Changing Culture III: Aperture & Resolution.

On the left, a 90mm apochromatic refractor and on the right, a 203mm f/6 reflector enjoying a bout of late evening sunshine.

On the left, a 90mm apochromatic refractor and on the right, a 203mm f/6 Newtonian reflector enjoying a spell of late evening sunshine.












One of the ABCs of telescopic optics is that resolving power scales linearly with aperture and light gathering power with the square of aperture. These are fundamental facts that are demonstrably true and have been used productively over two centuries of scientific applications. And yet, all the while, there has been a consistent drive in the last few decades within a section of the amateur community that somewhat erroneously links performance to absolute monetary value. This largely corrupt movement is most ostensibly seen in the refractor market, where amateurs are apparently willing to shell out relatively large sums of money for telescopes that, in terms of performance, are severely limited by their small apertures. This is a worrying trend indeed, and has led many astray within the hobby.

In this capacity, I decided to highlight the anomaly by devising a simple test which exposes this ‘peashooter’ mentality for what it is; a gross misrepresentation of basic optical principles.

Materials & Methods:

Two telescopes were set up in my back garden; a 90mm apochromatic refractor retailing at £1017 (tube assembly only) and a 203mm f/6 Dobsonian, with a retail price of £289, but with some basic modifications (97% reflectivity coatings and a smaller secondary giving a linear obstruction of just 22 per cent) which increased its cost to  approximately half that of the smaller telescope. The Newtonian was carefully collimated before use.

The telescopes were left out in the open air during a dry and bright evening when the temperatures had stabilised and were fully acclimated. Both instruments were kept out of direct sunlight. The refractor had an extendable dew shield to cut down on ambient glare, while the Newtonian was fitted with a flexible dew shield to serve the same purpose. To remove the complicating effects of atmospheric seeing, the telescopes were targeted on the leaves of the topmost boughs of a horse chestnut tree, located about 100 yards away.

Both telescopes were charged with approximately the same magnifications, in this case, a very high power was deliberately chosen; 320x. Next, the images of the leaves were examined visually, being especially careful to achieve the best possible focus, and the results noted.


The 203mm Newtonian images of the leaves were crisp, bright and full of high contrast detail. In comparison, the image served up by the refractor was much dimmer and a great deal of fine detail seen in the larger instrument was either ill-discerned or completely invisible in the smaller instrument. Though less dramatic, the same results were obtained when a larger refractor (127mm f/12) was compared with the 203mm f/6 Newtonian under similar conditions, with the latter delivering brighter, crisper images with finer detail.


This simple experiment, requiring nothing more than a few minutes of one’s time and no complicated formulae or optical testing devices, clearly showed the considerable benefits of larger aperture. The images served up by the Newtonian were brighter and easier to see than those served up by the smaller instrument. Resolving power and light gathering power work hand in hand; you need decent light grasp to discern fine details and vice versa.These results were largely independent of the surrounding atmospheric conditions, as the targets were located at close proximity to the telescopes and thus had to travel through a short column of air.

These experiments were repeated with larger instruments; a 127mm f/12 refractor and the same 203mm Newtonian, with the same results, that is, the smaller instrument runs out of light faster than the larger and shows less fine detail in the images served up.

These results confirm that larger aperture is superior to smaller aperture. No amount of claptrap can change the result either. Complications may arise when the same tests are performed on celestial targets, especially during bouts of turbulent atmospheric seeing, when the larger instrument will be commensurately more sensitive. In such instances, it is the environment that introduces anomalies. But when conditions are good, the benefits of larger aperture will be seen, clearly and unambiguously. Absolute monetary value has little or nothing to do with the end result, in direct contradistinction to what is claimed by those who promote small aperture refractors in an unscientific way.

See here for further reading.


De Fideli

Changing Culture.

Octavius: instrument of change.

Octavius: instrument of change.















As  I have commented on in previous communications, an urban myth has been cultivated over the years regarding the unsuitability of Newtonian reflectors in the pursuit of double stars. In the last six months or so, there are encouraging signs that more people are bucking this trend using Newtonian optics of various f ratios and in the examination of pairs of various difficulty, including the sub-arc second realm;

Exhibit A

Exhibit B

Exhibit C

Exhibit D

Exhibit E

These are but a few examples, and I can only hope that the changes will continue so that more people can enjoy this wonderful pass-time.

