to do it.

A little preparation and the determination to succeed is all that is required.

And one good night.

I contacted Bruce MacEvoy, who I had the pleasure of meeting in California a few years back. He will be editing a brand new edition of the Cambridge Double Star Atlas. Bruce followed my work with the Maksutov and, more recently, the Newtonian reflector. After congratulating him on his new role, I reminded him that he had a responsibility not to cultivate untruths about the types of telescopes that can and cannot do high resolution double star work. He assured me that the atlas will not endorse the fallacy that one type of telescope is superior to others.


Nota Bene: November 29, 2015: Dave Cotterell, based in Ontario, Canada, posted a string of high resolution images of double stars – some quite tricky for any telescope – using his 12.5″ f/6.5 Newtonian, thereby providing more evidence that these instruments can and do make excellent double star ‘scopes. In addition, he has reported his visual results here, using the same instrument, showing that he was able to cleanly resolve pairs down to 0.5″ or  0.6″. Well done Dave!

De Fideli


Optimising a 8-inch Newtonian for Visual Use

Based on an article which originally appeared in the peer-reviewed  Astronomy Now magazine (August 2015).

Telescopes 101

200POver the last decade or so, amateur astronomers have become increasingly obsessed with acquiring very expensive apochromatic refractors that offer near optically perfect views under good conditions, but are limited by their restrictive aperture. I have heard people claim that a 4- or 5-inch Apo refractor gives ‘better ‘images than an 8-inch reflector on planets for example. This is patently nonsense, as the 8 inch Newtonian has twice the aperture and much more light gathering power than any 4 inch refractor, no matter what its pedigree. So what is going on here? As I said elsewhere, this is a pleasant fiction.  The view through the smaller ‘scope might look nice from moment to moment but that’s only because it can’t resolve finer detail that the larger telescope can, but at the expense of being more sensitive to the vagaries of the atmosphere.  In other words, the smaller ‘scope conceals far more than it displays; its beauty merely skin deep. Rest assured though, when the Newtonian is working optimally it will not only yield ‘prettier’ images than the refractor but they will be a whole lot more detailed too. That’s just physics.

After many years of testing telescopes of every conceivable size and genre, I have come to the conclusion that a good 8-inch F/6 Newtonian provides the biggest bang for buck in today’s market. It offers decent aperture for both planetary work and deep sky observing, with a generous 2.25 degree field. It is portable and acclimates quickly, often without the need for cooling fans. With a focal ratio of f/6 it works quite well with even budget wide-angle eyepieces and is capable of being accurately collimated during daylight hours. One of the best examples comes from the SkyWatcher Skyliner range of Dobsonians, which can be purchased as an entire package for less than £300, including delivery to your door. Having purchased this telescope, I wanted to demonstrate ways in which a very good instrument can be further improved to give the best possible images – improvements that do not incur a large outlay of additional funds.

Tube modifications
Refractors tend to have very well baffled tubes that stop stray light from flooding into the optical train especially in comparison to economically priced Newtonians. But through some simple measures, you can help control this stray light reaching the eyepiece. One of the most important things that needs to be done is to flock the region of the tube immediately opposite the telescope focuser. Many astronomy retailers sell rolls of flocking material costing just a few pounds. I simply cut off a piece of this material measuring 6 x 8 inches and stuck it onto the inside of the tube immediately opposite the focuser. In addition, the drawtube of the focuser was similarly flocked.

Flocking the tube opposite the focuser is a good move.

Flocking the tube opposite the focuser is a good move.

Mirror Modifications
The secondary mirror in the Newtonian is usually elliptical in shape and is orientated such that its minor axis minimizes the size of the central obstruction that is all too important in producing images rich in contrast. The mirror supplied with the Skywatcher Skyliner 200P has a minor axis diameter of 50mm, thus providing a central obstruction of 25% by aperture. And while this is perfectly acceptable for all round use, a number of alterations can be made to the secondary to improve the telescope’s overall performance.

A new 44mm flat with edges blackened with matt black paint.

A new 44mm flat with edges blackened with matt black paint.










 I contacted Orion Optics UK, based at Newcastle Under Lyme, Staffordshire, who have a long-standing expertise in delivering quality Newtonian and Maksutov Cassegrain optics to discriminating observers. In particular, they have developed their highly regarded Hilux enhanced coatings with 97 per cent reflectivity and were also able to make to order any secondary size I wanted. At first, I had intended to get the existing SkyWatcher secondary and primary Hilux coated and to purchase an additional secondary with a 36mm diameter for high resolution work. But in the end I decided to settle on a single flat with a diameter of 44mm, thus providing a very modest 22 per cent central obstruction. This size of flat also means that I can employ wide-angle two inch eyepieces without imparting too much in the way of vignetting at the edge of the field. Finally, before mounting the new secondary, I blackened its edges with matt black blackboard paint.

Inserting a cooling fan to blow cool air over the surface area of the primary mirror would help to accelerate the telescope’s acclimation but, truth be told, I haven’t found the need for one. The telescope will acclimate in about 40 minutes if taken from a warm indoor room to the cool of the night air. What’s more, if the instrument is left in a dry, unheated shed, it will be in a permanent, ‘grab ‘n’ go state.

Performance in the field
The telescope was mounted atop an inexpensive water butt with the mushroom knobs on the base of the lazy Suzan mount slotted directly into two pre-drilled holes of the butt. Such a measure raised the telescope to a decent height off the ground and kept the base free from dirt and grime. The modified 8 inch Newtonian has the same contrast transfer as 6-inch refractor (200-44mm) and, owing to its ultra-high reflectivity coatings considerably greater light gathering power. All in, the telescope and its modifications came to less than £550. How does it perform? In a word; splendidly! But to elaborate, I’ve enjoyed some of my very best views of Jupiter with this telescope. Indeed, they are every bit as good as a 6 inch apochromatic refractor costing five to ten times more! And contrary to popular belief, a 8” f/6 Newtonian is no slouch on double stars. You just have to look at the superlative work done by astronomical artist, Jeremey Perez, who uses a similar telescope to see why. During a spell of good, clear weather I was able to cleanly resolve the tricky pairs , Iota Leonis, Mu Cygni and Eta Geminorum – systems that are more challenging with an excellent 12.7 cm  f/12 refractor and 17cm f/16 Maksutov. Subsequent work has shown that the same telescope can resolve sub arcsecond pairs, again, within the remit of its aperture.

Deep sky objects really come alive in an 8-inch telescope. I have enjoyed some beautifully crisp views of the Double Cluster (Caldwell 14) and its star-studded hinterland at 30x. Spring galaxies like M81, M82 in Ursa Major and M51 in Canes Venatici are very well presented and a joy to study at medium and high powers. All in all, this was an enjoyable and worthwhile project to undertake and has transformed a good telescope into a great one!

Octavius: instrument of change.

Octavius: on solid ground.















For further details see my related articles here and here.

Two more threads that you might find interesting can be viewed here and here.

In this thread, the poster uses a 12.5″ f/6.5 Newtonian to resolve pairs down to 0.5- or 0.6″, as well as posting actual images of other pairs here.

Post scriptum: The Premo-Dob manufacturer Teeter’s Telescopes are now using GSO mirrors in their Dobsonian line. As Rob Teeter openly acknowledges, the optical quality of these mirrors is generally excellent. These are the same quality mirrors that went into the telescope highlighted above. So, like I said elsewhere, I wouldn’t trade my 8-inch Newtonian for any 6-inch refractor on Earth! Why would I?

Neil English is author of Choosing and Using a Dobsonian Telescope.

De Fideli

A Little Prinz from my Youth

                              By Paul Brierley, Macclesfield, England
In 1976 my father bought from Dixons, a 60mm F11 “Prinz” refractor.  As a young boy, mad about astronomy, I thought all my dreams had come true!
Dad would take it outside and show me the Moon and stars. I well remember my first views of “Lunar” through it, and I was instantly hooked. The telescope came with a very rickety altazimuth mount, together with three Huygens eyepiece, a Barlow lens, Moon filter
and the dreaded solar filter.
A 60mm f/11 Prinze gets a new lease of life. All images courtesy Paul Brierley.

A 60mm f/11 Prinze gets a new lease of life. All images courtesy Paul Brierley.

The telescope was used on most nights during the winter of 1976 and during the latter years of the 1970s and early 1980s. I was able to see Jupiter and Saturn, and using projection, our Star.
I well remember trying to record my observations, but soon gave up. The mount was just too unstable. If you sneezed it would wobble. Eventually the scope fell into disuse.  I don’t know what happened to its mounting, but, I kept the optical tube assembly.
In  2015, I decided that I wanted to use this telescope again, and this followed an evening of astro-imaging, when  I was happily downloading CCD images from another telescope. I decided to dig out the “Prinz”  The Moon had risen and I was able to view it, with the telescope handheld.
The Prinz achromat on a sturdy modern mount.

The Prinz achromat on a sturdy modern mount.

I have an adaptor that allows the use of modern Plossls and Orthos. I looked and was stunned by the quality of the telescopes optics. I decided there and then, to restore it, and put into service again.
On August 22-23, I started work.I stripped down the optical tube assembly and re-painted it. I took the optics out of it’s cell and carefully cleaned them. I used Optical Wonder Fluid, from Baader. Now they have been cleaned. The doublet lens is as good as new, with no sign of fungus or scratches.
I took up play in the focuser and found a mounting bracket for the optical tube. The tube was originally white, but I didn’t have any suitable paint. So, I painted the tube matt black, using black pipe paint. It looks as good as new, and now the focuser slop has been removed. Images stay central during focus. I can now use this telescope again.  I  can mount the optical tube onto my Acuter Merlin mount, and I am happy to say, unlike 1976., it is very stable.
It saw first light once again on August 28 2015. Once again, It was the Moon that took the glory. The view through a 18mm Volcano Ortho was very impressive.  I believe the lens has a single magnesium fluoride anti-reflection coating but does a fine job.  The Moon was very sharp with no colour fringing visible. I would hesitate to say that I think this telescope, although only a doublet achromat, is similar to a modern ED Apo in optical quality. I will use this telescope from now on, for lunar and planetary observation, together with high resolution imaging of the Moon, using a QHY5II-M camera.
An August Moon, as captured by the 60mm Prinz.

An August Moon, as captured by the 60mm Prinz.















My sincere thanks to Paul for sending on this short article about a special little telescope that sparked his lifelong interest in astronomy. He is a member of the BAA, SPA, Webb DSS, as well as Macclesfield Astronomical Society.

De Fideli

Origins of Life: A Closer Look Part I

Some life scientists believe they can present a truly naturalistic scheme of events for the origin of life from simple chemical substrates, without any appeal to an intelligent agency.

Here is one such scenario, presented by Harvard professor, Jack Szostak.

I invite you to study the video at your leisure.

In this work, I wish to critically appraise each of the steps Dr. Szostak presents in light of the latest research findings that show that any such scheme of events is physio-chemically untenable from a purely naturalistic perspective.


Video Clock Time 00.00 -10.00 min

Here Dr. Szostak sets the scene for this thesis, exploring the varied landscapes and environments under which we find life on Earth. Dr. Szostak reasonably suggests that when life first appeared on Earth, it must have done so in an extreme environment with higher temperatures and in aqueous environments with extreme pH values and high salinity. What Dr. Szostak does not acknowledge is that life was already complex when the Hadean environment first cooled enough to permit life to gain a footing. For example, there is solid isotopic evidence that the complex biochemical process of nitrogen fixation was already in place at least 3.2 Gyr ago and possibly earlier still.


Eva E. Stüeken et al., “Isotopic Evidence for Biological Nitrogen Fixation by Molybdenum-Nitrogenase from 3.2 Gyr,” Nature, published online February 16, 2015, http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14180.html.
“Ancient Rocks Show Life Could Have Flourished on Earth 3.2 Billion Years Ago,” ScienceDaily, published online February 16, 2015, http://www.sciencedaily.com/releases/2015/02/150216131121.htm.

In a more recent study conducted by a team of scientists headed by Professor Von Karnkendonk, based at the University of South Wales, solid evidence for complex microbial ecosystems in the form of stromatolite colonies were established some 500 million years earlier at 3.7 Gyr ago.


M..J Van Krankendonk et al, Rapid Emergence of Life shown by the Discovery of 3,700 Million Year Old Microbial Structures, Nature Vol 537, pp 535 to 537, (2016).

Dr. Szostak claims the origin of life must have occurred via a Darwinian evolutionary mechanism, but the self-evident complexity of the first life forms strongly argues against this assertion, as there would not have been enough time to have done so. In other words, the window of time available for the emergence of the first forms of life on Earth is too narrow to entertain any viable Darwinian mechanism.

Dr Szostak continues by considering the vast real estate available for potential extraterrestrial life forms. Szostak presents the emerging picture; the principle of plenitude – that of a Universe teeming with planets. That is undoubtedly the case; there are likely countless trillions of terrestrial planets in the Universe.  However, new research on the frequency of gamma ray bursts (GRB) in galaxies suggests that such violent events would greatly hamper any hypothetical chemical evolutionary scenario. In December 2014, a paper in Physical Review Letters, a group of scientists estimated that only 10 per cent of galaxies could harbour life and that there would be a 95 per cent chance of a lethal GRB occurring within 4 kiloparsecs of the Galactic centre, and the likelihood would only drop below 50 per cent at 10 kiloparsecs from a typical spiral galaxy. What is more, since the frequency of GRBs increases rapidly as we look back into cosmic time, the same team estimated that all galaxies with redshifts >0.5 would very likely be sterilised. These data greatly reduce the probability that a planet could engage in prebiotic chemistry for long enough to produce anything viable.


In addition to GRB induced sterilization events, Dr Szostak completely ignores the remarkable fine tuning that is required to produce a planetary system that could sustain life for any length of time.




Dr. Szostak entertains the possibility that lifeforms with fundamentally different chemistry may evolve and that our type of life might be the exception rather than the rule. This reasoning is flawed, as the latest research suggests that carbon-based chemistry in a water-based solvent is overwhelmingly more likely to sustain any biochemical system throughout the Universe. Ammonia has been suggested as an alternative solvent to water but there are some( possibly insurmountable) issues with it.



Summary: Dr Szostak’s introduction presents a gross oversimplification of the true likelihood of prebiotic chemistry becoming established on Earth and other planets. Szostak does concede that our planet could be unique but is unlikely to be. The emerging scientific data however supports the view that life will be rare or unique to the Earth.

Video Clock Time; 10:00 – 32:00 min
The RNA World
In this section, Dr. Szostak presents the central dogma of molecular biology: DNA begat RNA and RNA begat proteins. Origin of life researchers were completely in the dark about how this scheme of events came into being, but in the mid-1980s, Thomas Cech et al discovered that RNA molecules could act catalytically.
Zaug, A. J & Cech, T. The Intervening Sequence of RNA of Tetrahymena is an Enzyme, Science, 231, (1986).

This immediately suggested a way forward; perhaps RNA was the first genetic material and over the aeons, it gradually gave up these activities to its more stable cousin, DNA. Szostak gives some examples of how this ‘fossil RNA’ has been incorporated into structures like ribosomes, the molecular machines that carry out the synthesis of polypeptide chains. His interpretation of these examples as ‘fossils’ is entirely speculative, however.

Szostak then explores hypothetical loci where prebiotic synthesis of biomolecules could have taken place, including the atmosphere, at hydrothermal vents and on mineral surfaces. For the sake of clarity, let’s take a closer look at RNA nucleotides, and in particular, the pentose sugar, ribose. Dr. Szostak mentions the Urey-Miller experiments where supposed prebiotic molecules were produced when an electric discharge was passed through a reducing atmosphere including water vapour. Though widely cited in college textbooks, its validity has in fact, long been discounted by serious researchers in the field. Urey and Miller assumed the atmosphere to be reducing in nature, but it is now known that it was neutral, consisting of nitrogen, carbon dioxide, carbon monoxide and water vapour.

The Early Setting of Prebiotic Evolution, Shang,.S
From Early Life on Earth, Nobel Symposium No. 84, Bengtson, S. (ed.), pp 10-23, Columbia University Press (1994).


Even in the complete absence of molecular oxygen, this atmosphere could not have sustained the production of prebiotic molecules, including ribose. Only in the presence of significant quantities of molecular hydrogen has some synthesis been demonstrated.

Schlesinger, G, & Miller, S. Prebiotic synthesis in Atmospheres containing methane, carbon monoxide and carbon dioxide. Journal of Molecular Evolution, 19, 376-82 (1983).

The problem with this scenario though is that molecular hydrogen would rapidly escape from the Earth’s gravitational field and thus is entirely irrelevant to the question of prebiotic synthesis.

An Aside:

Video Clock Time: 20:00 min: The Narrow Time Window:  Reconciling Dr. Szostak’s timeline for prebiotic chemical evolution with impactor bombardment history.

