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