De Fideli

Planetary Telescopes.

The author's plnetary telescope; a 8 inch f/6 Newtonian reffector.

The author’s planetary telescope; a 8 inch f/6 Newtonian Reflector.















But thou shalt have a perfect and just weight, a perfect and just measure shalt thou have:

Deuteronomy 25:15

Comments on planetary telescopes by established authorities** in the field over the last 130 years.

As a really efficient tool for systematic work on planets, telescopes of about 8 inch aperture cannot be surpassed. It is useless waiting for the two or three serene nights in a year when the whole diameter of a big instrument is available to really good effect. Amateurs urgently require the appliances most efficient under ordinary conditions and they will find a larger aperture of little avail until it is much reduced by a system of gagging and robbed of that very advantage which is extolled so much; namely grasp of light. The 18.5 inch equatorial of the Dearborn Observatory cost £3700, the great Washington refractor £9000, the great Melbourne Cassegrainean (reflector) of 4 feet aperture cost £14,600, and at first it would appear preposterous that a good 8.5 inch With or Calver mirror, that can be purchased for some £30 will effectively rival these expensive and elaborate instruments. Many people would consider that in any crucial tests the smaller instrument would be utterly snuffed out: but such an idea is entirely erroneous. What the minor telescope lacks in point of light it gains in definition. When the seeing is good in a large aperture, it is superlative in a small one. When unusually high powers can be employed on the former, far higher ones proportionally can be used with the latter. We naturally expect that very fine telescopes, upon which so much labour and expense have been lavished, should reveal far more detail than in moderate apertures, but when we come to analyze the results it is obvious such an anticipation is far from being realized.

From W.F. Denning’s, The Defining Powers of Telescopes, Anno Domini 1885.

The planet looks as if cut out of paper and pasted on [the] background of sky. It is perfectly hard and sharp with no softening of edges. The outline and general definition are much superior to that of a refracting telescope.

E.E. Barnard comparing the views of Saturn seen with the newly erected 60-inch reflector atop Mount Wilson, with the 36-inch Lick Refractor, Anno Domini 1908

Source: Sheehan, W. The Immortal Fire Within: The Life and Work of Edward Emerson Barnard, Cambridge University Press, pp 398. Anno Domini 2007.

Although something worth recording may be seen even with a 3-inch, the intending student of Jupiter should have available a telescope of not less than 6 inches aperture. With such an instrument a great deal of first-class systematic work can be accomplished and only the smallest of the really important markings will be beyond its reach; indeed, until only a year or two before his death Stanley Williams made all his invaluable observations with a 6-inch reflector. An 8-inch is probably adequate for all purposes; a 12-inch certainly is. The bulk of the author’s work has been done with a 12-inch reflector; and although it would not be true to say that he has never yearned for something larger when definition was superb, the gain would have been mainly aesthetic and he has never felt that anything important was being missed owing to the inadequacy of his equipment.

Peek, B.M., The Planet Jupiter:The Observer’s Handbook, Faber, pp 36-37, Anno Domini 1981.

If the aperture exceeds about 12 inches , the atmosphere will seldom allow the full aperture to be used……..Direct comparisons of performance on different occasions have revealed an 8-in refractor showing more than a 36-in reflector; an 11-in refractor surpassing a 12-in reflector; canali invisible in the Greenwich 28-in stopped down to 20 ins, but visible in an 8-inch by T.E.R. Philips; apertures less than 20 ins showing more than the Yerkes 40-in stopped to 30 ins.

From Mars by J.B Sidgwick, Observational Astronomy for Amateurs, (pp 118) Anno Domini 1971.

One of the greatest Jupiter observers, Stanley Williams, used only a 6-inch reflector, but most serious students of the planet now would look for at least an 8-inch, although a good 5-inch OG can reveal surprising detail. This is not the place to debate the relative performance of refractors and reflectors, but good resolution, high contrast and faithful colour rendition are essential. A good long focal ratio Newtonian , a Maksutov, or an apochromatic refractor is probably the best but, as in every field, the quality of the observer is the most important factor, and good results can be obtained with any reasonable instrument.