At 20.00min on his slide, Professor Szostak envisages the time during which prebiotic chemical evolution took place on the primitive Earth. He dates it to a period between 4.2 and 3.8Gyr ago (the supposed time of the beginning of the RNA world). Szostak presents a warm, aqueous environment during which all these reactions were taking place. But the planetary scientists modelling the impact history of the inner solar system have revealed a violent early history for the Earth. Extensive isotope analysis of terrestrial and lunar rocks, as well as cratering rate analysis indicate that the inner solar system was subjected to intense bombardment from the debris left over from the formation of the planets, which occurred between 4.5 and 3.9 Gyr ago. The cratering intensity declined exponentially throughout that era, except for a brief episode of increased bombardment between 4.1 and 3.8 Gyr ago. This is known as the Late Heavy Bombardment. One study has estimated that the total accumulation of extraterrestrial material on Earth’s surface during this epoch added a mean mass of 200 tons per square yard over all the surface of the Earth. Thus, Dr. Szostak’s relatively ‘gentle’ scenario is untenable. Realistically, the only oceans to speak of during this epoch are those of magma.


Anbar A.D. et al, Extraterrestrial Iridium, Sediment Accumulation and the Habitability of the Earth’s Surface, Journal of Geophysical Research 106 ( 2001) 3219-36.


Back to Ribose (a key component of RNA nucleotides discussed by Dr. Szostak). The only plausible mechanism for the synthesis of ribose is the so-called Butlerow reaction (also referred to as the formose reaction) which involves the coupling of the single carbon molecule, formaldehyde (methanal) in spark-ignited reactions forming sugars of varying carbon numbers, including ribose. However, many side reactions dominate formose chemistry, with the result that the atom economy with respect to ribose is very loww; up to 40 other chemical products being typically produced. This is the case in carefully controlled laboratory synthesis (read intelligently designed!), where the reaction is protected from contamination. Experimentally though, the presence of small amounts of ammonia and simple amines (which should be permissible in Szostak’s scheme) react with methanal to bring the formose reaction to a grinding halt.

Chyba, C. & Sagan,C., Endogenous Production, Exogenous delivery and Impact Shock Synthesis of Organic Molecules: An Inventory for the Origins of Life, Nature 355(1992): 125-32.

The concentrations of ribose would have been far too low to sanction any RNA world envisaged by Dr. Szostak. Compounding this is the added problem that ribose and other simple sugars are subject to oxidation under alkaline and acidic conditions, and since Szostak presents both hot and cold scenarios on the primitive Earth, it is noteworthy that ribose has a half life of only 73 minutes at 100C (near hydrothermal vents) and just 44 years at 0C.


Oro, J., Early Chemical Changes in Origin of Life, from Early Life on Earth, Nobel Symposium No. 84, Bengtson, S. (ed.), pp 49-50, Columbia University Press (1994).

But there are more serious reasons why Szostak’s scheme of events could ever have happened on the primitive Earth. This is encapsulated in the so-called Oxygen-Ultraviolet Paradox.
Szostak envisages prebiotic synthesis in warm aqueous environments, but on the primordial Earth, some 3-4 Gyr ago, the presence of much higher levels of radioactive nuclides such as uranium, thorium and potassium-40 would have presented another proverbial spanner in the works. These would have been more or less evenly distributed over the primitive Earth and when the radiation they produce passes though water, it causes its breakdown into molecular oxygen, hydrogen peroxide and other reactive oxygen species. Oxygen and the associated reactive oxygen species easily and quickly destroy organic molecules; not just ribose and other sugars but the other biomolecules mentioned by Dr. Szostak too, including fatty acids and purine & pyrimidine bases, required for the production of micelles and nucleotides, respectively .

The other part of the paradox pertains to the produce of stratospheric ozone, which requires ultraviolet light. The ozone layer was not present during the epoch in which Szostak’s scheme of events would have occurred. The intense UV irradiance on the primitive Earth would have sundered any exposed prebiotics, further compounding the problem.


Draganic, I.G., Oxygen and Oxidizing Free Radicals in the Hydrosphere of the Earth, Book of Abstracts, ISSOL , 34 (1999) .

Draganic, I, Negron-Mendoza & Vujosevis, S.I, Reduction Chemistry of Water in Chemical Evolution Exploration, Book of Abstracts ISSOL, 139 (2002).

Dr. Szostak appears to be completely unaware of Draganic’s work (though citing Hazen and Deamer’s hydrothermal synthesis work @ 31 minutes) and indeed, in and of itself, would preclude any further discussions of his scheme of events. But we shall nonetheless persevere with this analysis.


This work will be continued in a new post (Part II) here.

Test Driving the SkyWatcher Skyliner 200P

A Work Dedicated to Brian Carter


The cure for unobtanium is a good dose of speculum!

Here I shall provide an in-depth evaluation of the SkyWatcher Skyliner 200P; an 8 inch ( 203mm) f/6 Newtonian reflector.

Friday, January 30 2015: The telescope was ordered from Rother Valley Optics, UK. The complete telescope was purchased for £288, inclusive of delivery to my door. I chose this telescope owing to its almost universal acclaim by amateur astronomers of all levels of experience, from novices through to seasoned veterans. I was interested in getting something larger and more powerful than the most excellent 6″ (152mm)  f/8 model but  not something that would be too large and cumbersome. Many literary sources discussing Newtonian optics recommended as slow a mirror as one could get away with. As a purely visual observer, I was pleased to hear that SkyWatcher were offering their 8-inch Dob at a respectable f/6 relative aperture; a good compromise between portability, functionality and performance with a modest eyepiece collection.

An inexpensive Skywatcher laser collimator was purchased at the same time.

Monday, February 2: The telescope arrives in two boxes; one containing the telescope and various accessories and the other containing the flat-packed Dobsonian base. The telescope was presented in excellent condition, as were the accessories, but it was quickly discovered that the lazy Suzan rocker box supplied with the telescope was actually for the smaller 6 inch model. Oh dear; never mind!

A quick phone call to the retailer was enough to arrange for the delivery of the correct size base and the collection of the 6 inch base.

23:00h: A set of tube rings and a dovetail plate were found for the telescope and I was able to attach the instrument to my heavy duty SkyTee Alt-Azimuth mount. Turning to Jupiter, it was easy to see the optics were quite a bit out of alignment but I was taken at the sheer brightness of the planetary image. Star testing confirmed that the optics were mis-collimated.

The simple but sturdy focuser on the telescope can accommodate both 1.25 and 2″ oculars. The package came supplied with an 8x50mm straight through finder- always an impressive addition in my opinion.

Good but sturdy; the focusing unit on the Skyliner 200P

Good but sturdy; the focusing unit on the Skyliner 200P.













The innards of the optical tube were painted matt black and the parabolic primary mirror looked immaculate with no sleeks or flecks of dust/paint. Very nice indeed! Two eyepieces were supplied with the instrument; a ‘wide field’ 25mm focal length ocular delivering 48x and a higher power unit giving enlargements of 120 diameters.

The inner sanctum of the SkyWatcher Newtonian. All is well presented.

The inner sanctum of the SkyWatcher Newtonian. All is well presented.














Tuesday, February 3: The laser collimator arrived this morning and I quickly got to work aligning the optics. This device makes very light work out of precisely aligning the optical components. After placing the device firmly in the focuser, the secondary mirror was precisely aligned with the central spot of the primary and  then the primary was adjusted slightly to complete the collimation. This only took a few minutes to do but should make a huge difference to the images garnered by the telescope.

An inexpensive laser collimator makes collimating the instrument child's play.

An inexpensive laser collimator makes aligning the optical train child’s play.

23:30h: Seeing fair to good(Ant II-III).

With the telescope fully acclimated in -6C temperatures, I conducted a quick star test using my trusty 24-8mm Baader Hyperion zoom to confirm that the optics were properly aligned. The full Moon was easily framed with lots of room to spare at the 24mm (50x) setting of the zoom, the image being razor sharp and blindingly bright at the same time. Aiming at Polaris, I was very pleased indeed with the beautiful concentric diffraction rings either side of best focus at 150x. No astigmatism was noted with no signs of significant zones. This is a good, smooth mirror by most anyone’s standards.  In focus Polaris A was pinpoint sharp with delicate diffraction spikes consonant with a good Newtonian image. Its faint companion was easily picked off wide away.

I know; it's a Dob..... but I couldn't wait to test it.

I know I know; it’s a Dob….. but I couldn’t wait to test it.

Comparing the views of Jupiter with my 17cm f/16 Maksutov Cassegrain at~ 150x, I was very impressed how well the 8 inch Newtonian was performing. At a glance I could see finer detail than in the smaller aperture Maksutov, although the sky background in the latter was considerably darker.This could be attributed to stray light from off axis moonlight though and will require further investigation. I could also see the faint impression of the slender spider vanes superimposed on the in focus image, though after a few seconds, one could effectively ignore them and get on with enjoying the images.The brightness of the planet in the Newtonian at 150x will call for higher optimal magnifications to be pressed into service in due course.

Turning the telescope on Iota Cassiopeiae, the instrument was able to cleanly resolve the three components fairly easily. Almach, now fairly low in the northwestern sky was beautifully rendered, the golden and blue colours of the components coming through easily. The faint diffraction spikes around the stars were quite fetching to my eye. I don’t know what to make of them as yet.

All in all, an impressive first light for the econo-Dob.

Thursday, February 5: The replacement base arrived safely today and the 6 inch base collected. Assembly took about a half an hour. All tools were supplied with the package.

The Skyliner 200P Dobsonian telescope

The Skyliner 200P Dobsonian telescope.













18:30h: Seeing II, temperature -0.5C, clear, tranquil sky after dusk. Telescope kept outside all afternoon with optics covered. Little chance of dew as the air is still very cold and dry.

Theta Aurigae: companion easily picked off from the bright primary at 150x.


speculum vs speculum

speculum vs speculum

22:30h: Some low altitude cloud rolled into the valley for much of the evening while I was teaching. The telescope remained on duty throughout. Also fielded my 17cm Maksutov (acclimated) for comparison. Only a few targets visible at any one time. Seeing improved and now excellent, as judged on a very near perfect rendition  of Castor A and B through the Maksutov at 343x. I turned the instruments to Jupiter and studied the images delivered.

The winner: without a shadow of a doubt- the 8 inch f/6 Newtonian.

Both served up very fine views but the extra resolution and light gathering power of the Newtonian in these braw conditions put it comfortably ahead. An incredible amount of ultra-fine fine detail could be made out all across the planet but my eye was drawn to the high latitudes of the southern hemisphere, where a few curious white spots and a riot of linear structures quite literally jumped out at me! Where the smaller Maksutov suggested, the larger Newtonian plainly revealed.

The above was based on unfiltered observations, but I felt the icing on the cake was delivered with the addition of Al Nagler’s latest technical innovation: the Televue Bandmate BPL. It diminishes the glare around the planet, darkens the background sky and brings out all kinds of reds, oranges, yellows and whites within its enormous atmosphere. To my eye, the image at 170x with the filter was truly magnificent! I didn’t try any higher powers as there was no need to do so.

I would invite others to pull out their GSO/Synta 8 inch Dobs, spend a few minutes lining up its optics, letting it fully acclimate (I didn’t use a fan) with its cap on and comparing its views with a more ‘revered ‘scope of similar aperture.

To the rich man I speak: the man who has ‘seen’ it all in the cold light of day.

Maybe you could buy one in for a while at least? Nothing to lose, right??

But be prepared to be shocked!

SkyWatcher Skyliner 200P: planet killer extraordinaire!

Sic transit gloria mundi!

Friday, February 6: I am still somewhat in awe of what I saw last night and no one can take that away from me. It only takes one good night to reveal the truth about any telescope, whatever preconceptions one may have had about it. Would I have been better off with a larger mirror; a 10- or 12-inch maybe? I believe that the difficulty in producing a fine mirror scales with the square of aperture. A 12 inch reflector takes four times more effort to make well compared with a 6 inch. Larger apertures may have some advantage over the 8 inch but my previous experiences with these larger Dobs suggest that I would not get as many nights where everything comes together like it did last night. Mileage counts for me.  No, the right choice was the 8 incher since I now know that it will play ball with the environment in which I reside.

I spent some time this morning mulling over old books on planetary astronomy. One choice example is Bertrand M. Peek’s tome on Jove; The Planet Jupiter: The Observers Handbook (1981 edition) in which he states categorically that “An 8 inch [reflector] is probably adequate for all purposes.”

Fred W. Price and a host of other authorities I respect, say much the same thing.

Fortune continues to smile my way, as the settled conditions look to last at least another night.

Thank goodness for small mercies!

18:50h temperature -1C, seeing II

Had the ‘scope out for a couple of hours while I was teaching. Before the Moon rose above the horizon, I managed a very competent split of Eta Orionis (1,6″) at 225x. The Great Nebula in Orion (M42) was jaw-droppingly fine; a great heaving mass of ‘chlorescent’ gas at 120x. The Trapezium stars, including the more elusive E and F components, were easily sighted.

19:30h In a race against time, I assessed its wide field capability with two of my eyepieces; a Mark III 24-8mm Baader Hyperion zoom and 40mm Erfle. Both eyepieces performed extremely well in the very forgiving f/6 Newtonian. Cruising through central Orion at 30x in a 2+ field is a joyous experience, with stars staying acceptably sharp over the majority of the field with only the extreme edges showing coma and field curvature. I noted identical behaviour with the zoom eyepiece at the 24mm ( x50) setting.

You'll need to use the supplied adapter to get 2 inch oculars like this 40mm Erfle to work.

You’ll need to use the supplied adapter to get 2 inch oculars like this 40mm Erfle to work.











Caldwell 14, the Great Spiral in Andromeda (M31), the Auriga open clusters and M35 were simply spellbinding in the telescope. I have a 8 inch f/6 doublet achromat awaiting return to its manufacturer but I can tell you this Skyliner 200P package – costing 11 times less and weighing half as much – is a much superior telescope in almost every conceivable way!

Is a 8" f/6 rich field refractor really worth it?

Is a 8″ f/6 rich field refractor really worth it?













Oh vanity of vanities!

Later tonight it’s Jupiter watching time. Away in to watch the rugger the noo.

23:30h:Smashing opening performance by England against Wales in the Six Nations championship.

Low waning gibbous Moon in the southeast. No clouds.

Seeing deteriorated a little this evening (Ant III), as judged by the appearance of slightly swollen seeing disks in Castor A and B in the acclimated Maksutov. I decided to test how fast it would take the 8 incher to come to temperature by taking it directly out from a warm indoor room to the cold of pre-midnight air at -3C ( so ~ 25 degree C swing). The optics were left covered throughout. When I returned to the telescope an hour later, the images had stabilised. That’s no surprise though, at 8.6 kilos (19 pounds) fitted with its finder, the Newtonian actually weighs less than the 20 pound Mak and enjoys a much greater area over which to rapidly radiate heat.

I again compared the views of Jupiter served up by the Maksutov (17cm) with the 20.3cm Newtonian and the result was the same; the Newtonian shows finer details than the compound, confirming that aperture wins.

I’m not really known for having great affection for Newts, but this dapper telescope has got my pulse racing.

Saturday, February 7: This afternoon I leafed through my old observing notebooks from the years 1996 through 1997. Back then I was living in the city of Aberdeen, Northeast Scotland, and enjoying my first salaried job as a lecturer at the local university. In the summer of 1996, I purchased a Celestron 8″ f/6 Star Hopper Dob and at a much higher relative cost than today. “Wolfgang,” was big and awkward, with a cheap, oversized sonotube OTA. I observed Jupiter throughout the summer and autumn of 1996 with this telescope and my drawings reveal a good amount of detail; rather similar to the details I have recorded in more recent years with my 5″ f/12 achromatic. I was very pleased with the images Wolfgang served up – it was the base that failed first – but the planetary detail revealed by my new Skyliner 200P is easily better (on all but the worst nights) than any five incher can deliver. I was puzzled by this, and it got me digging a little deeper into my notes. Turns out Jupiter was fairly low  in my sky during those years, sprinting through Capricorn (1996) and then Aquarius(1997). Now, Jupiter is considerably higher in the sky and the differences between these instruments are much more apparent. That’s one theory. Here’s another one: Maybe the SkyWatcher mirror is truly better than the one I got with Wolfgang? Can technology improve in 20 years? It is not unreasonable to think that it could.

Man and his technology!

22:15h: Sky clearing of clouds, -2C, good seeing (Ant II): One final comparison between the 17cm Maksutov and the Skyliner 200P. Target: Jove: Both telescopes serving up super fine images. The 8 inch image is more detailed though. GRS approaching the planetary limb…..cool.

23:00h: Since the conditions were quite good, I decided to push the telescope by examining the diffraction images of stars at very high powers. Using the 2.25x Barlow with the zoom, I dialed in a maximum power of 338x and examined the brilliant white pair Castor A & B. In this cold weather, the heat from your body can affect the image, especially when you place your warm hand in front of the optical train to reach for the handle. This quickly dissipates though and when you are wrapped up warm and well insulated the effects are minimised.

I was really quite impressed at how clean and together the morphology of the Airy disks presented at these high powers. The diffraction spikes are quite charming and have an aesthetic grace all of their own. I have a very good mirror; a mirror that will surely see a lot of use over the months ahead.

Monday, February 9: With the moon now out of the evening sky, I spent a few minutes yesterday evening enjoying some dark, transparent skies with the Skyliner 200P. Using the low power (50x) setting on my zoom, I turned the telescope towards Cassiopeia now nearly overhead, and spent a few minutes drinking up the views of a few favourite open clusters including NGC 457 (with its ET eyes staring back at you), M103, M52 and a real corker: NGC 7789 which, at higher powers (75x), resolves into a plethora of faint stars. The traction on the mount is quite good and it’s easy to find the ‘sweet spot’ for most any application, from low power scanning to tracking planets and smaller deep sky objects at higher powers.

