Optical Quali

A word or two here about optical quality may be appropriate. Manufacturers quite often use the term "diffraction limited." This means that the optics are good enough that they are only limited by the laws of physics, and the property known as "diffraction" limits the resolution of all instruments, whether optical or radio telescopes.

The larger the mirror is in relation to the wavelength, the better the resolution you will get. To be "diffraction limited," an optical telescope needs to have optics that, as a minimum, deviate from the perfect shape by no more than a quarter of the wavelength of light. In other words, from the deepest valley below the perfect curve, to the highest peak above it, should be less than a quarter of a wavelength. Green light has a wavelength of about 550 nanometers, so a quarter-wave is about 140 nanometers. This is what the term quarter-wave PV means (PV = peak to valley). If the mirror is less accurate than this, then planetary detail will get noticeably soft and mushy. A quarter-wave PV equates to plus or minus one-eighth-wave surface accuracy. Another factor here is RMS, or root mean square. This is an indicator of how smooth the telescope mirror is, on average, rather than just taking account of the most extreme peaks and troughs. Typical commercial Schmidt-Cassegrain mirrors tend to be one-quarter-wave PV or one-sixth-wave in really good examples (such as Celestron's C9.25 models). In other words, they are diffraction limited. Commercial lemons occasionally crop up though, with half-wave PV optics. Commercial mass-produced optics usually have RMS figures of 1/20th to 1/30th wave, but a really good set of Newtonian optics may have an RMS of 1/40th wave and a PV figure of 1/8th or better. These are optics to be proud of.

In the 1970s and 80s, SCTs had quite a poor reputation for optical performance. One factor often cited by SCT critics was the size of the secondary mirror, i.e., the central obstruction. The traditional advice of experts was that a telescope should have an obstruction less than 20% the diameter of the main mirror, if diffraction effects were not going to degrade the view. The effect of any obstruction in the telescope light path is to reduce the contrast at the telescope limit. In an unobstructed telescope the vast majority of the light from a point source like a star ends up in a central point with the remainder distributed in a series of rings, becoming increasingly fainter as you move out from the central point. As you increase the central obstruction, the intensity of the central point is reduced while the intensity of the rings increases. How does this affect the planetary performance? Well, without going into complex analyses involving mathematical terms like "MTF" (modulation transfer function) and Strehl, visual observers often quote a rule of thumb that a telescope of aperture x with an obstruction of aperture y reveals subtle planetary features as if it were an instrument of aperture (x - y). In other words, a 30-cm SCT with a 10-cm obstruction will behave like a 20-cm apochromat refractor (a refractor with no visible color aberrations). Most planetary observers think this rule is about right, although when observing the Moon, where there is so much contrast available, a large central obstruction is much less of a problem. Of course, there is another factor here. A 30-cm instrument with a 10-cm obstruction still has twice the light grasp of a 20-cm refractor. With more light there is a higher signal-to-noise ratio in the webcam image (for the same image scale) and you can choose to have a shorter exposure to better freeze the seeing turbulence. But there are downsides to larger apertures in that you have a heavier less-friendly instrument, one with more thermal mass and one that will rarely be able to resolve to its theoretical limit anyway. If you want to really simulate a telescope's optical performance, I would strongly advise downloading another excellent piece of freeware written by Cor Berrevoets. This software package is called Aberrator (available at http://aberrator.astronomy.net/).