Moseley T., from the chapter on Jupiter in The Observational Amateur Astronomer, (Moore, P. ed), Springer, pp95, Anno Domini 1995.

To recapitulate: Mars is not an easy target. Because the disc is generally small, it is essential to use a fairly high power telescope if it is hoped to see anything except for the most prominent features. Of course a small telescope such as a 7.6cm refractor or a 15cm reflector will show something under good conditions, but for more detailed work a larger aperture is needed. A 20cm telescope is about the minimum for a reflector; I would not personally be happy with anything below 20cm, though opinions differ, and no doubt observers more keen sighted than I am will disagree.

Moore, P., from the chapter on Mars in The Observational Amateur Astronomer, (Moore, P. ed), Springer, pp78, Anno Domini 1995.

A 3-inch refractor with a magnification of around 50x will show the planet and its ring system, but an aperture of no less than 6-inches is needed for observations to be of value; ideally one should aim for an aperture of at least this size – the larger the better. It has been claimed that the best magnification for planetary observation is about equal to the diameter of the object glass or mirror in millimetres. To see the fine details of Saturn’s belts and ring structure, a magnification of 150x to 300x is necessary, and therefore, according to the above rule, telescopes of 150mm or more are clearly required.

Heath, A.W., from a chapter on Saturn in:The Observational Amateur Astronomer, (Moore, P. ed), Springer, pp113, Anno Domini 1995.

Seeing varies from 0.5 arc seconds on an excellent night at a world class observatory site to 10 arc seconds on the worst nights. On nights of poor visibility, it’s hardly worth observing the Moon with anything but the lowest powers, since turbulence in the Earth’s atmosphere will make the lunar surface appear to roll and shimmer, rendering any fine detail impossible to discern. For most of us, viewing rarely allows us to resolve lunar detail finer than 1 arc second, regardless of the size of the telescope used, and more often than not a 150mm telescope will show as much detail as a 300mm telescope, which has a light gathering area 4 times as great. It is only on nights of really good visibility that the benefits of the resolving power of large telescopes can be experienced. Unfortunately, such conditions occur all to infrequently for most amateur astronomers.

Grego, P., The Moon and How to Observe It, Springer, pp244, Anno Domini 2005.

As a choice for planetary observations then, there is a lot to be said for the Newtonian reflector in the 6- to 10-inch aperture range.

F.W. Price, The Planet Observer’s Guide (2nd Edition), Cambridge University Press, pp 41. Anno Domini 2000


It allowed visual scrutiny with very high magnifications, each time it was necessary.

Adouin Dollfus (2002) in a comment pertaining to the efficacy of the Great Meudon Refractor.

A high quality Newtonian reflector is a very powerful instrument, fully capable of superb performance in viewing the planets, when the optics are kept clean and properly aligned. They have been among the favorite instruments of serious planetary observers for many decades.

Bengton, J.L., Saturn and How to Observe It, Springer, pp57, Anno Domini 2005.

As good as my 6-inch f/9 is, the 8-inch f/6 I built soon after is crushingly superior in virtually every way — including planetary performance. This is something to keep in mind if you’re considering a long-focus Newtonian. A long f-ratio helps, but aperture is much more important. Would an 8-inch f/9 be better than my f/6? Probably. But mounting and using a scope with a tube more than 6 feet long is would be a challenge. And when the aperture gets much bigger, it’s easy to keep the secondary size small without resorting to extremely long focal lengths.

From an article entitled,The Big Red One, Sky&Telescope Associate Editor and veteran ATMer, Gary Seronik, commenting on the superiority of a 8-inch f/6 reflector over an optically superlative 6-inch f/9 reflector ‘Planet Killer,’ Anno Domini 2009.

I was once loaned a 4.5 inch refractor by the British Astronomical Association back in the 1990s; it was an excellent instrument, but the optical tube was longer than me! These days refractors come with much shorter tubes, but at considerable cost and apertures of 5 in., or more, however the cost of smaller refractors have come down in recent years. Although they look splendid, remember it is aperture(size of the telescope) that is the most important. Ideally you should get the largest telescope you can for your money.