The SkyWatcher model in my possession probably has a similar mirror to the US branded Orion XT8 Dobsonian. US-based telescope reviewer, Ed Ting, has very interesting things to say about it here:

“It’s not a Zambuto,” he says, but it’s “shockingly good!”

How true!

These telescopes have been used to great effect by very well established observers on both sides of the pond. Dr Paul G. Abel, a professional astronomer and BBC Sky at Night presenter, uses the same model as my own – albeit on a motorised equatorial platform –  to make all his detailed observations of the planets. I contacted Paul to enquire about whether he had any modifications done to the optics. He told me he had not and saw absolutely no need to do so. A true draftsman, his portfolio of work is of the highest quality.

Arizona based astronomical artist, Jeremy Perez, needs no introduction in amateur astronomy circles. Using only an XT6 and XT8, Perez provides first rate illustrations to all the major astronomy magazines including Sky & Telescope, Astronomy and Astronomy Now,and has contributed to several major books on the subject of astronomical sketching. In addition, Jeremy maintains an excellent website where his versatile work with the XT8 is clearly demonstrated. Worth checking out I’d say.

What inspirational blokes we have here!

21:45h After enjoying some gorgeous views of M36, M37 and M38 at low and moderate powers, I turned the telescope to Jupiter, not yet at an optimal altitude, and was delighted to see that a satellite transit is currently underway. Will report back again later.


Me drawin's

Me drawin’s













Seeing wasn’t great tonight (Ant III mostly) and it was a bit on the breezy side, which made the image shake a wee bit. Fairly mild (+5C) though. I decided to keep magnifications down at 150 diameters. I followed the eclipse until it reached the middle of the disk and then noticed the GRS had reappeared. A fantastic experience!

Pure dead brilliant!

Geein’ it laldy!













Thursday, February 12: On family vacation until next week.


Monday, February 16: My short time away from this telescope got me thinking about its enormous potential. As it is, the Skyliner 200P is an excellent general purpose instrument; a veritable ‘diamond in the rough’. Yet, it can be improved still more and with only a modest additional investment. The reflectivity of the mirrors(worst case scenario) is ~87 per cent, so combined produce an overall light gathering power of 0.87^2 or 76 per cent. Light loss due to the area of the secondary mirror reduces this by just a few more per cent.  That’s  not bad at all:- still significantly more light gathering power than a 6 inch refractor but with greater resolving power.

Curiously, there are services available to increase the overall reflectivity of both mirrors to 97 per cent. For example, Orion Optics UK offer their Hilux coating which would bring its light gathering power into line with a similar sized refractor and maintain this ultra-high reflectivity for two or three decades!

The secondary mirror in the Skyliner 200P has a diameter of about 50mm, giving a 25 per cent central obstruction. This provides full illumination over ~ 25mm of the central region of the 46mm diameter field of view; a good compromise for all round use. Yet, Orion Optics UK also offer the amateur the opportunity to buy a smaller secondary with a diameter of 36mm, thus reducing the central obstruction to just 18 per cent, in order to optimise the instrument for planetary and double star work. Heck, one could keep both flats and inter change them as and when required!

Man and his technology!

Small modifications to the existing secondary, such as thoroughly blackening the sides of the mirror and the supporting structure would cut down on glare from bright objects. Flocking the tube with low reflection material, especially opposite the focuser, would also improve the overall contrast of the image reaching the eye.

Black paint skillfully applied to the secondary flat mirror and supporting structure would be a worthwhile project to improve overall performance.

Black paint skillfully applied to the secondary flat mirror and supporting structure would be a worthwhile project to improve overall performance.











What exciting telescopes these 8-inch Dobs are; lucis in caelo!

23:00h: With all the moving about the ‘scope has thus far endured, I decided to check collimation of the Newtonian, discovering that it was a wee bit off perfection. That’s how accurate the laser collimator is! Set it upon my kitchen table and tweaked collimation until it was spot on again and then set it out to cool.

Tuesday, February 17:


Ambient: +1C, blustery northwesterly sleet showers coming and going all evening.

Seeing quite poor (Ant III-IV). En passant, neither the fully acclimated 17cm Maksutov or the Skyliner 200P were giving good stellar images this evening. Only the easier doubles were resolved; Gamma Leonis was pretty at 150x and Mizar & Alcor lovely at 120x, their seeing discs large and ‘boiling’ in both instruments, but sightly more so in the bigger Newtonian. I attribute the latter to its greater sensitivity to atmospheric turbulence

Jupiter looked rough even at 150x but I was still able to watch the GRS whirl across its disk in both telescopes, the slightly larger Newtonian showing it that little bit more easily throughout.

Thursday, February 19: Having discussed the tremendous value of the Skyliner 200P with my editor, I decided to embark on a project to make the aforementioned modifications to the instrument in order to maximise its potential. The project will be featured in an up-and-coming issue of Astronomy Now and, at some later stage, on this website too.

Thanks for reading my story so far.

I leave you now with a short video on Sir Isaac Newton’s original reflecting telescope, its speculum mirror and the ingenious design of the tube and mount.

Man and his technology!

Wednesday, April 1, 2015:

I got both mirrors re-coated but in the end decided on just one secondary of 44mm diameter, and not two as I considered previously. This secondary provides a very decent 22 per cent obstruction – less than my Maksutov and a good compromise between a smaller secondary and a larger one. This still provides excellent illumination even towards the edges of the field with a low-power 2-inch eyepiece. I bought a water butt that perfectly fits the mushroom knobs under the base of the lazy suzan mount and provides both a stable and more elevated platform for more comfortable, seated visual use. I have undertaken all the modifications to the tube mentioned previously.

23:15 hours.

After putting the whole thing back together this afternoon, I set the telescope outside with its caps on about an hour before my teaching finished. As luck would have it; the sky was both clear and tranquil (seeing I-II) and I trained the instrument on Jupiter. Wow! Incredible detail such as I’ve never seen in the smaller Maksutov! I watched a beautiful shadow transit of one of the Galilean satellites. The GRS was near the centre of the disk and the satellite shadow was directly north of it.

I made a sketch to record those moments.

Time: 20:40-50h UT

Seeing: Ant I-II

Object: Jove

CM II: 236 degrees

Magnification 170-190x

Jupiter as seen in an 8" f/6 Newtonian reflector o the evening of April 1, 2015.

Jupiter as seen in an 8″ f/6 Newtonian reflector on the evening of April 1, 2015. North is at the bottom.














By 22:00 BST, I had successfully split Theta Aurigae and the magnificent triple system, Iota Cassiopeiae.

This is my best telescope, no doubt about it, but it’s also considerably less (even with the mods) expensive than the 180mm Mak and my 5 inch refractor.

In time, it will become my most used telescope.

Aperture wins. Period!

Don’t let any  wooly thinking cloud your judgement!

And so, it only remains for me to bestow a name on my powerful speculum; eight inches of Newtonian bliss.

Only one appellation trips off the tongue: Octavius!


Sailing away with young Octavius.













Saturday, April 4, 2015

An Evening with Octavius & Cornelia

Time: 22:00 BST

After a beautiful bright and reasonably warm day, I set up both my 17cm Maksutov and Octavius side by side and allowed them to fully acclimate over a couple of hours with the optics capped. I watched the Moon rising in the East and brilliant Venus in early gibbous phase setting in the West. When dusk arrived, I discovered that the seeing was near perfect! Hardly a quiver on Theta Aurigae at 340x in the Maksutov. Comparing the Jovian images one more time, I got the same result; two lovely instruments capable of rendering excellent images but one clear winner; the 8″ f/6!

The images are more stable in the Maksutov, owing to its greater insensitivity to atmospheric turbulence but that said, when the images settle down in the Newtonian, it easily shows its superiority to an attentive telescopist.

I would welcome people to test this claim and report it to as many astro folk as possible.

The biggest surprise though was the efficacy of the reflector in resolving the tricky binary system Iota Leonis. At magnifications of 225x, the 8 inch speculum showed me the clearest view of its elusive secondary that I have yet seen in any telescope. It’s raw resolving power is a law unto itself!

This is very encouraging as it means that there will be nights where I can push the 8 inch on pairs that are deemed very tough for my smaller instruments.

There is something very sweet and pure about this telescope; it’s understated power and charm shine through when just a little care and attention is afforded it.

My exchange with the public.

Battle o' the Specula.

Battle o’ the Specula.














A Triumph for Octavius!

For the defeat of badness with goodness!

For reason over darkness.

For diligence over laziness.

For vanquishing bluff with truth!

For heart filled humility!

And the ostrasization of hubris.

A Triumph for Octavius!

Go in Peace Octavius!



 Tuesday, April 7, 2015

Continuing to learn from Octavius in good seeing. Started a blog on my adventures on CN to save some space on my site. Thank you CN!

I am eternally grateful to Cornelia for teaching me the ways of Octavius.

On vacation until next week.

Wednesday, April 15 2015

Time 20:45-55UT

Work on CN aborted owing to account suspension.

After a rather grotty few days compared with what our neighbours south of the wall have enjoyed, the clouds cleared in the late afternoon here and it stayed that way for the rest of the evening. After work, I set Octavius outside to cool for about 40 minutes from my heated office. That was enough time to get it serving up excellent images of Jupiter in very good to excellent seeing (I-II). My favourite magnifications are about 170x with the Televue BPL filter in place. Though less than 50 degrees altitude at this time, the 8 inch speculum rendered some really fine details within the planet’s vast belt system. The NEB was a heaving mess and the SEB distinctly bifurcated. When I began my observations at dusk at 20:30UT, the GRS was just coming round the eastern limb of the planet and over the next half hour I watched it grow more and more distinct as it edged its way into the disk.

I made a sketch of my impressions at 20:45-55 UT CM II 182 degrees.

Jove 15.04

It is hard to describe the image of the planet in the Newtonian. It ‘flits’ back and forth between slightly blurry and magnificently sharp over the seconds and minutes. I have learned to completely ignore the spider vein shadows superimposed on the planet’s disk. They are largely cosmetic in nature and have little or no impact on the resolution of fine detail.

I have not encountered much in the way of the ‘tube currents’ that other observers have reported. Indeed, I have had the instrument long enough now to judge that I do not need a cooling fan. The mirror is relatively small and of low mass and cools very quickly, even if taken out of a warm room. I am mindful also of many distinguished observers from yesteryear (Philips, Webb, Browning etc), who produced some very fine work over many years using nothing more than passive cooling to the night air. Fans did not exist during their lifetimes.

 Friday, April 17, 2015

Time: 21: 20h UT

Seeing: II-III, fully clear sky.

I set up my fine 5″ f/12 achromatic refractor (Tiberius) as well as Octavius to again compare the views in each telescope when I pointed them toward Jupiter. I also compared the views of Theta Aurigae in both telescopes.

Will report back; a shadow transit has now commenced on the eastern limb of the planet.

Saturday, April 18, 2015.

Time 23:25 UT

Both telescopes easily split Theta Aurigae at about 150x. The refractor view had a darker sky background and the pair more stably presented. The Newtonian showed the pair more easily but less stably. The explanation is probably complex; the smaller aperture of the refractor and its greater thermal stability were probably at play here. But if I were not measuring the system, the Newtonian would be the better choice; simply because everything is brighter and better resolved.

I enjoyed a wonderful shadow transit and recorded my impressions of the event at 22:45h CM II 192 degrees.

Jupiter as it appeared in the 8" f/6 Newtonian x 170x w/TV BPL on the evening of April 17.

Jupiter as it appeared in fair seeing in the 8″ f/6 Newtonian , 170x w/TV BPL on the evening of April 17.

Both the 5 inch glass and the Newtonian revealed the event well but the superior resolution of the Newtonian made it far more compelling. ‘Perfect’ linear features in the 5-inch come alive in the 8 inch speculum and were transformed into a sea of ‘imperfections,’ as a feast of micro-contrast details – entirely invisible in the refractor – popped in and out of view. And all the while, the image always flits in the Newtonian and I did experience a minor tube current that lasted for about ten minutes before it vanished.

A 8-inch f/6 Newtonian has not received much acclaim as a double star telescope but I wonder if this is yet another urban myth. I am very encouraged by the ability of the instrument to split some tricky double stars – I added Epsilon Lyrae (observed just after local midnight) and Izar to my tally this evening. I would like to flesh this interesting topic out further after I have performed more tests, but in the meantime I encourage you to look at Jeremy Perez’s website to see how well an 8-inch Newtonian can perform on binary star systems. I am also mindful of the work of the English amateur astronomer,Thomas Teague, who has measured many double stars with a quality 8″ f/5 Newtonian and a Celestron Astrometric Eyepiece.


15:30 UT

I would now like to quantify the overall quality of the 8″ f/6 reflector in light of the modifications I have made to the instrument.

Here are some pictures of the modifications made.

Upgrading of the reflective coatings on both mirrors to 97 per cent. These should remain durable for at least 25 years, even in our rather damp climate.

Replacement of the secondary by a smaller one of semi-major diameter 44mm. Mirror edges blackened.

A new 44mm flat with edges blackened with matt black paint.

A new 44mm flat with edges blackened with matt black paint.











The interior of the tube opposite the focuser was flocked as well as the drawtube of the focuser itself.

Flocking the tube opposite the focuser is a good move.

Flocking the tube opposite the focuser is a good move.

These modifications give the instrument the contrast transfer of a 204* – 44 = 160mm unobstructed aperture – so a little better than a 6 inch refractor. Light gathering power will be significantly better than a 6 inch refractor though. ( *The diameter of the primary mirror as measured by Orion Optics UK)

The central obstruction does reduce the system Strehl in a quantifiable way (see below).

Graph showing the effect of a central obstruction on the Strehl ratio and Airy pattern.

Graph showing the effect of a central obstruction on the Strehl ratio and Airy pattern.

Taking a 22 per cent central obstruction and reading off the graph we get a Strehl of ~0.9.

All in all, I have a very  fine telescope in my possession, as experience at the eyepiece testifies.

Cost Breakdown

The Skyliner 200P: £288 delivered.

Water Butt: £20.00

Paint: £5.00

Flocking paper: £9.00 (including post and packaging)

Secondary mirror replacement with enhanced Hilux coatings ( including optical work to check surface flatness, post and packaging): £81.40

Primary mirror re-coating with Hilux (including post and packaging): £147.20

Total Cost: £550.60

Is it any wonder why I like to wax lyrical about Octavius!

20:00h UT

My sister in law came to visit yesterday and my wife and I  treated her to a view of Jupiter at sunset. At this precise time, the Great Red Spot was prominently on display and had just moved into the eastern hemisphere of the planet but the image was beautifully sharp. “Wow!” she explained, ” I have never seen the planet so well!

The superior coatings on the mirrors seem to have reduced light scattering in the image, improving the overall view.

When I ventured out later the same evening, I obtained a great and easy split of Iota Leonis at 339x. My first attempt at Delta Cygni was unsuccessful at 22:00 UT but it was rather low in the east at this time. When I tried again at 22:30 UT, the companion as well resolved at 225x.

Sunday, April 19, 2015

Yet another clear night tonight (remember you can always check my weather!) and time to explore some of the glories of the deep sky with Octavius.

I will report back on those activities later but for now I wish to share other user testimonies of the unmodified SkyWatcher Newtonian. I do not trust the judgement of those who express unbridled enthusiasm for apochromats. I believe their judgement in these matters is  ‘coloured’.

Here are 12 independent assessments of the 8 inch Dobsonian for your perusal.

In addition, I would like readers to compare and contrast the Jupiter drawings of Dr. Paul Abel (mentioned earlier) – a highly experienced planetary observer – who regularly uses the same telescope (albeit entirely unmodified!) as my own.

You can see a drawing of Jupiter he made with his 8″ f/6 in good seeing here.

Now, compare that drawing to one he made with the venerable 6″ f/15 Cooke refractor at Hampstead Observatory, London, just a few days afterwards (also in good British seeing).


Would you concede that the drawings reveal similar amounts of detail?

Tuesday, April 21, 2015

21: 40 UT

The Lyriads have been dandy this year! Hoping for the best of them tomorrow evening!

I have been very busy preparing my students for their science and mathematics exams, which are starting in just a few short weeks. But Octavius is so easy to set up and get working that I can put him to good use more or less immediately after I finish with them. The key is preparation; leave the telescope out with its caps on for about an hour and you’re cooking with gas.

Sunday evening presented near ideal conditions and I set the telescope to work showing me many deep sky objects. As dark skies will only remain for a few more weeks, time is against me and so I will refrain from providing that report until I have visited all the targets on my list.

Last night was interesting. At 21:10 UT, I had both the five inch refractor and the 8″ speculum out in the open air and fully acclimated. The seeing was unexpectedly grotty though (Ant IV), as I discovered while looking first through the refractor. I had to look very hard to see the GRS in the long glass; the details having been smeared out by the turbulent air. Remarkably though, while the image in the Newtonian was roiling in the bad seeing, I could much more easily pick it out on the Jovian disk. A modest aperture advantage is a boon on planets like Jupiter even in rough seeing. A good lesson learned!