When I first came into amateur astronomy I was convinced that bigger was better. I wanted the largest telescope I could acquire. Eventually, I owned a massive 49-cm aperture Newtonian. However, as the years have gone by it has become more and more apparent to me that a user-friendly telescope is the best telescope to have. Specifically, a quality user-friendly telescope, with a reliable drive and good optics, that can be easily collimated, is what is required. This is especially true in planetary observing where the atmosphere rarely allows features much smaller than 0.5 arc-seconds to be seen. Over the years I have consulted many leading planetary observers and imagers on what they think is the largest useful aperture that you need. Many of these observers have used extremely large telescopes at high altitudes with mirrors up to 1 meter in aperture. The general consensus is that a 25-cm telescope will show you all that you need to see on 95% of nights, even on a planet that is at 50 or 60 degrees altitude. Yes, there are freaky nights, maybe once a year, when a 30-, 35-, or even 40-cm instrument could benefit the observer, but the user-unfriendliness of such instruments will probably have a negative effect on the observer's enthusiasm. In addition, the thermal properties of such large instruments and the likely quality of their optics will work against such telescopes giving any benefit. One of the "holy grails" in planetary observing is seeing the elusive Encke division in Saturn's A ring. The elusiveness of this feature, even in large apertures, is a testimony to the idea that large apertures rarely have any benefit to the planetary observer. The Encke division (not to be confused with the Encke minimum, which is simply a subtle shading effect between inner and outer parts of the A ring) is a feature that resembles a human hair, right on the limit of visibility in amateur instruments. It is 1/20th of an arc-second across, so well below the resolution of instruments less than 2 meters in aperture. However, it can be glimpsed, even in instruments as small as 15 or 20 cm, because it causes a contrast drop in that part of the A ring. Despite this fact, it took a night of perfect seeing on the 36-inch Lick refractor in January 1888 to confidently record the feature for what it was. That definitive and undisputed observation was made by James Keeler. The Encke division is still an elusive feature, even in the webcam era. However, in perfect seeing it has easily been recorded, at Saturn's ansae (the east and west ring tips) and with the rings wide open, with 20-cm instruments and a webcam.

I think I have provided quite a bit of evidence here that large 35- and 40-cm aperture instruments are just not needed for planetary observing. Indeed, their user unfriendliness can be off-putting. But is there actually a perfect planetary telescope? Well, all I can do is list a few examples of commercial telescopes that have been used to good effect by some of the world's keenest webcam and CCD imagers.

As I have already mentioned, Damian Peach has achieved staggeringly good images with Celestron's 11 and 9.25 inch SCTs (28 and 23.5 cm in metric). The C 9.25 has something of a legendary status as the best planetary SCT you can buy. Part of the reason for this is that it has a longer f-ratio primary mirror than other SCTs (f/2.5 compared to f/2.0), which significantly improves the chances of there being less optical defects and aberrations. It is also of an ideal aperture for exploiting the best the atmosphere can offer, while packaged in a lightweight, low thermal mass optical tube. I know a lot of observers who have owned C 9.25s and none have been disappointed. The French planetary imager Thierry Legault has achieved very impressive results with a 30-cm Meade LX200 mounted on a Takahashi German Equatorial mounting, thus combining good optics with a superb, smooth drive. SCTs only have two main disadvantages: the "flopping" of the primary mirror out of collimation and dew formation on the corrector plate (when it occurs on the inside of the corrector, in very damp conditions, this can be especially frustrating as it leaves a haze long after the dew has gone). One of the best-known "connoisseur"

instruments for planetary imaging is the Takahashi Mewlon 250 Dall-Kirkham Cassegrain. Unlike with SCTs, Dall-Kirkham telescopes have no corrector plates, so their optics are exposed to the night air and cool down more quickly. The Mewlon 250 also has a detachable rear mirror cell plate, enabling the primary mirror to cool down very swiftly to the night air. Takahashi's Mewlon 250 is a superb performer, although a lot more expensive than an SCT of the same aperture. The Singapore observer Tan Wei Leong has produced exquisite results with his Mewlon. Veteran Florida amateur Don Parker, best known for his massive 40-cm f/6 Newtonian, has a Mewlon 250 too. In the U.K., Orion Optics produces an excellent planetary Maksutov called the OMC 200, which is well worth considering (Figure 3.15). Their newest open-tube Maksutov Cassegrains (the OMC 300/350 models) are intended both for planetary and deep sky work. For those with a love of Russian telescopes, the company Intes-Micro produces some superb modest aperture Maksutov-Cassegrain and Maksutov-Newtonian instruments (Figure 3.16). The Maksutov-Newtonian is a particularly interesting design, combining the quality of a slow Newtonian primary mirror with a corrector plate to reduce Newtonian aberrations. Such an instrument has a sweet spot of a similar size to a long-focus Newtonian. However, just as important as the optical specification is how an instrument performs in nightly use. An instrument may have perfect optics, but, if, for example, the collimation process is almost impossible, it is as useful as a chocolate teapot, an inflatable dartboard, or a rubber pick-axe. Some of the larger Maksutov-Newtonians in circulation have excellent Russian Intes-Micro optics mounted in optical tubes made by less competent dealers . . . buyer beware! The website www.cloudynights.com is a very useful site to visit when assessing what telescopes are actually like to use, in reality, away from all the advertising hype.