Abel, P.G, Visual Lunar and Planetary Astronomy, Springer, pp 13, Anno Domini 2013.

All in all, if you can afford it, and if you have the room to house it in some sort of observatory, I would say go for a Newtonian reflector of 10 inches -14 inches aperture and as large a focal ratio as you can reasonably accommodate…..My second choice would be a 5 inch refractor…..having a focal ratio of f/12……or an ED apochromat ( f/8).

North, G., Observing the Moon, Cambridge University Press, pp 52, Anno Domini 2014

When Mars was closest to the Earth in August 2003, the Macclesfield Astronomical Society held a star party at Jodrell Bank Observatory with quite a number of telescopes set up to observe it. As the evening progressed a consensus arose that two scopes were giving particularly good images; my FS102 4-inch Takahashi Fluorite refractor (at around £3500, or $5000, with its mount) and an 8-inch Newtonian on a simple Dobsonian mount newly bought for just £200($300). I personally preferred the view through the f/6 Newtonian but others thought that the FS102 gave a slightly better image, so we will call it a draw. It is worth discussing why these performed so well and, just as importantly, why perhaps the others did not.

From A Prologue of Two Scopes: Morison, I. An Amateur’s Guide to Observing and Imaging the Heavens, Cambridge University Press, xiii, Anno Domini 2014.

** The author chose these individuals based on both published and unpublished observations of planets available from historical archives and/or books, and having (ostensibly) sustained these observations over many years.

                                   Relevant Physical Principles



A telescope of diameter D cuts off a wavefront and blurs a point source to an image size, I,  given by I = lambda/D (radians). This can be converted to arc seconds by multiplying this result by 206265 giving I = (lambda x 206265)”/D.

Making both units of diameter and wavelength (arbitrarily set to 5.50 x 10^-9 m)  the same we obtain:

I = 0.116/D

This is similar to the more familiar Dawes formula (expressed in inches given by 4.56/D)

Thus resolution scales linearly with aperture e.g. a telescope with a diameter of 20cm will have an angular resolution twice that of a 10cm instrument.

Contrast Transfer:

Optical engineer, William Zmek, in the July 1993 issue of Sky&Telescope magazine, analysed the effects of a central obstruction on contrast transfer, arriving at this simple rule:

Contrast Transfer of an Obstructed Telescope = Full Aperture – Aperture of Obstruction.

Consider this author’s chosen planetary telescope, a 203 mm Newtonian with a secondary minor axis of 44mm (22% linearly), the resulting contrast transfer will be the equivalent of a 203-44 = 159mm unobstructed aperture, the effects of the spider vanes being essentially negligible (~1-2 %).

This result has been amply borne out by the author’s extensive field testing.See here for details.

Larger apertures also allow the observer to enjoy a larger exit pupil, which is of paramount importance in studying low contrast details at magnifications typically employed in planetary studies. See this link to see how a consideration of the size of the exit pupil can radically change the direction of a discussion about two very different telescopes.

Light Gathering Power:

Image brightness is proportional to the number of photons collected, which in turn scales as the area of the optical surface. Thus a 20cm telescope collects four times more light than a 10cm, all other things being equal. Refractors, having no central obstruction and multi-coatings applied to the glass surfaces have the greater light transmission. Reflective surfaces exhibit proportionally less transmission to the eye due to less efficient reflection off optical surfaces. In the same article highlighted above, the author described the acquisition of ultra-high reflective coatings (and greatly reduced light scatter) to both mirrors (97 per cent). Thus the overall transmission is (0.97)^2 =0.94 and subtracting the obstructing area of the secondary reduces the overall light gathering power to ~0.9. Compared with an almost perfectly light transmitting refractor object glass, this represents a 10% reduction in light, a value that even a seasoned observer would be hard pressed to see. Thus, the author’s 20cm Newtonian has a broadly equivalent light transmission to an unobstructed refractor of equal aperture.