Beginning at 20:10UT this very evening, I uncapped Octavius, turned him toward Jove and was delighted to see that the seeing had once again improved. The telescope was placed on its Lazy Susan mount an hour earlier and I gave Ciaran, my final student this evening, his first ever telescopic gander at a beautiful crescent moon dominating the lower western sky and Venus to its upper right. But first, I turned the telescope on Jupiter, dialled up 170x in the Baader Hyperion zoom (with TV filter attached) and focused the planet in the centre of the field. Ciaran peered in and after just a moment of eye-brain coordination shouted out, ” That’s so amazing! It looks like a spacecraft image!”

And it was! No GRS this time but plenty of palpable activity across its vast globe. The planet literally comes alive in the large aperture Newtonian.

He asked to look at Venus and I obliged. “This will look different,” I explained, as I unscrewed the filter. Choosing a power of 150x or so, I centred the planet and invited him to take a look.

” Wow!, he exclaimed, ” its dazzlingly bright! What’s that cross spread over the disk?

“Ah, that’ll be the shadow of the telescope’s spider veins on the planet,” I explained, turning the telescope to show him the secondary assembly.

” Oh I see!”, he said.

We ended by taking a quick look at the Crescent Moon and though quite low down, was wonderfully sharp and filled with astonishingly fine details (magnification about 80 diameters).

“Oh my God!” Ciaran said, ” that’s so beautiful!”

They say that what goes into a person cannot defile them, but what comes out, can. Ciaran spoke from the heart and revealed the truth about this telescope. And he did not defile himself.

Wednesday, April 22, 2015


The spell of fine weather continues and the telescope performed flawlessly all night. Jupiter was magnificent. The Crescent moon was eye-wateringly beautiful! I was able to effortlessly push the magnification to 340x without any degradation of the image.

Collimation has held remarkably well thus far; f/6 is clearly a good place to be in this regard.

I visited a suite of double stars to assess the efficacy of the Newtonian. Eta Geminorum Iota Cassopeiae, Alula Borealis & Australis, Iota Leonis and later Izar. All of these are repeat observations – routine but important work in the true assessment of any instrument.  All were easily split with the 8 inch speculum. I was most impressed at the calm and well resolved images of Iota Leonis at powers up to 500x. Out of curiosity, I threaded a neutral density filter onto the eyepiece and studied the images of Iota Leonis at 340x. The filter cuts off quite a bit of the glare and attenuates the bleeding of light from the brighter primary. The background sky was also darkened producing images that were incredibly refractor like!

The attenuation of light was modest though, as judged by my ability to easily detect (with the same filter in place) the light from the exceedingly faint (magnitude 10.1) companion to Alula Borealis. I tried this trick because of what I learned from reading the works of  skilled observers from a by gone age – vis-a-vis on the large American refractors – and I believe it will be an exceedingly good and inexpensive way to help ferret out faint and tight companions to a legion of double stars within reach of my modern 8 inch reflecting telescope.

The air was so dry here that even though the telescope was left out for several hours, it did not dew up and even after I brought the instrument in shortly after local midnight, only the merest veneer of moisture settled on the outside of the tube; the mirrors being entirely dew free! That doesn’t happen very often round these parts.

My growing scepticism of many of those who have dismissed the Newtonian as a capable double star ‘scope impelled me to, once again, seek the knowledge of my ancestors. Though regrettably, their work has been summarily ignored by many of my contemporaries – I have ‘discovered’ for myself that truly dedicated observers achieved some mighty impressive results with instruments not too dissimilar to my own.

I speak especially of the work of the great British Victorian astronomer, Thomas William Webb (Anno Domini 1806-85), who resolved many sub arc second pairs with a Newtonian reflector – and had no reason to mislead others. Earlier this afternoon, I received a copy of a book chronicling his life and work.

Webb owned many smaller refractors in his early days – a 3.7 inch Tulley refractor, a similar sized Dollond and even a fine 5.5 inch Clark refractor – but eventually he settled on a 9.3 inch With-Berthon silver-on-glass reflector, which he used  with great enthusiam for the last 20 years of his life. It is especially noteworthy that despite having the privilege of observing through similar sized refractors on magnificent equatorial mounts (owned by fellow astronomers in England, including those of Dawes and Huggins), he expressed no desire to acquire one of his own.

In the words of  Robert A. Marriot, author of Chapter 8 of the book, The Stargazer of Hardwicke: the Life and Work of Thomas William Webb;

Having observed with these large telescopes, with which he was obviously impressed, Webb must have had at least some desire to own such an instrument himself, and yet for the last two decades of his life he was perfectly content to use his 9.3 inch With-Berthon reflector, which, although beyond his definition of a ‘common telescope,’ always served his needs ’till the dappled dawn doth rise’.

pp 141.

Believe me when I tell you that I know exactly how the Reverend Webb felt!

Oculus historiae

Oculus historiae















Saturday, April 25, 2015

19:40h UT

Seeing III, excellent transparency, cool northerly airflow.

Octavius enjoying some late evening sunshine.

Octavius enjoying some late evening sunshine.















I am thoroughly enjoying the views of Jupiter with Octavius but the planet is now well past the meridian when full darkness falls upon the landscape. I can locate the planet just before sunset and with the TV filter in place can still make out some very fine details. The GRS is now in full view on the eastern hemisphere and can be very clearly made out with the generous aperture. Soon, I will show my boys the wonders of the first quarter Moon.


Temperature 4C

The boys were blown away by the clean, crisp images delivered by the Newtonian. They had great fun zooming from 50x to 150x with the Baader zoom.

I returned to Venus and enjoyed looking at its marble white, slightly gibbous form. 150x is about as high as I’d like to go on this planet. It’s hard to imagine that a world that looks so comely from our clement vantage is the most hellish place that one can conceive of.

I find the diffraction spikes on Venus to be a source of endless fascination. Like the presence of modest levels of secondary spectrum in achromatic telescopes, they have a singular beauty all of their own.

A Cruciform Venus

A Cruciform Venus















Sunday, April 26, 2015

Time 22:25h UT

Ambient: Seeing poor (IV), transparency excellent, temperature -2C, light north-northwesterly winds bringing in frigid polar air.

We had all four seasons in one day today. A freak Arctic blast of cold air brought hail and a light dusting of snow which quickly dissipated in the bright April sunshine.

Northerly air flows generally bring poor seeing to this valley and I could only manage 150x at the most on a handful of objects visited. Jupiter was poor this evening, the Moon reasonable at the lower powers. Caught a brief glimpse of Theta Aurigae’s faint companion though at 150x, which gave me a lift. All four components of Epsilon Lyrae also reasonably well (though still low down in the north east) resolved but the swollen seeing disks made the sight far less compelling. Ditto with Castor A & B and Gamma Leonis.

Called it an early night; glad to embrace the simple comfort of an open fire.

Thursday, April 30 2015

11:15h UT

With every passing hour, my admiration for the Newtonian grows. Myths cultivated in refractor forums have served to steer people away from them too long and my contempt for those who advocate apochromatic refractors in the larger sizes grows stronger day by day.

Disillusioned by contemporary telescopic culture, I am finding myself spending more and more time reading the literature of older, accomplished observers. Just this morning, I spent some time leafing through Fred W. Price’s book, The Planet’s Observing Handbook.  Like every other author of his generation (and many before him), he recommends – for sound, practical reasons – the Newtonian as the instrument of choice in the long term study of planets. The book is filled with beautiful drawings made with his own 8-inch reflector. There is something very pure and uncorrupted about his work that very much appeals to me.

In recent years, there has been a movement in the direction of ultra fast Newtonians, mainly to maximise aperture and portability. But not so long ago, opticians were making Newtonians with slower f ratios – in the f/7-12 range. With their very small central obstructions (CO) and amazing resolution and light gathering power, they would have beaten the pants off those pretentious large aperture Apos being offered to the unwary today.

Those who want that little bit more from a Newtonian would do well to consider models produced by smaller firms such as Mag1 Instruments, Orion Optics UK, and R.F Royce, to name but a few. Though more expensive than mass produced products, they are still far less pricey than their refractor equivalents and deliver excellent results that will serve you for a lifetime.

Personally, I am entirely sated by the performance of my 8 f/6 speculum, but I would love to see Synta or some such come out with a 8 inch f/7 or 10″ f/6 Newtonian models with enhanced coatings and a ~20% CO at a reasonable price. A big optics house like that should be able to pull this off without much alteration to its existing manufacturing ethos.

Check out this thread for encouraging signs of cultural change within the hobby.

Octavius: instrument of change.

Octavius: instrument of change.















20:30- 21:00UT

Another very cold and clear night, 2C, bright gibbous Moon in the south, light northerly airflow. Seeing lower than average (Ant III-IV).

Conducted some more observations of Jupiter and a drawing made at dusk as it appeared at 20:45h.

CM II 271 degrees. Altitude ~45 degrees.

Giant Jove.

Giant Jove. 170 diameters w/TV BPL


I hope milder weather will come soon and with it the return of better seeing. Then I can begin investigating some sub-arc pairs.

Friday, May 1, 2015

Cornelia, my large Maksutov, has been put up for sale to raise funds to donate to the Nepal emergency appeal.

Saturday, May 2, 2015

Playing with numbers: Last night, the seeing improved quite a bit and I was treated to a beautiful image of Jupiter with the Newtonian at 170x despite its lower altitude. Later, I visited some of the show piece doubles in Bootes and Lyra.  The Newtonian has thus far proven to be very powerful in this regard. All in all, I’m very impressed with its efficacy; quick cool down, excellent performance on the full panoply of celestial targets and relative insensitivity (compared with larger specula I’ve owned) to the vagaries of the atmosphere.

That got me thinking about an old chestnut oft pondered upon by amateur astronomers; is there, in any objective sense, an ‘optimum’ sized telescope for the pursuit of my double star adventures? Then I re-visited knowledge garnered by my diligent forebears.

According to the work of John Sidgwick in his Amateur Astronomer’s Handbook,  the maximum useful magnifcation (M) is given by M = 28D^0.5 where D is expressed in millimetres. Setting  D= 200mm gives  M = 396x, so about 50 per inch of aperture for my 8″ speculum. This seems like a reasonable upper limit for most applications conducted by amateurs and over quite a wide range of fielded apertures.

The late professional double star astronomer, Paul Couteau, considered the minimum magnification needed to best see and measure doubles to be M~ > D, where D is again expressed in millimetres.

Since magnification M is F/f, where F is the focal length of the telescope and f is the focal length of the eyepiece we have:

F/f > D,  so F/D > f.  F/D is the focal ratio (relative aperture) of the telescope and the formula predicts that this minimal magnification will be achieved with an eyepiece of focal length approximately equal to the the f ratio of the telescope  you use; an observation born out well in my field experiences over the years with many kinds of telescopes.

But over what range of apertures does this hold true? It stands to reason that Couteau’s minimum magnification ought to be less than Sidgwick’s maximum magnification. This can be expressed mathematically as an inequation:

D < 28D^0.5

Squaring both sides yields;

D^2 < 748 D

Thus, one solution yields D < 748mm.

That is a most interesting result, as 748mm translates into an aperture of the order of 30 inches! Bigger sizes are (presumably) too perturbed by the atmosphere to be used( without modern technology).

Can it be a coincidence that virtually all double star work has been conducted with instruments of this size and smaller?

I think not!

But, we may go still further. Is there in any sense an optimum sized telescope between zero and 748mm? Interestingly, we may also express this mathematically.

Specifically, what aperture, D, provides the biggest difference between the maximum and minimum magnifications employed, that is, what is the optimum value of D for the function arbitrarily defined as f(D)= 28D^0.5 – D?

Differentiating the function with respect to D and setting the result equal to zero yields:

f'(D) = 14D^-0.5 -1 = 0

Solving for D yields  D = 14^2  = 196mm.

Thus, in this simple analysis, the optimum aperture turns out to be 8 inches!


Bigger is better, of course, but not nearly as often!

Octavius; Optimus!

References: Argyle, R (ed.) Observing and Measuring  Visual Double Stars (2012).

Tuesday, May 5, 2015

A spell of unsettled weather has befallen us. All day long, sullen rainclouds drenched the landscape with life sustaining moisture. It was good, proper rain.

I’ve become much more curious about the potential of the Newtonian to explore hitherto uncharted territories in my observing experience. One of the posters on this thread was kind enough to share his experience with me regarding his success with the sub-arc second pairs; Eta Coronae Borealis and  Zeta Bootis, using a 8″ f/6 speculum.

Literature describing those kinds of activities conducted with Newtonian reflectors is well nigh scarce in comparison with the documented achievements of astronomers who have used the classical refractor. But I did find one interesting contemporary reference: Observing and Measuring Visual Double Stars (2nd Edition), 2012. In Chapter 11, written by Christopher Taylor, he crystallizes his thoughts on that very subject – The Newtonian Reflector in Double Star Astronomy.

Good with numbers, Taylor presents a proper (and I mean proper!) theoretical consideration of the Newtonian telescope in the pursuit of double star astronomy. Though he rightly acknowledges the superior stability of the images in long focus refractors, he nonetheless concludes that, given f/6 or slower relative apertures, “All the supposed optical defects of the reflector are removable or fictitious“, and further reminds us that, “a good 0.3m reflecting telescope is a far less expensive item than an equally good 0.3m refractor!

But there’s more. Taylor, rather admirably, backs up his conclusions with the results of his own observational experience using a ‘disembodied’ (my choice of word) old 12.5 inch f/7  Calver reflector with a 16.3 per cent central obstruction, erected in a “good spot” in his garden in rural Oxfordshire. There is no tube; the mirror is completely exposed to the night air and, perched seven or so feet above it, sits the secondary assembly. The mount, massive though it is, turns out to be an altazimuth! LOL!

The magnificent apparatus weighs in at an incredible 1500 pounds (680 kilos)!

On page 140, he describes his achievements with this mirror with which he employs a power of 825x on nights of excellent seeing. Though he does not mention the frequency with which he enjoys such nights, Taylor presents tables and sketches of drawings he made of  exceedingly close pairs like Delta Equulei and Beta Delphini (and many others); pairs that have orbital periods of just a few years or decades! Taylor describes the essence of why these investigations are so invigorating:

An unforgettable demonstration of this was provided by the 2005 periastron passage of the famous 169-year pair gamma Virgini…. In May of that year, gamma Vir swept through 0.3 arc seconds approach of the two stars with an apparent rate of revolution which briefly peaked at 2 degrees per week! This superb phenomena was closely followed by the author’s 12.5 inch right through closest approach, at which it was still found possible to make formal measures of p.a., yet it appears that sadly, few observers equally well equipped saw much of this spectacular double star event of the century.

His enthusiasm has rubbed off on me. I understand why he felt so passionate about seeing these things; for like ships passing in the night, it is only in the sub-arcsecond realm that amateurs can truly hope to enjoy  a ‘ringside seat’ on the dynamics of distant suns sailing far beyond the Solar System.

We cannot perceive what we cannot conceive.

Friday, May 8, 2015

Even though I have quite an extensive library on astronomical topics, of which I am very proud of, I came to realise that it was heavily biased towards the classical refractor. But I have become much more interested in the work of historical figures who used the power of the speculum mirror to advance the cause of astronomy. Accordingly, I have began to redress that imbalance by buying up several books – old and new – chronicling the lives of the Herschels who brought the reflecting telescope to the fore.

The Herschel Collection.

The Herschel Collection.















Saturday, May 16, 2015

I have not had much time to use Octavius of late owing to my teaching commitments. But I hope to redress that soon. I have examined quite a few deep sky objects on and off over the last month and will report back on my findings in due course. The length of true darkness at this latitude is now very short, heralding the onset of summer twilight which will persist until late July.

I have been reading some of the articles of Sky & Telescope associate editor, Gary Seronik, who is a long-time Newtonian enthusiast. In particular, I found Gary’s article on his 6 inch optimised Newtonian to be a good read. Seronik shows us how to get top quality views from a Newtonian at a fraction the cost of a refractor of the same aperture. And he derives even greater pleasure from the fact that the telescope is entirely home built.

Perhaps most importantly though,Seronik reminds us of the importance of aperture, noting at the end of that article that despite building and using a super duper 6 inch f/9 Newtonian, a 8 inch f/6  is, in his own words, “crushingly superior” on all targets.

No matter how much you mollycoddle a telescope, increasing its aperture is a better way to go.

Monday, May 18, 2015

CN account un-suspended but I will not be contributing there any more for personal reasons.

Friday, May 22, 2015

More musings on numbers: This thread has alerted me to the joy of establishing a theoretical basis for my experiences with Octavius. ‘Aperture’, you have heard, ‘wins’. Many know this to be true, as I do, within certain limits. But is there a physical basis for believing this? Why does resolving power scale inversely with aperture?

This leads us to Airy’s result for a circular aperture where  he showed that the resolving  angle, theta = 1.22 lambda/D, where lambda is the wavelength and D is the diameter of the aperture.

The derivation of the Airy formula is really quite involved but an email from a gentleman in the States (thanks Jim!) prompted me to look at a simpler approximation based on one rendition of Heisenberg’s Uncertainty Principle. Airy himself did not have knowledge of such physics, as this was only developed in the early 20th century. What follows is an elaboration of the approach of Dick Suiter, in his book, Star Testing Astronomical Telescopes.

