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!
Weather still rather unsettled, very humid with lots of heavy down pours, so little else to report from my own observations.
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.
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.
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?
Nuff said, eh?
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
Ambient: Mostly clear, tranquil, cool (10C), twilit.
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
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
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!
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?
July 9, 2015
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.
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.
July 13, 2015
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.
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!