Atmospheric Turbulence, Seeing Error & Viewing Altitude

The astronomical seeing conditions at a given site can be well described by the so-called Fried parameter r0. We need not wade into the mathematical details to understand the basic ideas behind this model. In this scheme of events the air consists of moving cells which form due to small-scale fluctuations in both the density and temperature above the observer, resulting in the blurring and/or moving of the image. The larger these cells are (which is a measure of r0) the greater the aperture that can be profitably employed. For telescopes with diameters smaller than r0, the resolution is determined primarily by the size of the Airy pattern (which scales as 1/D) and thus is inversely proportional to the telescope diameter. For telescopes with diameters larger than r0, the image resolution appears to be determined primarily by a quantity known as seeing error and scales as (D)^5/6. So, for example, a doubling of aperture results in a 1.78x i.e. (2D)^5/6 increase in seeing error. Interestingly, while the seeing error does scale with aperture, the rate of increase is not nearly as rapid as one might anticipate. This implies that large apertures can work at or near optimally, though maybe not as frequently as smaller apertures.

Reference here.

The best estimates of r0 for typical observing sites used by the amateur astronomers seems to be in the range of 5–20cm (2-8 inches) and generally larger in the better sites at high altitude, where bigger telescopes are pressed into service. Intriguingly, r0 also appears to scale somewhat with wavelength, being as high as 40cm at 900nm(near infrared).

Reference here.

Seeing is also dependent on the altitude of the planet owing to the variation in air mass through which it is viewed. If one observes a planet at the zenith, one looks through 1 air mass. At 30 degrees altitude, the air mass through which the observer views is fully doubled and at 10 degrees altitude it shoots up to 5.6 air masses!

Reference: Morison, I., An Amateur’s Guide to Observing and Imaging the Heavens, Cambridge University Press (2014), pp 22.

In general, a long-held tradition recommends waiting for the planet to rise above 30 degrees altitude to begin to exploit the potential of any given telescope, large or small.

Taken together, these physical parameters can be used to adequately explain all of the aforementioned comments made by celebrated planetary observers over the decades and centuries.


Unbiased testimonies provide a bedrock upon which sound conclusions can be formulated. It is self evident that aperture plays a crucial role in seeing fine detail and it is reassuring that basic optical principles reaffirm this.

The list of British observers quoted above; Denning, North, Moore, Abel, Grego, Heath and Sidgwick etc, highlight the efficacy of moderate but not large apertures in divining fine detail on planets. The consensus appears to be that apertures of between 6 and 10 inches are most efficacious in this regard. This may be explained in terms of the size of the atmospheric cells that move over British skies, which allow telescopes in this aperture range to be exploited. My own discussions with many experienced planetary observers abiding in Britain affirm the truth of this; British skies seem to favour these moderate apertures. It is important to note that this conclusion has little to do with planetary imaging, which often employs significantly larger apertures to excellent effect.

The testimony of Gary Seronik shows that an optimised 6-inch f/9 Newtonian – which presumably would provide views rivaling a 6-inch apochromatic refractor, was comfortably outperfomed by an 8-inch f/6 Newtonian, again confirming the superiority of a little more aperture.

The testimony of E.E Barnard at Mount Wilson and Adouin Dollfus at Meudon shows that larger apertures can be used to much greater effect if seeing conditions allow. Both Meudon and Mount Wilson have enabled telescopes of 30 and 60-inches to be used visually, indicating that the atmosphere can be particularly good there and for long enough periods to permit a meaningful program of visual study.

There evidently exists regions on Earth where the seeing is poor (small r0) for prolonged periods of time, explaining why amateurs in these regions stick to smaller apertures. This in part explains the popularity of small refractor culture.

The most intriguing testimony is offered by Professor Ian Morison, also based in the UK, which, on the face of it, seems to lend more credence to small refractor culture. The reader will recall that during the August 2003 Martian opposition, a large number of amateurs, fielding various telescopes, were present at Macclesfield, England. Morison claimed that two telescopes were doing particularly well; a Takahashi FS102 Fluorite refractor and a mass produced 8″ f/6 Dobsonian and that there was no clear consensus on which was delivering the better views. Having owned several econo- and premium 4 inch apochromatic refractors (and even a gorgeous 4-inch f15 classical refractor), this author (also based in the UK) has become intimately familiar with their performance. And while they all provided good views of the planets, they come nowhere near the performance of the author’s 8-inch f/6 Newtonian, which, despite its very modest cost, proved ‘crushingly’ superior to the former.