Starting with the momentum/ position inequality.

delta p x delta y~ h where delta p is the ‘fuzziness’ in the momentum in the y direction and delta y is the fuzziness in position y. h is Planck’s constant.

Rearranging this formula we get;
Delta p/p ~ h/(p x delta y)

The momentum of the photon can be expressed as E/c and since E = hf  where f= c/lambda we get E = hc/lambda (wavelength). Delta y can be considered to be the diameter of the circular aperture, D.

So p = E/c = hc/(lambda x c) which simplifies to h/lambda

so delta p/ p ~  h/ (p x delta  y) ~ hc/fhD and since c/f = lambda the formula reduces to lambda/D.

Thus, the resolving angle, theta, or resolution of the telescope, can be expressed as
theta ~h/(p x delta y) = h x lambda/ (h x D) or just lambda/D.

It is interesting that this approximation is very close to Airy’s result of 1.22 lambda/D!

So it really is true; larger aperture resolves finer detail!
What is less well known to amateurs is that resolving power scales with wavelength, as shown. The shorter the wavelengths observed under, the finer the detail we ought to see.

Neat huh?

Octavius has eight inches of aperture; Tiberius, only five!

Wednesday, May 27, 2015

With the chemistry examinations taking place tomorrow, my academic work has come to an end for another year, so I have more time to finish this particular blog.

So, let us reason again, you and I.

I have already stated that I believe Octavius would give very similar planetary views to a 6- inch refractor and have a light gathering power at least equivalent to, or better than, a 7-inch  refractor.

One other way to establish the veracity of these claims is by comparing CCD images of a tough target like Jupiter.

In this capacity, I searched tinternet for comparison images taken with a 8″ f/6 Newtonian and a refractor that has garnered quite a bit of prestige in the amateur world; the TEC 140 (a 5.5 inch triplet refractor).

Here is the best sequence of images of mighty Jove I could find taken with the TEC 140.

Now, please compare these images with those taken with an unmodified 8″ f/5 SkyWatcher Newtonian. You can see some good ones here.

I have no reason to suspect any foul play,  as both imagers would have striven to show off the best attributes of their instruments. After all,  ’tis only human nature to do so!

Would you say that they are broadly equivalent?

I think the TEC images are a bit over processed and were probably (that’s a hunch, I can’t prove it however), captured using a more sophisticated CCD camera than that used by the owner of the Newtonian. The f/5 SkyWatcher Dob was completely unmodified though; so it will have a (larger) central obstruction and standard mirror coatings than my own telescope.

And yet it produces similar or even greater amounts of detail than the prestigious TEC!

But let’s now factor in the cost of these telescopes;

Suppose I were to order up a 8″ f/6 Dob from this retailer.

And then buy the TEC 140 (optical tube assembly only) from the same retailer.

The price differential is 20x!

Is there something I’m missing here?

Mein Gott im Himmel, ich verstehe nicht!

The ‘majesty factor’ perhaps?

A more ‘transcendent’ image maybe?

I can’t quantify either of those things, unfortunately, but I’m sceptical, as I know, having owned my speculum for a few months now and having personally tested many refractors of similar or larger size over several years, that both types of telescopes produce beautiful images of planets.

Octavius, with its 22 per cent central obstruction and light gathering power broadly equivalent to a refractor of the same size, must be a better all-round telescope than the famous TEC 140. Not just because I want it to (I do admit to disliking these telescopes for logical reasons); but because it has to!


Octavius the Meritorius!

Sunday, May 31, 2015

A note on further experiments

The primary mirror of the 8-inch SkyWatcher is very good. Very smooth optics with no astigmatism. I have experimented with removing the mirror from the cell a few times now to explore the effects of tightening the three clips that hold the mirror in its cell. A word of caution:-

Over-tightening the mirror will induce marked pinching in the optics that can ruin high power images.

I have found that they should be tightened just enough so as to hold the mirror rigidly in place but only so tight as the clips can still wiggle a little. The difference between getting it right and over doing it is like night and day!

Is your primary over tightened?

I have not thought much more on improving the thermal properties of Octavius because I could possibly be blissfully happy with its native performance without a fan. But I have learned that in days gone by some reflector enthusiasts have lined their tubes with cork or, more recently, with polystyrene flocking.

Will I give this a go?

Maybees aye, maybees naw!

Some notes on low power eyepieces:

I’m not one to fuss about eyepieces, especially these days when even inexpensive stuff is so good. Of all the astronomical kit discussed by amateurs, it is arguably eyepiece discussions that produce the most heat and the least light. Don’t get sucked down that black hole what ever you do LoL!

Since Octavius is destined to become my most used telescope in the coming years, I have thought some more about getting the best bang-for-buck, low power ocular for Milky Way sweeps and for framing the largest deep sky objects. At f/6, the 65 degree AFOV Erfle gives good performance but my attention was piqued by a new eyepiece marketed by Explore Scientific. As part of their Maxvision range, the company offer a 40mm model with improved coatings and eye relief, an adjustable eye cup and a larger 68  degree AFOV.

The Antares 40mm Erfle and the Explore Scientific 40mm eyepiece.

The Antares 40mm Erfle  (left) and the Explore Scientific 40mm eyepiece.










After testing the unit for a review (you can see that in Astronomy Now magazine) I decided to buy one as it offers enough of a performance boost to justify the switch, eventhough it weighs in at nearly three times that of the Erfle (1.25 kilos LoL)! And at £117 plus shipping, it wasn’t an extravagant splurge. Now I can play with a 2.25 angular degree field with my 8-inch speculum. M31 here I come!

Testing an' that.

Testin’ an’ that.














Man and his improving technology!

Tuesday, June 2, 2015

As I briefly touched on previously, some amateurs have found that using a fan which blows air over the primary to be a useful tool in bringing the optics to thermal equilibrium with the outside air.  Where I live though, there are never great temperature swings at any time of the year and my months of testing suggest that it would not really be necessary.

I’m a very lucky man!

Besides, I am not overly enamoured about the prospect of having some electrical appendage  grafted onto Octavius. It just wouldn’t feel right to me.

Wednesday, June 3, 2015

I would like to give you more.

I would like to describe my deep sky experiences with Octavius over the last few months.

Thursday, June 4, 2015
Octavius is a very impressive deep sky telescope. With a good 40mm wide angle eyepiece, I can coax fields of view in excess of two angular degrees out of it. On the evening of Sunday, April 19, I caught sight of the Pleaides (M45) just above the north-western horizon, where it seemed to hover above a distant rooftop. Excitedly, I trained my 8 inch speculum and focused the stars down to tiny, crisp pinpoints. I was delighted to see that the entire cluster fitted neatly into the field of my six element Erfle! I have since replaced this unit with a significantly better Explore Scientific Maxvision ocular offering a larger field of view (~2.25 degree) with slightly better contrast and edge-of-field correction. From a dark and transparent sky, my 8-inch speculum provides lovely, contrast-rich views with only a slight drop off of light at the extreme edges of the field. This edge-of-field vignetting is entirely acceptable though.

The improved coatings on both mirrors came into play when I visited the great globular cluster, M13 in Hercules. I could push the telescope to 200x before the image became too dim. The view was spectacular though, more reminiscent of what you would get with a standard 10-inch Dob. The cluster was beautifully resolved with hundreds of stars cleanly resolved to the core. The widely reported ‘propeller’ streams curving out from its main body were very obvious with a direct gaze. When I swung the telescope over to M92, I wasn’t disappointed either; a veritable swarm of white stars at 200x, more condensed than M13 but still well resolved almost to the centre.

Spring galaxies also benefited from the improved coatings. The celebrated M81 and M82 were easy pickings for this telescope, with both ‘Island Universes’ revealing excellent structural detail at powers of 150x or so. The improvement was obvious compared to a 17cm Maksutov set up along side it.

More exciting still, I could more easily trace out the spiral arms of the Whirlpool Galaxy, M51, in the constellation of the Hunting Dogs. The difference between the views in the smaller Maksutov and the larger Dob was subtle but quite obvious. The same result was observed when I swung the telescopes down to the Sombrero Galaxy (M104) in Virgo. The greater light grasp and resolving power both came into play to deliver a noticeably superior view in the Newtonian than in the Maksutov, where it was easier to see the prominent dust lane bifurcating this classic edge-on spiral at powers of 150x or so.

Open clusters are a pure joy to observe with Octavius. The Double Cluster (C14) in Perseus was beautifully framed at 30x. I spent a good twenty minutes in the wee small hours of Monday, April 20, in sheer awe observing this spectacular communion of stars. The superior light grasp of the reflector over either my 17cm Maksutov (now gone) or 127mm refractor was painfully obvious, my eye being able to differentiate white, blue-white, creamy, orange and downright ruddy constituents at moderate powers (100x or so).

Having become intimately familiar with the appearance of the three bright Messier clusters in Auriga in my 5-inch refractor, I was simply stunned at the difference manifested in the 8 inch speculum. While they are beautiful in the 5 inch glass, the view of M36, M37 and M38 in the larger speculum are quite simply in a completely different league!

The same was true when I compared and contrasted the views of the famous planetary – the Eskimo Nebula (NGC 2392) in Gemini – using the 8-inch speculum to my smaller Maksutov and refractor telescopes. The superior light grasp and resolution of the Newtonian allowed me to push the magnification to significantly higher powers. I found 250x presented the best views, with the bright, 10th magnitude central star being clearly differentiated from not one, but two diffuse, concentric shells of luminous matter. In contrast, the smaller ‘scopes maxed out at powers at or slightly below 200x. Shell detail was also a notch down in these smaller instruments.

All in all, I am positively delighted with the deep sky views served up by my 8-inch light bucket. From my relatively dark country sky, it will provide a lifetime of wonderful sights to visit, some for the first time, but many to be returned to again and again with the passage of the seasons, like faithful old friends.

Haste ye back the dark skies of August!

Friday, June 5, 2015

Having sold off some gear that I no longer need, I raised some more funds to purchase a better finder for Octavius. Nothing fancy; a SkyWatcher 9 x 50 right angled model, which can be bought new in the UK for a very reasonable price (£59). Such an upgrade will enhance my observing experiences by reducing neck strain and should make finding objects that little bit easier.

With June now upon us, the skies never get truly dark here. At such times, I turn my attention to double stars since they are little affected by twilight. Summer can present quite excellent seeing here (as revealed in many previous blogs), especially if its settles down for any length of time. I am very excited about examining progressively tighter pairs  to see how much I can push my 8 inch speculum. Once this study is completed I shall bring my public telescope reviewing to an end.  I pray that the weather will be kind to me in the coming weeks.

Saturday, June 6, 2015

Time : 00:05h BST

Ambient: 10C, brisk westerly wind, good transparency. Seeing fair to good  (Ant II-III).

I got off to a solid start this evening. Octavius was placed outside and left to completely equalize with the night air. No dew as the wind is too strong.

Izar ( Epsilon Bootis) beautifully split at 225x. Faithful colours – yellow and blue.

Epsilon Lyrae 1 & 2 easy at all powers above 100 but best seen at 225x

Alula Australis: lovely near equal magnitude split at 225x

Delta Cygni; easy pickings at 225x

Zeta Herculis – unsuccessful, strongly elongated at 225x and 340x. Will need calmer conditions to bag this puppy.

00:30h: The bonnie triple system, Iota Cassiopeiae, beautifully resolved at 225x, despite its fairly low altitude in the northern sky.

No’ bad ken.

Telescope put away. Time tae hit the hay.

21:00h BST: Heehaw else tae report today.

Dreich weather ken.

Get it roond ye!

Get it roond ye!










Sunday, June 7, 2015

Time: 19:50h BST

The Lord works in mysterious ways. After a day of downright rotten weather, today has been much better and this evening promises more clear skies! I have no idea how good they will be but at least I’ll get a chance to field my best telescope once again.

In addition, it occurred to me that I have another witness to my own weather conditions, someone impartial, who lives near me, and has been a contributor to this blog. He can vouch for my blue skies.

Indeed, maybe the same gentleman can vouch for most, or even all the observing-related weather entries I have made on this blog/review since its inception at the end of January last?

Hey, how lucky am I?

Just a few hours to go before my next adventure.

Fingers crossed eh!!

A white Knight upon a dark horse.

A White Knight upon a Dark Horse.















22:40 BST

Ambient; westerly wind has all but abated at ground level but with some isolated cloud patches moving swiftly across the sky at high altitudes. The cursed wee midgees came out earlier but now the temperature has fallen to 7C, so not likely to pose a problem going forward. Some good images of Arcturus.

Midgees eh!

23:30 BST

More cloud encroaching, seeing a notch or two down on last night (Ant III). But still managed a clean split of Izar at 170x. Seeing disks more turbulent tonight though. Ditto for Epsilon 1 & 2 Lyrae. The quartet were fairly stably held at 340x but with frequent morphing of the image in and out of focus as the stars swept through the field. Don’t see much point in continuing higher resolution targets tonight. Need to wait for better nights.

Monday, June 8, 2015

A Note on Collimation:

As anyone who uses a Newtonian regularly will tell you, alignment of the primary and secondary mirrors is absolutely critical  to gaining the best possible images. Some folk  get in a right guddle with this procedure though, while others border on being OCD, checking it two or three times per night. LOL!

As briefly touched on earlier, collimating a f/6 Newtonian is fairly straightforward and is rendered almost completely pain free using an inexpensive laser collimator (I use an inexpensive SkyWatcher unit). F/6 or slower systems can be accurately aligned  during the day but I tend to always back this up with a star test before beginning serious observations.

You can find a very good demonstration of how to quickly collimate your Newtonian here.

I would say however that it is better to do the collimating on a horizontal bench/table so as to avoid the small risk of one of the Allen keys falling down the tube and hitting that all-important primary mirror – yikes!

Looks like the weather is good again for tonight, so perhaps some better opportunities to  split some tougher pairs.

New finder should be with me tomorrow- yeehaw!

 22:00h BST

At last, a fairly warm and bright day. And the day has given way to what looks to be a clear and tranquil night.

I walked through the garden in the cool of the evening. With a song on my heart and a spring in my step, I retrieved Octavius from my office and set it out to cool.

The warmer temperatures have brought out more insects. The swallows feast on them high in the sky – a good sign that fair weather is here for a while. Nearer ground level, the bats are busy too. No wind. Only the sound of a distant cuckoo breaks the silence.

Will report back later.

23:45h BST

Ambient: 8C, twilit sky, clear and calm, seeing appears very good (Ant II).

I have just come in from examining the images of my first two targets.

Epsilon 1&2 Lyrae: Magnificent tonight! Much greater stability to the images, four tiny Airy disks surrounded by well structured diffraction rings at 340x.

Delta Cygni: Companion very easily seen and perfectly formed at 340x!

Just waiting for the sky to get that little bit darker……

Tuesday, June 9, 2015

00:20h BST

Lambda Cygni resolved tonight! Couldn’t see anything except the primary at 340x but the image was calm and very well defined. I ran to the office to get my 1.6x Barlow. This yielded a power of 544 diameters. Swung the star to the eastern edge of the field, refocused until it was at its best at the centre of the field – both members (0.9 arc seconds) well resolved! Mirror holding up really well at these very high powers!

Absolutely astonishing!

8-inch Newt owners please give this system a try, and, if successful, post it where everyone can see it!

Will talk more about it in the morning.


My new finder arrived today. Looks cool!

A nice comfortable finder for Octavius.

A nice comfortable finder for Octavius.

I had to get up early and leave for the city this morning, so was not able to make any more observations last night. But that’s beside the point; my modified 8 inch f/6 Newtonian resolved a sub arc second pair – Lambda Cygni – and with relative ease!

Why was I astonished by this result? Well, for one thing, being used to the domination of this arena of observational astronomy by the classical refractor, it was quite a reality check to see that a Newtonian could perform so well in this regard. But there is absolutely nothing in this observation that violates any known principle of physics; I mean, according to the Dawes limit (4.56/D) my Newtonian ought to do considerably better – down to 0.57 arc seconds.

I know from previous experience with a 17cm Maksutov that in some locations I can split pairs as low as 0.7 seconds of arc. And if I can do that with a Maksutov, I can also do it with a Newtonian. Let’s just say I have faith in Octavius; not a blind faith but one that is based on reason and experience.

The fine weather is continuing but this would be a suitable place to finish.

I commend this economical telescope to you and invite you to test everything I have said about it.

It is, by some considerable margin, the best bang for buck in the entire hobby! It is a most excellent, all-round performer, an instrument that will serve up a lifetime of wonderful views and require little in the way of maintenance.

Three Cheers for Octavius: the People’s Telescope!

Thank you and God Bless!


De Fideli

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The Joy of the Maksutov Telescope

The author's superlative 7 inch Maksutov Cassegrain.

The author’s superlative 18cm f/15 Maksutov Cassegrain.

















A work began in December 2014.


Dedicated to Asbytec.

Among the catadioptrics, the Maksutov Cassegrain has justifiably earned a solid reputation as an excellent high resolution telescope. The Maksutov design combines a spherical mirror with a longer native focal length (slower f/3 relative aperture) than a typical Schmidt Cassegrain (f/2) with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly spherically symmetrical. The negative lens is usually full diameter and placed at the entrance pupil of the telescope (commonly called a corrector plate or meniscus corrector shell). The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting for chromatic aberration. It was patented in 1941 by Russian optician Dmitri Dmitrievich Maksutov after a five year spell of careful ray tracing and prototype building, which culminated in the first working model produced in the autumn of the same year. He based his design on the idea behind the Bernard Schmidt’s camera, which used the spherical errors of a negative lens to correct the opposite errors inherent of a spherical primary mirror. Because the design utilises all-spherical elements, it greatly aids in mass fabrication.

Similar independent meniscus telescope designs were also patented in 1941 by Albert Bouwers (his 1941 concentric meniscus telescope), K. Penning and Dennis Gabor (a catadioptric non-monocentric design).A culture of secrecy during World War II kept these inventors from knowing about each other’s designs, but rightly or wrongly, the design was named after Maksutov and the rest as they say is history.

This design appeared commercially in Lawrence Braymer’s 1954 Questar telescope and in Perkin–Elmer designer John Gregory’s competing patent for a Maksutov–Cassegrain. Commercial use of Gregory’s design was explicitly reserved for Perkin–Elmer but was published as an amateur telescope design in a 1957 issue of Sky and Telescope in both f/15 and f/23 iterations. Most Maksutovs manufactured today are this type of ‘Cassegrain’ design (called either a “Gregory–Maksutov” or “spot-Maksutov”) that use all-spherical surfaces and have, as a secondary, a small aluminized spot on the inner face of the corrector. This has the advantage of simplifying construction. It also has the advantage of fixing the alignment of the secondary and eliminates the need for a ‘spider’ that would cause diffraction spikes. The disadvantage is that, if all spherical surfaces are used, such systems have to have focal ratios above f/15 to avoid aberrations. Also, a degree of freedom in correcting the optical system by changing the radius of curvature of the secondary is lost, since that radius is the same as that of the rear meniscus face. Gregory himself, in a second, faster (f/15) design, resorted to aspherization of the front corrector surface (or the primary mirror) in order to reduce aberrations. This has led to other designs with aspheric or additional elements to further reduce off-axis aberration. This type of Maksutov-Cassegrain’s high focal ratio and narrower field of view makes them more suitable for lunar and planetary imaging and any other type of observing where a narrow field high power view is just fine for resolving tightly packed globular clusters and double stars.

The elegant Questar 3.5

The elegant Questar 3.5

The Rumak
The Rutten Maksutov–Cassegrain (also called a Rumak or Sigler Maksutov) has a separate secondary mirror mounted on the back of the meniscus corrector, sometimes similar to the corrector/mirror holder configurations found in commercial Schmidt–Cassegrains. This provides an extra degree of freedom in correcting aberration by changing the curvature of the corrector and the secondary independently. Specifically it allows the designer to aspherize the secondary to provide a flatter field and slightly better colour correction than traditional spot Maksutovs, with less off-axis coma. Mounting the secondary on the corrector also limits diffraction spikes. This version is named after the work of the Dutch optical designer, Harrie Rutten.

Are Rumaks something to aspire to over the Gregory Mak? That’s something you’ll have to decide for yourself. To my way of thinking, a Gregory Mak is more than up to the task.

Some Historical Models

Perhaps the most iconic of modern telescopes is the beautiful Questar 3.5, a classic all metal, 90mm Gergory Maksutov that has changed very little in over half a century. While expensive, it is a great work of art and is still highly favoured by telescopists in the 21st century. The company also make larger Maksutovs (the Questar 7, for example) at (you’ve guessed it) much higher prices. Celestron was the first to respond to the high cost of the superlative Questar and marketed their orange tube C90 Maksutov in the late 1970’s for less than $500.

Optically, these were said to be quite variable, from mediocre to excellent. The big game changer came in the 1990s when Meade revolutionized amateur astronomy by introducing the ETX, first in the form of the RA, which had a built in clock drive and then shortly afterwards with the ETX EMC which featured full go-to capability. As an owner of the original ETX 90 RA, I can vouch for the excellent optics – on par with that of the far more expensive Questar 3.5. And though my unit is approaching twenty years of age, the mirror is in excellent condition, as are the coatings on the front corrector plate.

The venerable C90. Image credit; Celestron.

The venerable C90. Image credit; Celestron.

Shortly after the launch of the ETX 90, Meade introduced two larger instruments from the same family – the ETX 105 and the ETX 125. I spent a considerable time looking through the 125 and can vouch for the razor sharp optics on these units when conditions allow.
Meade also produced a 7inch (178mm) f/15 Gregory Mak as part of their highly successful LX-200 series of computerised telescopes which received very high praise from discriminating lunar and planetary observers who raved about their apo-like optics. These telescopes are now considered highly collectible classics from the late 20th century.

Celestron has recently revamped their venerable C90 in a neat black-tubed spotting scope. Costing less than $200, it provides excellent optics in an ergonomic package, eminently suitable for general nature studies and astronomy. It has to be one of the best bargains in the hobby today.

Cheap as Chips: the all new C90 spotting scope from Celestron.

Cheap as Chips: the all new C90 spotting scope from Celestron.

Despite these innovations it is arguably the range produced by Orion (USA) and SkyWatcher that has made most heads turn in the Maksutov camp in recent years. Following fast on the heels of the better established small companies, Synta churned out an exciting suite of Gregory Maks in the 90mm – 180mm range which could be purchased as complete packages including a mount or as optical tube assemblies. Over the last six months or so, I have been carefully evaluating an Orion re-branded version of Synta’s 180mm model – the telescope that has really opened my eyes to the tremendous versatility of the Maksutov design as a visual instrument.

She sure is purdy: the ETX 90 RA.

She sure is purdy: the ETX 90 RA.











Fit ’n’ Finish
The Orion 180mm Mak is no Questar 7 that’s for sure. But it’s got a few things going for it that makes it an exciting prospect even in comparison to the legendary Questar. For one thing, the corrector plate is made from the highest quality Schott optical glass and has the very latest in multi-layer anti-reflection coatings. The primary spherical mirror is over-coated too which will ensure its longevity over many decades if properly looked after. As a result, it may surprise you that it will yield slightly brighter images than older Questar 7s what with their more primitive magnesium fluoride based anti-reflection coatings.

The instrument is focused in a completely different way to a refractor and involves moving the primary mirror either closer or further away from the front corrector plate. I received the unit in perfect collimation after a long road trip and all of the components have remained in perfect alignment despite me taking the entire instrument apart to flock the inside tube as well as the long, slender baffle tube leading to the eyepiece. There is not many telescopes on God’s Earth that would allow such license. Remarkable!

The telescope equipped with tube rings and a 50mm finder tips the scales at under 20 pounds and is less than half a metre long, so it can be used on a light weight mount so saving quite a considerable cost to the user. This makes the instrument super portable, much more so than the equivalent refractor.

Maksutovs and Acclimation
Large catadioptrics can take some time to acclimate, especially if taken from a warm indoor environment to a chilly night outside. But this is not unique to this telescope genre; all large telescopes will struggle for a while before they stabilise in their new environment. Thankfully there are ways to ameliorate this. I’ve often taken my 17cm Orion Maksutov out from the warmth of my living room into my dry, unheated shed and after just a few hours it was delivering pinpoint stars in temperatures just a few degrees above zero. I’ve also had it working perfectly well in sub-zero temperatures.

The same instrument can be made permanently grab ‘n’ go by keeping it in the same environment while not in use. Others resort to active cooling using fans that blow cool air across the optical components. Some have suggested using cheap ice packs to create strong thermal gradients to draw heat out of the instrument. All these measures will accelerate the process of thermal acclimation. That being said, there are a few individuals (the ‘poodle pushers’), who have persisted in wilful scaremongering about these telescopes by asserting that they won’t acclimate in many locations. I was alerted to this after discovering that a seasoned observer located in La Union in the Philippines, who has done great work with a 6–inch Maksutov, over many years, enjoys diurnal temperature changes of ~10 degrees Celsius. This prompted me to do research into diurnal temperature variations and what I discovered was quite revealing; the vast majority (pick a location, any location LOL) of locations where humans live and observe enjoy annual diurnal temperature variations of the order of 10C, so telescopes of most any design will acclimate. At my location, these variations amount to little more than a few degrees (check out Glasgow climate and its average high and low temperatures throughout the year) and I have never had any significant issues with my 18cm f/15 Maksutov. The worst places, which exhibit diurnal temperature swings of the order of 20C or more, are located in deserts and at high altitude.

Maksutovs will work nearly anywhere on Earth. Laziness and ill-preparedness have prevented many from discovering this. Others have been led to believe that they won’t acclimate. But I wonder if this is the result of cultivating an elaborate lie; a  bad meme spread on tinternet. I remember a well known American astronomy couple who boldly wore T-shirts emblazoned with a  ‘No to Catadioptrics’ logo. How myopic is that? How misleading is that?

Recurring interest in the Maksutov Design over the Decades
There seems to be a recurring interest in this design, its elegance of form, extreme portability for its aperture, rigidity of the various components in the optical train and so on and so forth. It’s interesting that three of the leading amateur ‘scope makers in the USA; TEC, Astrophysics and D&G have offered Maksutov or classical cassegrains to their customers. The leading UK telescope manufacturer, Orion Optics, Newcastle under Lyme, England, also produce their own versions – the OMC series;

Zeiss too seem to have offered Maksutovs for amateur astronomers back in the day. And then there are the various incarnations from Intes and Lomo etc etc.

Why do you suppose the Maksutov Cassegrain has garnered such interest from these opticians of skill? The reasons are clear to me. The extreme portability and ergonomics of the design is a major plus of course, and the not too inconsiderable fact that they serve up images midway between an SCT and a fine refractor, has led leading telescope makers to maintain an interest in building them and bringing them to market.

Having said that, in independent bench tests, the mass market Maksutov fairs very well in comparison to custom designed units made by leading telescope makers.

I found one test on Mr. Rohr’s website, where he evaluated the 6 inch SkyWatcher model;

Herr Rohr also evaluated a 8-inch TEC Mak.

You can see two further  tests on the SkyWatcher 180 Maksutov here and here.

Not bad quality from the mass market Chinese Mak eh?

That kind of quality is more than enough to achieve  superlative visual results as is showcased in the next section.

A Case Study; Asbytec’s Work with a 6 inch Orion Maksutov

As mentioned previously, dedicated observers using the Maksutov have produced some very high quality work. Based in La Union in the Philippines, Asbytec has faithfully used his 6 inch Orion Mak over several years to produce an excellent portfolio. Many of his drawings have appeared on the online telescope site Cloudynights. His work highlights the high resolution capabilities of the Maksutov under good seeing conditions and his trained eyes have really pushed the envelope in terms of what can be seen. My own but less extensive work with its larger Orion sibling dovetails very nicely with his.


Jupiter. Image Credit: Asbytec



Mars. Image credit: Asbytec










Saturn. Image credit: Asbytec

The Eskimo Nebula

The Eskimo Nebula. Image credit: Asbytec


Elongation in 72 Pegasi. Note the angular separation!

Elongation in 72 Pegasi. Note the angular separation! Image credit: Asbytec.














 Imaging with the Maksutov

The long focal length of the Maksutov makes it especially suited to lunar and planetary imaging. The majority of other instruments require powerful Barlow lenses or Powermates to boost the f ratio to about f/20 ( generally considered to be the sweet spot for imagers) but with many Maks having relative apertures of 15 or so, little in the way of auxiliary amplifiers are needed to get to that optimal imaging speed. Richard Garrad, an imager from Utah, USA, has used his Orion 180mm Mak to great effect capturing detailed images of the bright planets and the vast lunar regolith. You can see examples of his work and gain an appreciation of the excellent resolution and light gathering power of the same instrument here.

Notes from the field

To get the best performance out of the telescope, I flocked its main tube as well as the long baffle tube connected to the primary mirror. This resulted in a small increase in the contrast of daylight images (which can ‘flood’ the tube with off axis light) as well as on bright objects like the Moon and bright planets. After testing a few different types of diagonals, I came to the conclusion that a good quality prism diagonal was preferable to its dielectric mirror based counterpart. Well made prism diagonals seem to do a better job curtailing stray light and improve contrast. That said, the model used was not one of the expensive prism diagonals but a no frills 1.25” Celestron model # 94115-A, which I consider an excellent value in today’s market.

Like the classical refractor, one of the great joys of the Maksutov telescope is that it can be used with fairly inexpensive eyepieces owing to its high f ratio. Simple Plossl and orthoscopic  eyepieces give excellent edge to edge performance in this telescope so the user will not incur a large monetary sacrifice in using the instrument. For low power work,  I elected to use a SkyWatcher 32mm Plossl delivering a power of 84x and a half a degree field. For higher power applications I employ a 24-8mm Mark III Baader Hyperion zoom, the performance of which is excellent.

One might think that such a large aperture telescope would be unsuitable for nature studies but I found it to be excellent in this capacity. What makes it so versatile in this respect is its low mass and tremendous back focus. You can focus on flowers just a few metres away and examine their glory at powers up to 300x. I found this to be quite an enjoyable pastime during the summer months. Without the addition of various extenders etc, you simply can’t do that with a refractor of the same size.

Comparing its double star efficacy with that of my fine 5” f/12 achromatic refractor, I found the Orion Maksutov to be noticeably superior at ferreting out sub arc second pairs such as the 0.9” Lambda Cygni and on one occasion, a big surprise from the star O Sigma 507 (RA 23h 49 min, Dec +64 degrees 54 min). The A/C components (mag 6.8/8.6), separated by about a Jupiter diameter and arranged roughly north-south, were easy pickings at low power but I was more interested to see what happened to the primary as I cranked up the magnification to 340x. So I swung the system to the east end of the field and let the vibrations settle down. To my sheer amazement, I glimpsed (often for several moments at a time) the secondary (A/B; mag 6.8/7.8) just (and only just) touching the primary, and extending away to the northwest! I repeated this several times within a few minutes to make sure I wasn’t seeing a diffraction artifact. As I have described elsewhere in my double star surveys, it looked for all the world like a “a tiny little snowman in the sky” morphed time and again by the vagaries of the atmosphere. Now, my records show that A/B is currently of the order of 0.7 arc seconds apart! This is truly an extraordinary result, as the components were not merely elongated but very nearly separated to my average eye. Clearly, the Maksutov was operating real close to its theoretical limits (so far as is known conventionally). Let me tell you you’ll struggle to get this kind of performance out of  the finest 6 inch refractor money can buy!

This is a new ‘personal best’ for me. If anything, it shows that I can go beyond the 0.9 arc second barrier under the best conditions which were clearly on offer at this location (Torphins in Northeast Scotland) on this evening. That said, my notes show that I already enjoyed excellent seeing here before, albeit using smaller instruments.

In other tests, I turned both the Orion Maksutov and my high quality 5″ refractor on Psi Cassiopeiae. The primary is a 5th magnitude K spectral class star and just east of it lies the faint magnitude 9.1 and 10.0 (C & D components) separated by 2.3”. Looking first through the large Maksutov, I could see the exceedingly faint pair at 170x. The challenge here is that the C and D components are both very close and very faint and the bright orange glow from the primary right next door doesn’t help. In comparison, the 5 inch refractor really struggled. I convinced myself that it was doable – but only just! Thus, there was a clear performance difference between the instruments here.

The faint companions to Psi Cassiopeiae as sketched by the author.

The faint companions to Psi Cassiopeiae as sketched by the author.

The same is true of its lunar and planetary performance. If fully acclimated and under good conditions, the Orion Maksutov will comfortably outperform the 5 inch refractor. This was made apparent by studying the craterlets on the floor of Plato. The largest – A, B and C – can be seen in the 5-inch refractor but are better defined in the larger Maksutov. The D craterlet, which was distinctly seen in the Maksutov, was invisible in the refractor under the same conditions.

Preliminary tests comparing the views of the 5 inch refractor with the Orion Maksutov confirms that the latter can also resolve significantly finer atmospheric details on Jupiter than the former. The brighter image of the 170mm Maksutov allows greater magnifications to be pressed into service and under good conditions shows the true colour of ovals and barges.

Though opinions differ, like many larger aperture telescopes, I believe the Maksutov does benefit from the use of filters to bring out very subtle planetary details on the precipice of visibility. Blue filters (the 80A and 82A) are excellent for bringing out belt details, while the Baader Neodymium, Contrast Booster and TeleVue Bandmate planetary filters show great promise in enhancing low contrast details on the Jovian disk.

Some amateur astronomers consider the Maksutov to be a rather specialised, high power, high resolution instrument, but that does not mean it can’t be put to good use as an effective instrument on deep sky objects. Truth be told, the vast majority of these objects are well framed within the smaller field of view of the Maksutov. In this capacity, I enjoyed many evenings studying the glories of the late summer Milky Way through Cygnus and Cassiopeia. Small open clusters are excellent targets for this telescope, as are globular clusters, owing to the telescope’s extra light grasp over a mid-sized refractor. The finest 5-6 inch refractor money can buy will not give you an image of M13 like this economical Maksutov. Everything is easier to see and better resolved. The Orion 18cm Maksutov is also a wonderful telescope for studying planetary nebulae. The views I had of M57, M27 and NGC 6826 were simply spell binding, exploiting the natural, high magnifications achieved by this instrument.