So, Morison’s testimony presents an apparent contradiction, which must have a rational explanation.

Further investigation revealed that during the August 2003 Martian opposition, the maximum altitude of the Red Planet was just 23 degrees at meridian passage as observed from London (51 degrees North latitude).

Reference here

Since Macclesfield (53 degrees North latitude) is further north than London, the maximum altitude of Mars would only have been 21 degrees and thus was significantly below the minimum altitude recommended – 30 degrees – for planetary study. Thus, it is not at all surprising that Morison et al reached the conclusions they did.The Newtonian being more sensitive to the vagaries of the atmosphere would not have been performing optimally at that low altitude, while the smaller refractor was performing much as it always does. In addition, this author observed Mars during the same August 2003 opposition using a 20cm f/10 Schmidt Cassegrain. At 56 degrees North, the planet was only 18 degrees above the horizon at meridian passage. Needless to say, the images of Mars were nothing to write home about.

Interestingly, this author reached the same conclusion whilst comparing visual drawings of Jupiter conducted with a Celestron 8″ f/6 Dobsonian during the mid-1990s with those delivered by a 5-inch refractor in much more recent apparitions. It was subsequently discovered that Jupiter was low in the sky in Aquarius at this time, while the 5-inch refractor enjoyed views of the Giant Planet situated much higher in the sky. Last year’s Jovian apparition revealed the clear superiority of the 8-inch f/6 Newtonian over the 5-inch under these more favourable conditions.

Thus there is no contradiction; aperture rules when conditions are reasonable to good. Anomalies only arise under sub-optimal conditions – persistent bad seeing, low altitude viewing etc – or if one telescope has not fully acclimated when the other has etc, hardly a fair test.

This author has brought the reader’s attention to the efficacy of a modified 8-inch Newtonian on all types of objects; deep sky, planets, lunar and double stars. These testimonies provide further evidence that such an aperture – 20cm – is probably optimal for British skies and many other environs besides.

De Fideli

An Evening with Octavius.

Octavius, my 8 inch f/6 Newtonian in the Cold of Mid-Winter.

Octavius, my 8 inch f/6 Newtonian in the cold of mid-Winter.











Prove all things; hold fast that which is good.

                                                                                1 Thessalonians 5:21

Date: Saturday, December 12, 2015

Time of observation: 22:00-23:30 UT

Temperature at commencement: -3C

Temperature at termination: -4C

Seeing: Ant I -1.5, transparency excellent, no wind.

Instrument: Well-collimated 20cm f/6 Newtonian (22 % CO). Standard Skywatcher Dob focuser. No fans employed.

Materials & Methods: A suite of double stars observed with the Newtonian, being mindful of magnifications used, aesthetics of the image garnered, and efficacy of the splits on a range of systems of varying degrees of difficulty. Mark III Baader Hyperion Zoom and dedicated 2.25x Barlow, 1.25″ Baader Neodymium filter, Parks Gold 7.5mm ocular. Instrument sat upon a Lazy Suzan Dob mount (undriven). Instrument moved from a warm and dry domestic setting to a cold and dry unheated shed, where it remained for several hours until its deployment at 22:00 UT. Flexi-dew shield attached.

Observations: One good night can dispel a myth and expose a fallacy. This time it pertains to the notion that small refractors are better suited to winter viewing and that reflectors or compound, catadioptric instruments can only do useful work in the warmer months of spring and summer.

This is a misleading notion and simply untrue. Nor is it supported by the weight of historical evidence, which shows that large Newtonians were used to great effect by some of the finest and most admired observers of past generations. The work of the great Victorian, Reverend T.W. Webb, is just one example. As we have explored previously, Webb chose to use a 9.25” With-Berthon silver-on-glass reflector to carry out observations in all weathers, including freezing winter nights. Nor did Webb employ active cooling to his telescope as no such device was available for him to use.