There is nothing preventing a determined observer from sketching larger swathes of sky than can be captured in the small field of view of the Maksutov. Here is a modest sketch I made of the Double Cluster (Caldwell 14) in Perseus. Because the maximum field of view presented by the 32mm Plossl is only of the order of 0.5 degrees, it cannot wholly capture both clusters in the same field of view. Both NGC 869 and NGC 884 individually span some 18’ of sky and are separated by about a Moon diameter (25’). Nevertheless I wanted to include both in the sketch, so I took to ‘stitching’ them together by moving the telescope slowly eastward from the core of NGC 869 towards NGC 884.

In this way the traditional limitations of the Maksutov’s small field can be overcome; in just the same way that imagers have done with their CCD cameras.

The Double Cluster in Peeus as drw by the authorr using his 17cm Orion Mak and a 32mm Plossl eyepiece.

The Double Cluster in Perseus,  as drawn by the author using his 18cm Orion Mak and a 32mm Plossl eyepiece.

The Orion 180mm Maksutov Cassegrain represents an excellent alternative to a medium aperture apochromatic refractor but is more closely akin to what you would expect from a long focal length classical refractor of the highest quality. The telescope will need some time to acclimate in winter, especially if taken from a heated inside room to the cool of the night air, but storing it in a dry unheated outhouse should alleviate any problems in this regard. Its ultra-compactness and relatively light weight for its aperture will allow you to transport the instrument safely in the back of your car to a dark sky site. In today’s market, where some amateurs obsess over high quality refractors costing a second mortgage to acquire, this magnificent, ergonomic telescope represents an exciting breath of fresh air! A telescope like this would have astounded an observer in my father’s time and he would have needed the wealth of a Sultan to acquire one of this quality. To think that one can get this kind of performance out of a telescope that cost just a few hundred pounds, is only half a metre long and weighs a mere 20 pounds, is a joyous revelation.

Why would anyone want anything more from a lightweight, ultraportable visual ‘scope?

Update: March 19, 2015

Having spent a fairly cold winter with this instrument, I am delighted to report that it has earned my deep admiration. Out of curiosity, a second time: I deliberately unscrewed the back, removed the flocking from the long baffle tube on the primary and reapplied fresh stuff LOL. When I put it back together, it still held perfect collimation as judged by a high power star test on two separate nights! This telescope is amazingly resilient to mis-collimation! Try it out for yourself! I think the rigidity of the aluminised spot on the secondary helps make this magic happen.

I have managed its ‘alleged’ thermal problems throughout this time and never once have I needed to resort to some kind of active cooling. I will re-state what I said previously; if the telescope is kept in a dry unheated shed, it is effectively in a permanent ‘grab n’ go’ state. Only the local seeing conditions will curtail its efficacy.

My family and friends have enjoyed some positively charming views of Jupiter, the Moon and a variety of deep sky objects with the telescope.

Here is a drawing of Jupiter I made on the evening of March 24 during a spell of fine weather.

Jupiter, as it appeared in the telescope at 190x on the evening of March 24, 2015.

Jupiter, as it appeared in the telescope at 190x on the evening of March 24, 2015.















In addition, the telescope has continued to provide excellent views of tricky double stars. Challenging pairs such as Eta Geminorum (Propus), Iota Leonis and Eta Orionis have been successfully split under good seeing conditions.

Since first beginning my assessment of this large Gregory Maksutov, I have been made aware of two independent tests, both of which suggest that the optical quality of this telescope is very high.

You can see one such test here and another here.

I have had many opportunities to compare the views of Jupiter through my fine 5-inch f/12 achromat the 17cm Maksutov during bouts of fine seeing. The latter shows a good bit more detail than the long glass. My conclusions mirror this gentleman’s findings when he compared a 5″ f/15 refractor and a 7″ f/15 Intes Mak on the Moon and planets.

Cornelia Africana, my 180mm f/15 Maksutov Cassegrain.

Cornelia Africana.










I have named this telescope ‘Cornelia’ and she will remain in my stable, serving as a powerful and ultra-portable telescope.**

My Initial Exchange with the Public

And its Follow Up

Update: April 23, 2015

More Mak varieties have now hit the market.

Meade Instruments and Explore Scientific announce exciting new 6 inch Maksutov telescopes which were showcased at NEAF. Explore Scientific plan to shortly launch an even larger 8-inch model.

This is an exciting time for the Maksutov Cassegrain!

** The instrument was eventually sold on and the funds raised were given to a charity supporting the earthquake victims of Nepal in mid-2015. These days the author makes do with an excellent Skywatcher 8-inch f/6 Dobsonian.


De Fideli

The Mark III Baader Hyperion Zoom

The Mark III Hyperion Zoom comes with loads of useful accessories.

The Mark III Hyperion Zoom comes with loads of useful accessories.










The eyepiece market is booming. Every other month, a new line of oculars hit the shelves promising better eye relief, field of view or some other desirable feature. Zoom eyepieces – which offer a range of magnifications in one neat package – have also followed this trend, with a raft of new models available to the discriminating observer. But not all zooms are created equal, as you’ll discover by using them in the field.

Earlier zooms were blacklisted by many observers owing to their inferior performance compared with eyepieces offering a fixed focal length. They complained of decreased sharpness, loss of light and questionable mechanical construction among other things. But recently a new line of zooms promising much improved performance over their predecessors has helped to buck that trend. One such product is the Mark III Baader 8-24mm Hyperion zoom, which I was eager to put through its paces.

First impressions
The Baader Hyperion zoom arrived well packaged inside a sturdy cardboard box, which included a soft leather pouch to store the eyepiece and a variety of adapters that allow you to use the zoom in either 125 inch or 2-inch mode. All the adapters are threaded for filter attachment. The package also includes an alternative rubber eyecup with a raised shade to block off stray light. No written instructions are provided with the product, although the box very clearly illustrates the way all the accessories relate to each other. I elected to order up the dedicated 2.25x Barlow lens with the eyepiece in order to coax a larger range of magnifications from the eyepiece. At the time of writing, retailers offer both as a package at a slightly reduced price compared with buying each item separately.

The zoom is a seven element, fully multicoated design and possesses click stops at 24, 20, 16, 12 and 8mm settings. The motions are smooth and the field of view ‘opens out’ as one dials in the shorter focal lengths from 50 degrees at the 24mm setting to 68 degrees at the 8mm setting. What’s more, the eyepiece is continuously variable, allowing you to get additional magnifications between the click stop settings. The eye cup can also be adjusted in height so as to make viewing the entire field as comfortable as possible. Eye relief is a generous 12-15mm, enabling those who wear eyeglasses to make use of the zoom. Overall, the Baader zoom has the look and feel of a quality product but how did it square up under actual use? To answer this question, I tested the eyepiece on a 80mm f/5 short tube achromatic refractor as well as a 180mm f/15 Maksutov Cassegrain.

Continuously variable magnification can be achieved by twisting the upper part of the zoom eyepiece.

Continuously variable magnification can be achieved by twisting the upper part of the zoom eyepiece.











Notes from the field
The Mark III Hyperion zoom worked well when coupled to the fast achromatic refractor. Images were clean and bright with good colour fidelity and sharpness across most of the field of view. The field stop at the 24 and 20mm settings looked a bit soft compared to that observed at the higher power settings. Testing on a flat roof a couple of hundred yards distant, showed excellent sharpness and contrast. Some pincushion distortion was noted in the outer 10 per cent of the field at the 24mm setting (17x) but was noticeably improved when greater magnifications were dialled in. Although a little bit on the heavy side, the Baader zoom would make an excellent eyepiece for spotting ‘scope enthusiasts. Indeed, when coupled to the short tube 80 it makes for a good quality 17-50x spotting ‘scope, thus providing a decent range of magnifications for terrestrial use. The various adapters accompanying the zoom will also allow you to couple it to a DSLR for daylight photography. When coupled to the dedicated 2.25x, I could extend the magnification range on the 80mm f/5 to 113x, a nice upper limit for a rich field telescope. No significant deterioration in the image was noted using the Barlow.

The majority of my tests were carried out on celestial targets and once again the Hyperion zoom did not disappoint. In the fast achromatic refractor, stars focused down to tiny pinpoints and remained sharp over most of the telescopic field. Only the outer 15 per cent or so could I see significant distortion at the 24mm setting. Increasing the magnifications improved the edge of field performance quite a lot. Indeed at 50x (the 8mm setting), stars remained crisp and sharp nearly to the edge of the field. Unlike other zooms I have experienced, there was no sign of lateral colour. I consider this a very satisfactory result for such a fast telescope.

When I switched to the 180mm Maksutov, the performance of the Baader zoom was noticeably better. Stars resolved to tiny round Airy disks across the entire field of view under good conditions. No sign of ghosting was seen at any magnification indicating that the proprietary phantom anti-reflection coatings were working effectively.

The eyepiece is not parfocal between click stops. A considerable amount of refocusing is necessary when switching between the high power (8mm) and low power ( 24mm) settings of the eyepiece.

The traditional drawbacks of zoom eyepieces are best revealed when they are compared with high quality, fixed focal length oculars. In this capacity, I compared a good quality 20mm Plossl in the 180mm Maksutov to the view obtained using the 20mm setting of the zoom. My target was the bright star Vega and its hinterland. Switching between eyepieces the views were more similar than they were different. The fixed focal length Plossl showed slightly less scatter and the sky was just a shade darker but I was genuinely impressed by how well the zoom eyepiece held its own in comparison.

The Baader phantom coatings on the Hyperion zoom produce lovely contrasty images devoid of internal reflections .

The Baader phantom coatings on the Hyperion zoom produce lovely contrasty images devoid of internal reflections.











In praise of ergonomics
The Baader Hyperion zoom is an excellent value in today’s market, offering versatile optical performance satisfyingly close to fixed focal length oculars. It is incredibly handy for finding celestial objects at lower powers and then one can dial in the right amount of magnification that best frames the target. Fumbling about in the dark interchanging eyepieces will become a thing of the past and that in itself is a worthy reason for using it. Highly recommended!

Dr. Neil English’s latest book, Grab ‘n’ Go Astronomy, has just been published by Springer.

De Fideli



The SkyWatcher Evostar 90mm f/10 Achromat.

Soon, I’ll be taking delivery of a 90mm (3.5″) achromatic refractor with a focal length of 900mm, which I picked up for the princely sum of £55, plus shipping. And though it has not yet arrived, I expect its performance to be very good. The instrument obeys (or very nearly so) the so-called Sidgwick Standard of colour correction, where its focal ratio (determined by dividing its focal length by its aperture) divided by its aperture in inches is approximately 3. For many decades, this relatively simple relation has been used to good effect to create many choice achromatic refractors.

Here, I want to demonstrate that its derivation is based on sound optical principles. Virtually all achromatic doublets are of the C-F variety, meaning that red and blue wavelengths are brought to the same focus with green wavelengths usually coming to focus slightly ahead of the coincident red (Fraunhofer C-line) and blue (Fraunhofer F-line).

In the absence of significant spherical aberration, the depth of focus of the telescope is approximated by +/- 4 lambda (f/D)^2 where f is the focal length, D is the aperture of the instrument and lambda is the wavelength. Typically, the longitudinal chromatic aberration for a standard crown/flint object glass is ~ 0.0005f i.e. about 1/2000th of its focal length.

Let us now see what happens when we constrain the colour aberration within the depth of focus of the telescope.

Equating both expressions, we have;

0.0005f = 8 lambda (f/D)^2

from which,

f = D^2/16000lambda.

Setting lambda = 550nm (green light) and then converting to inches yields 2.17 x10^-5.

Substituting the value of lambda in inches into the above formula yields  f= 2.88D^2 or ~

f = 3D^2.

So, plugging some familiar apertures into the formula (outlined in bold above) gives a minimum focal length necessary to give wholly negligible secondary spectrum.

Here are some examples:

Aperture(“)                     Minimum Focal Length (“)                        Focal Ratio (f/D)

     3.0                                                      27                                                                9

     4.0                                                      48                                                               12

     5.0                                                      75                                                                15

     6.0                                                    108                                                               18

If we divide the focal ratio by the aperture we come up with 3; thus the origin of the Sidgwick Standard for achromatic doublets.

First hand experience with a wide variety of achromatic refractors will quickly convince  you of the soundness of this result. The images served up by instruments with the above specifications give beautiful images. Indeed, the Sidgwick Standard can be further relaxed to 2 or even 1.5 without greatly affecting the information garnered by the telescopic image (the unfocused light being effectively attenuated or completely removed by modern minus violet filters).

The 3.5″ Skywatcher has a focal ratio of 9 and so is just under the Sidgwick Standard (~2.6) Thus, if properly executed, it ought to be quite a delightful telescope.

Well, it finally arrived this arvo (Tuesday, July 9); the Evostar D= 90mm f =910mm achromatic refractor.


The guy whom I bought it off packed it rather well and my first impressions were very good indeed. There were no signs of much mechanical wear and tear, the lens looked pristine and the focuser did what it ought to.

I scurried to affix a dovetail plate to the tube rings that came with the instrument and had just about had it mounted on the Vixen Porta II when I heard the thunderous sound of my wife boom from the kitchen.

“ What’s this? Another telescope? You said you were done buying and selling.”

 “Yes dear, I am”, I replied, “it’s strictly for a review. It’ll be gone to a good home as soon as I’m done with it.  Any way, folk need to know when there are bargains to be had,” I continued, “remember my poem about champions?”

“Of course!” she knew immediately what I was talking about.      

Her menacing look turned to an enormous smile and I knew that all was well once again.

So what did my hard earned pecunia deliver?

It’s a lovely, light-weight tube; very portable. The dew shield is plastic but not a problem in itself.

 The dust cap (also plastic) is neat looking with a 60mm aperture mask, enabling it to operate as a 60mm f/15. Cool!


Removing the dew shield reveals a pristine 90mm air spaced doublet objective with very nice and durable coatings applied to all surfaces. The instrument is not collimateable however, but neither is an ED80 or a Televue 85.


Checking collimation with a Cheshire eyepiece under a bright blue sky revealed good but not perfectly aligned optics but neither was it enough to worry about in the slightest. Unlike some early Meade 90mm f/11 OTAs I’ve examined, the paintwork on the inside of the tube was immaculately applied and the baffling was more than adequate, as evidenced by the shot I took, looking up the tube with the diagonal removed. evostar4

The focuser is the regular 1.25” Synta rack & pinion. Nothing to write home about but more than adequate for the task; it’s f/10.1 remember!. It handled heavy 1.25” eyepieces very well. Pity it didn’t come with a 2-inch focuser for use with modern wide angle eyepieces. But you have to remember this puppy cost me £55 plus shipping.


The ‘scope can be used with a regular 90 degree diagonal for astronomical use or a 60 degree, correct-orientation diagonal for terrestrial applications, like this one.


Day time assessment of the optics revealed nothing out of the ordinary. Indeed it fulfilled my expectations. Low power views of the corbies seeking refuge in the trees nearby were sumptuous; tack sharp and rich in detail, free of any secondary spectrum.

When the temperatures stabilised in the early evening of this scorching, July day, I inserted my 4.5mm Delos ocular delivering 202 diameters. Carefully focusing on the horse chestnut tree I was treated to an excellent image. The leaves were nice and sharp. Some small amount of secondary spectrum could be seen around their edges but nothing that could seriously detract from the excellent high power views this telescope was clearly delivering.

Did I mention that it only cost me £55 plus shipping?

Very happy with what I had thus far seen, I grabbed a bite to eat and awaited the semblance of darkness to fall on the landscape.

Wednesday, July 10

Unfortunately, while I was blogging last night, thick, low altitude clouds moved in from the east making any observations of the heavens a no no.

The clouds persisted right up until the late afternoon today, when the strong July Sun finally managed to burn them away. Eager to test the Evostar 90mm refractor out on ANY celestial object, I placed a white light solar filter on the instrument. It fitted snugly once the dew shield was removed.


Inserting a medium power eyepiece, I was delighted once more with the sharp, high resolution detail I could see on the solar photosphere. A string of sunspot groups were observed in a diagonal line across much of the Sol’s southern hemisphere.

Still awaiting a star test but I’m confident it will show well corrected optics.

From where I’m standing, even those ‘bargain’ ED ‘scopes in the same aperture bracket are beginning to look ridiculously over priced, especially if you’re a dedicated visual observer.

                                                           Under a Clear Sky

Shortly after midnight on Thursday July 11, I got an opportunity to star test the instrument under excellent seeing conditions. Contrary to what you’ve heard before, it only takes one good night to assess a telescope’s optics. Only on imperfect nights does one need to pass it through multiple star tests.

Centring the bright star Vega in the field of view of an eyepiece generating 150x, the Evostar produced a clean, white, sharply focused airy disk with a nicely formed first diffraction ring. The star was surrounded by a beautiful halo of purple; absolutely normal for this type of telescope and perfectly acceptable for visual use.  Defocusing ever so slightly showed no signs of astigmatism. Examining the Fraunhofer diffraction pattern both intra- and extra-focally using a green filter revealed smooth, well corrected optics with only slight under correction. All in all – a very good result for a telescope that cost so little. Having had the privilege of testing out a number of small refractors of similar focal length, I am more confident than ever in claiming that such instruments are very easy to manufacture to very high standards.

Epsilon 1 and 2 Lyrae were perfectly resolved into fours stars. The greenish companion to Epsilon Bootis – now sinking lower into the western sky- was nicely separated from its orange primary at 150x.  The same was true of Delta Cygni. Pi Aquilae, while certainly not resolved, was plainly shown to be strongly elongated. All in all, this budget refractor delivered up everything an unobstructed 90mm aperture ought to.

Its deep sky prowess was decent too. During the darkest period of twilight, I saw the beautiful incandescent annulus that is the Ring Nebula, M57, sharply defined at 101x. Moving into Hercules, I tracked down the globular cluster M13 and was rewarded with a bright, highly condensed image. With a concentrated gaze I could make out quite a few individual stars on its periphery. Switching to a quality low power ocular, I swung the telescope over to Cassiopeia and to the open cluster M52, now fairly high in the northeast. The telescope did not disappoint, revealing several dozen stars in a kidney-shaped arrangement, the eye being drawn to a striking 8th magnitude orange sun on its flank. The lack of field curvature of this ‘slow’ object glass makes examining these bright clusters a particular joy.

In summary, the SkyWatcher Evostar 90mm achromat is arguably one of the best buys in the small refractor market. Usually touted as a beginner’s telescope, it is much more than that. The Evostar 90mm would embarrass instruments costing ten times its modest cost new. It is super light, super portable and delivers fine views at low and high power. If I were in the market for a no-nonsense 90mm telescope, this would be the one I would unreservedly recommend. Because of its low cost and excellent durability, it would be especially useful to astronomy clubs undergoing public outreach. It would also serve as an excellent telescope for the instruction of undergraduates in astronomy/astrophysics.

Available in the UK for as little as £139 UK complete with AZ 3 mount, or as part of a slightly more expensive package complete with a lightweight equatorial mount. See here for more details.

Also available in the US under the Orion rebranding. See here for details.

January 13, 2014 update:  Over the last few weeks I have thought about acquiring a modest additional family telescope for use while I’m on vacation in southern Ireland. In this capacity, I had an opportunity to re-consider the Evostar 90 achromat as a suitable instrument. After discussing the issue with my wife, I decided to pick one up on the used market for £85. The previous owner had the focuser re-lubricated, and flocked the interior of the tube and dew shield. I had a chance to evaluate its optics and, as I had come to expect, it more than delivered the readies. I will have the instrument shipped over to my sister’s home, together with a Vixen Porta II alt-az mount,  in the coming week.

There! You see!

Truly a laird wi’ my telescopes three.

My 'Irish' telescope.

My ‘Irish’ telescope.

PostScript: I’ve since negotiated a better way forward with my CEO. I can buy items so long as I raise the money from selling some of my existing kit. Fair’s fair, I suppose.

Dr. Neil English is author of Choosing and Using a Refracting Telescope.

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Taking Back Visual Astronomy- Filters (An Update on Colour Blindness)

Are serious visual observers becoming colour blind?

Are serious visual observers becoming colour blind?

This work is dedicated to Rutilus

Do not let yourself be tainted with a barren skepticism.

Louis Pasteur (1822-95)

Filters work.

Sunglasses show up details in a bright, washed out image that are nearly invisible in its unfiltered counterpart. Narrow band nebula filters allow you to more easily see faint deep sky objects despite removing vast amounts of other information from the image. Atmospheric haze turns a good achromatic image into a great one. And colour filters, judiciously selected and attached to the eyepiece of a telescope, help you to see planetary details more clearly.

Filters are useful.

But  as I write these words, a new generation of amateur astronomer is running roughshod over tried and trusted traditions of visual observing. The condition is particularly perspicuous amongst those who delight in owning high-end apochromats and premium Newtonian mirrors. In love with aesthetic images, these uber emotive souls are shocked and horrified by the suggestion that adding a filter to the eyepiece might actually enable them to see more than their pretty unfiltered images render. Here are a couple of statements I’ve heard from two supposedly serious planetary observers

“I tried out various colour filters with my 12″ Zambuto mirror, but in the end, figured I was seeing everything without the filters than with them.”


” My TEC 160 image is so good, I wouldn’t dare cheapen it by placing a $5 filter between it and my eye.”

Or, how about this quote oft parroted on tinternet:

“Filters are unnecessary as it has more to do with the observer’s eye being ill-adapted to photopic mode observation.”

Well, try saying that to a bloke wearing polaroids on a bright summer day.

You see, these three statements amount to complete and utter nonsense!

Take a quick look at these three full moon images.



Green filter

Green filter

Red Filter

Red Filter






Are you going to sit there and tell me filters don’t do anything?

Simply put, filters can improve what we see by removing what we don’t want to see from the view. We can all understand that most contemporary observers desire the most aesthetically pleasing image possible from a ‘scope. Adding a colour filter won’t do anything to enhnace that world view but what it will most definitely do is exaggerate differences in brightness between the various features of a planetary or lunar image.

Colour filters have many uses, including;

1. Glare reduction, which almost invariably leads to an increase in perceived image quality.

2. Overcoming to a greater or lesser degree, the image distorting effects of the atmosphere

3. Enabling observers to study different levels of a planetary atmosphere.

4. Increasing contrast between areas of different colour.

5. While not eliminating optical defects, improving image definition even with bad or mediocre optics.

If you are in the slightest way sceptical about any of the claims above then you’ll be helping to topple the photographic industry. The most commonly used Wratten system, for example, was developed by Kodak in 1909 and has been the standard ever since. The Wratten number, usually found around the circumference of the colour filter gives precise information about the properties of that filter.

Colour Filter                   Wratten #                  Light Transmission (%)

Light Yellow                      8                                         83

Yellow-Green                   11                                       78

Yellow                               12                                       74

Deep Yellow                    15                                        67

Orange                             21                                        46

Light Red                         23A                                      25

Red                                   25A                                      14

Dark Blue                         38A                                      17

Violet                                 47                                         3

Light Green                       56                                        53

Green                                58                                        24

Blue                                   80A                                     30

Light Blue                          82A                                     73

As you can see from the table above, filters vary considerably in their ability to transmit visible light. But, get a load of this, they do it without sacrificing resolution (and may in fact increase it). A very important point I should think. In general, filters work better with larger instruments which have more light gathering power. That said, one of the most versatile filters – the light yellow Wratten # 8 – can be used productively with even the smallest apertures.

Most of the attributes of filters highlighted above are well known, with the possible exception of attribute 2. Meteorologists have known for quite some time about the scattering effects of particles in the atmosphere. Known as Rayleigh Scattering, it predicts that for a given sized particle, light is scattered in inverse proportion to the fourth power of wavelength. Thus, it can be shown that violet light (wavelength ~400nm) is scattered some 16 times more effectively than deep red light (800nm). That’s why the sky is blue and sunsets are red.

And, so the theory goes, employing a red filter during turbulent atmospheric episodes might mitigate to some degree the deleterious effects of bad seeing. Although I have not explored this as vigorously as I’d have liked to, I once tried to see Sirius B using a light red Wratten # 23A with my 4″ Televue 102 refractor some years back, and if memory serves me well, the results were encouraging. Weather permitting, I shall attempt resolving the Sirian Pup – always very low even at culmination from my northerly latitude – using a similar strategy early in the new year.

A violet (47A) filter is very useful for observing cloud features on Venus and although its light transmission is painfully low, it can be pressed into service with larger aperture ‘scopes.

Mars is a great planet to learn how good colour filters can be in extracting atmospheric and surface features. A simple light yellow (#8) reduces glare and increases contrast in smaller apertures (5-inches and less). An orange (#21) is great for pentrating haze and cloud in the Martian disk,as well as increasing contrast between the light and darker areas of the planet. A light green # 56 filter darkens both red and blue features, enabling the observer to prize the morphology of the polar cap more easily.

Jupiter and Saturn also benefit from coloured filtration. Blue and green ones are just dandy for bringing out the belts of the planets. A yellow filter can help reveal bluish features( festoons), while a red filter can help bring out the white ovals so cherished by planetary observers. The icy Saturnian ring system too can look majestic using a red filter.

I’d be willing to bet good money that a patient observer, sketching planetary details in red, green and blue light will see more than one observing a ‘luminance’  image. Every dedicated planetary observer should have a set. And while dyed glass filters are perfectly adequate ( and cheap as chips), one might gain some additional benefit from the newer interference colour filters manufactured by companies like Baader Planetarium., Germany.

The study of colour filters on the lunar surface is an unexplored frontier, as far as I’m aware, but think geologically (minerals and that)!

                                          Improving resolving power with colour filters

The resolving power of a telescope (in radians) is approximated by Lamda/D, where lamda is the wavelength and D is telescope aperture. The Dawes limit is closely matched to a wavelength of 562nm. Converting radians to angular degrees, we can easily compute that for a 4-inch instrument (0.1m), the Dawes limit is ~1.15 arc seconds. Yet, as I have shown elsewhere in my work, there are quite a few instances where this value has been exceeded. An overly sceptical person might doubt the veracity of these claims, but if the eye has a peak sensitivity at a lower wavelength, resolution can be improved.

Individuals who have a form of colour blindness called protanopia perceive red hues as essentially dark and have peak spectral sensitivities shifted to shorter wavelengths (typically 520nm) – quite similar to where a normal, trichromatic eye would shift when fully dark adapted (~507nm). Thus, even if these individuals were observing in photopic or mesopic mode, they would have no sensitivity to longer, red wavelengths but with no loss of acuity. A simple calculation shows that such an individual might derive a ‘new,’ lower Dawes limit of 1.0 arc second with the same 10cm ‘scope. What happens as the protanopic eye dark adapts – does the peak sensitivity fall further back as in the trichromatic eye? If that happens, even greater resolution feats are conceivable!

And what of deuteranopia (another fairly common form of dichromatism where the retina lacks green cones)? Under typical night time viewing conditions, wouldn’t their red light sensitivity decrease, inducing them to rely on their blue-sensitive (peak spectral response ~440nm) cones. Could these individuals resolve finer details still?

All this serves to illustrate is that we still know far too little about the human eye (in all its enormous variety) to devise over-restrictive rules that only serve to tell folk what they can and can’t acheive. I for one don’t want to be told what I can and cannot see. Away with the Universal, away with the ‘thought police’!

While all of this sounds like pie in the sky, it can be handidly demonstrated with colour filters. A violet filter working at 390nm will improve the resolution of a telescope by up to 30 percent. A blue filter; less so. Noted CCD imager Damian Peach, produced a cool illustration of this effect on his website. You will note that the binary system is unresolved at red wavelengths, elongated at green wavelengths, and cleanly resolved at blue (lower) wavelengths. See here. Neat huh?

Note addded in proof: While researching the life of the 19th century observer, G.V. Schiaparelli, I came across a curious account of colour blindness in William Sheehan and Steve O’ Meara’s book, Mars; The Lure of the Red Planet(pp 117):

That color blind individuals possess superior vision, at least for certain  types of observations, is attested  by at least one other case known to me. According to Donald Osterbrook, Lick Observatory, astronomer Nick Mayall was colour blind., ” and he believed that it made his eyes more sensitive to faint light so he could find and observe fainter stars, nebulae and galaxies than other astronomers  with normal eyesight. Certainly, when I visited him at the Crossley reflector one night around 1955, he was taking a spectrum of an object  that was too faint  for me to see, though he evidently could see it well and the spectrum  was a good one when he developed it the next day. Several other astronomers have told me that color blind observers can see fainter objects at night than those with normal eyes -WS.

Indeed, knowedge of this sort has helped resolve a few issues I have had with my own telescopes. I only recently discovered that my eyes are particularly red sensitive and I appear to have less sensitivity at shorter (bluer) wavelengths. While using my 5″ Russian achromatic refractor, I can see the faint companion to Eta Geminorum better using a fringe killer than without it ( it blocks off  deep red wavelengths very effectively, as you’ll see below). I find that while I can achieve 1 arc second splits quite easily, 0.9 arc second pairs remain beyond my abilities, possibly because I see too much of the red end of the diffraction pattern. By using a blue filter, I hope to finally smash that 1.0 arc second barrier, if anything, to prove to myself that it is my eyes that are found wanting and not my telescope.

Nosce te ipsum.

                                                            Polarizing Filters

The light that reaches us from the depths of space vibrates in every conceivable plane. Plane polarised light, on the other hand, vibrates in only one plane, greatly reducing scattered light in the eye (irradiance) and increasing contrast. You only need look at the effects of a polarising filter from a medium focal length lens to see how dramatic an improvement to a daylight landscape it can make.

Single polarising filters have been used successfully by double star observers , especially in cases where one component is significantly brighter than the other. The theory is that the glare from the brighter primary is reduced enough to render visible a faint secondary.

Many observers have employed polarising filters to observe deep sky objects during full Moon nights. There is apparently a big contrast gain when the telescope is pointed about 60 degrees away from Luna and amazing results when swung away to 90 degrees. Larger apertures and low powers naturally benefit from this more than smaller instruments.

Neutral density filters are often cross polarising in effect, where two polarisng layers are mounted in such a fashion that one can be rotated relative to the other, empowering the observer with ability to vary the brighness of an object. Moonwatching with a large Dobsonian  can be cool with one of these.

                                                         Minus Violet Filters

Minus Violet or anti-fringing filters have been round for over a decade now. By and large they have a tried and trusted reputation for improving the performance of achromatic refractors. They achieve this through selectively blocking (via destructive interference) selected wavelengths at both the violet (short wavelength end) and deep red (longer wavelength) end of the visible spectrum. Typically it is at these extreme ends of the spectrum that most of the unfocused light(secondary spectrum) arises in achromatic refractors. Because they block specific wavebands of visble light, they usually impart a yellowish tint to the image, which seems to bother some more than others. In addition to blocking off unwanted secondary spectrum they greatly help with focusing the instrument, particularly during turbulent bouts.

The most aggressive minus violet filters have a tendency to dim the image a little too much, especially if you’re in the habit of using smaller instruments, but there is one filter that I have studied at length that is particularly useful; the Baader Fringe Killer. Although more expensive than traditional yellow filters, the fringe killer works more effectively in my opinion. The Baader Contrast Booster is also excellent although it cuts off too much light for productive use in smaller instruments.

I have used the fringe killer quite extensively with my 5″ f/9 achromatic refractor, observing Jupiter at or near opposition as well as in the pursuit of double stars. It deals effectively with the unfocused blue halo round the planet but also greatly increases the contrast between the darker belts and surrounding bright areas of the atmosphere. Focusing the planet is child’s play too. If you’re a student of Jupiter using a moderate aperture achromatic refractor, then this filter is highly recommended.

These filters are also excellent for star testing refracting telescopes. They invariably clean up the spherochromatism that oft attends the intra- or extra-focal images of stars, allowing you to more accurately assess the quality of the optic.

But it’s not only achromatic images that can be improved by this filter, ED scopes seem to respond very well too. This is particularly the case with the new breed of econo-model ED scopes now on the market. One way these manufactuers get round the issue of producing a fairly short focal length ED doublet without introducing spurious blue fringing round bright objects is to over correct at short wavelengths at the expense of more lax correction at deep red wavebands. This reduces blue fringing alright but serves up slightly washed out images of planets like Jupiter. When I examined the image in one of these units, I discovered that it was quite dramatically improved by using the fringe killer. I attribute this to the cleaning up (by blocking off) of the loosely focused deep red colours. It was just easier to see the details in the planet’s belt with the filter in place. The filter transmits enough light that telescopes as small as 80mm can benefit from its effects.

Double stars too benefit from this filter. One of the most effective things the filter does is cut down on glare (irradiance) which can make seeing a faint, close companion more easy to pick off, especially when located right up next to a much more brilliant companion. While evaluating a 6-inch ED instrument, which produces very bright images of systems such as Delta Cygni, I found the fringe killer reduced the glare round the primary quite a bit making the companion easier to keep in view during a vigil.

And on my 5″ f/9 achromatic refractor, I discovered that the fringe killer was a fantastic tool to render tricky systems like the devilish companion to Eta Geminorum (Propus) much easier to see. On most nights I have trouble with this system, whether observing through an apochromat or achromat. Reducing the red glare of the red giant primary with this filter was a real eye-opener to me though.

Some might object that using such a filter reduces the aesthetic appearance of a pair, but that’s not really been my experience at the eyepiece. Famous colour-contrast pairs, such as Albireo and Gamma Andromedae (Almach) are just as beautiful with the filter as without it and the rich colours actually seem more enhanced to my eye compared to the unfiltered view. Only whiter pairs seem to give way to a yellowish cast.

If you’ve been round the block a few times, no doubt you’ve heard the show-stopping mantra of the apophiles whenever the subject of these filters comes up.

“It won’t turn your $300 achromat into a $3,000 apochromat.”

What does that mean exactly?

If they allow you to use optimal magnifications on the moon and planets with your modest achromat isn’t that enough?

These filters will often improve the images of both telescopic genres by teaching your visual system to concentrate on the most important wavebands – where the vast majority of the information from an image is imparted – over yellow green wavelengths.

Filters are tools; pure and simple. Find the time to use them skillfully.

References and Further Reading

Some additional background on filters here



Bakich, Michael E, 2003, The Cambridge Encylopedia of Amateur Astronomy, Cambridge University Press.

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