Octavius, my 8-inch f/6 Newtonian (also without active cooling fans) performed flawlessly this evening when turned on a variety of double stars, the Airy disks of which were observed as tiny, round, calm and beautifully resolved. Despite the large temperature differential between my body and the surrounding air, a warm insulating coat, gloves and hat greatly reduce heat loss. Attaching a dew shield to the end of the tube increased the distance between my body and the entrance pupil. All these measures and a steady atmosphere conspired to produce arguably the finest images of double stars I have seen in any telescope.

theta Aurigae: Very easily resolved at 160x but even more compelling at 360x, both the primary (magnitude 2.6) and secondary (magnitude 7.6) separated by about 4 seconds of arc. This system can be quite tricky, owing to the large magnitude differential between the components. A magnificent sight!

Rigel: Despite its low altitude, the exceptionally calm air made seeing the feeble spark from its companion child’s play at 60 diameters or above.

The theta1 Orionis complex: The 8-inch speculum showed all six of the Trapezium stars A through F at magnifications of 360x, the magnitude +11 E and F components becoming ever more distinct as one’s eyes became better adapted to the dark.

Alnitak: The easternmost star in Orion’s belt. The 8-inch telescope at 150x showed its whitish, magnitude 3.7 companion just 2.5” away to the south southeast of the magnitude 1.9 primary.

Mintaka: Very easy at all powers. The primary shines in a soft white colour but the companion, located a decent 52” away to the north presents as a beautiful, pale blue cast in the 8-inch speculum.

eta Orionis: Much more challenging but the decent aperture made very light work of this system at 360x. The telescope showed its very tight companion a mere 1.8” east northeast of the primary. This is a fetching colour-contrast double, faithfully rendered in the perfectly achromatic reflector, with the primary appearing yellow and its companion blue.
32 Orionis: The pair was perfectly resolved at 360x.The primary (magnitude 4.4) and its secondary ( magnitude 5.7) separated by 1.8”

52 Orionis: a classic Dawes pair of 6th magnitude luminaries, brilliant white and well separated by 1” using a magnification of 360x.

42 Orionis: Very impressed with this system, which was not as difficult as I had anticipated! Though only separated by 1.1”, the difficulty here is the very large brightness differential between the primary and secondary (4.6/ 7.5), the raw resolving power of the 8-inch aperture (360x) in these excellent conditions rendering the split easily.

eta Geminorum: The icing on the cake on this frigid evening! I have given mention to this orange variable star (magnitude ~3.5) in previous communications. Although I have glimpsed (and I mean glimpsed) the very faint secondary in 4- and 5-inch refractors as well as 17cm f/16 Maksutov at very high powers, the image in the 8 inch reflector was in a completely different league! My notes from the evening of January 29, 2015, using a 17cm Maksutov Cassegrain revealed the companion using a glare-reducing variable polarising filter at 340x and at a recorded temperature of -5C. That being said, the superior resolution of the 8 inch speculum was all too obvious to my eye, for it revealed the dim (magnitude 6.5) bluish secondary just 1.6” off to the west-southwest of the ochre primary more plainly than I have ever seen before in any telescope! And this was true even though the system was still a few hours away from meridian passage! The superiority of aperture being abundantly apparent, I watched eta for many minutes, savouring the sight which I had dreamed of seeing for many years.

Concluding Comments: Despite the cold, the Newtonian telescope worked perfectly well. Laziness and wilful scaremongering have prevented many from using their larger Newtonians to good effect under these conditions. The observations made this evening reaffirm the importance of aperture-dependent resolving power working under ideal meteorological conditions. They are supported by three tiers of evidence; physics, history and personal experience. Don’t let anyone stop you from discovering the genius of Newtonian optics! If you don’t try, how will you ever know? And as to the wisdom of confining one’s observations to aperture-limited refractors in winter, that’s all well and good, but bear in mind that you’ll be missing out on golden opportunities to see more, much more!


De Fideli


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,
“Ancient Rocks Show Life Could Have Flourished on Earth 3.2 Billion Years Ago,” ScienceDaily, published online February 16, 2015,

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.

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