Orion Maksutov
Figure 3.15. Orion Optics excellent 20-cm f/20 Maksutov, the OMC 200. A superb and compact planetary telescope. Image: Orion Optics.
Intes Micro
Figure 3.16. Another excellent planetary telescope: Intes Micro's MN78, an 18-cm f/8 Maksutov Newtonian with a tiny secondary obstruction and closed tube. Image: Jamie Cooper.

If you have very deep pockets, a 20- or 25-cm apochromatic refractor is, perhaps, the ultimate planetary observer's status symbol. But only consider this option if you are happy spending $20,000 or $30,000 on the optical tube alone. A number of specialist large refractor companies will be only too happy to empty your wallet and provide you with such an instrument.

From my own perspective, as previously mentioned, my favorite planetary instrument is currently my 250mm f/6.3 Orion Optics Newtonian (Figure 3.17). It has superb, easy to collimate optics (1/45th wave RMS), a very convenient eyepiece height, and a cooling fan at the mirror back. The 1-mm-thick tube cools to ambient temperature quickly and yet is rigid enough to maintain the system in collimation. The telescope also features an unusual double circle secondary holder that eliminates diffraction spikes around bright stars (Figure 3.18). The only modification I have made to it is to add a motorized JMI focuser. The optical tube only weighs 11 kg. However, even this system is not quite perfect. The Sphinx mount that the telescope was supplied with is right on its weight limit and long-tube Newtonians are quite susceptible to wind-shake and vibration, especially if the observer is changing filters while the system is in use. A Newtonian of a certain weight needs a much heftier mounting than a Schmidt-Cassegrain of the same weight to avoid rigidity problems. One solution to this sort of rigidity problem, and far cheaper than a bigger mounting, is the so-called "Hargreaves Strut." This modification, named after the historical British amateur astronomer, telescope-maker, and wartime BAA president F. James Hargreaves, employs a strut (with universal joints) between the end of the telescope tube and the German equatorial counterweight arm, to brace the system.

Figure 3.17. The author and his 250-mm f/6.3 Orion Optics SPX Newtonian. Image: Martin Mobberley.
Hargreaves Strut
Figure 3.18. The unusual secondary mirror (spider) on the author's Orion Optics Newtonian. This design eliminates diffraction spikes around bright stars. Image: Martin Mobberley.

Undoubtedly the best "ultimate planetary telescopes" of all are those designed and built by the amateur astronomer for his or her own use. ATM, or amateur telescope making, has declined as a hobby throughout the 1980s and 1990s and into the 21st century, largely due to the mass-production of amateur telescopes and the pressures of modern working lives. There is another factor that is relevant too. Many ATM fanatics are far more interested in building telescopes than actually using them. ATM is their all-consuming passion, not observing. With me it is the opposite. The telescope is the tool to get the job done. My hobby is imaging astronomical objects, not building telescopes and I would rather spend time at the eyepiece or working on images than working on a lathe. However, while the average amateur with a job and family simply does not have the time to make telescope optics or a telescope drive from scratch, the telescope tube is something that lends itself well to customizing. The Newtonian is the easiest tube to modify and dramatic enhancements can be made simply by adding cooling fans, making a thin vane secondary support system, and, most important of all, making the system easy to collimate and rigid. As I have previously mentioned, in days gone by, amateur astronomers used to make Newtonian telescope tubes from mahogany rather than aluminium to avoid the metal tube radiating more tube currents into the light path as it cooled down. Often, the inside of the tube was lined with cork to further reduce tube currents. If the Newtonian tube is only fractionally wider than the mirror, tube currents when cooling are more likely. A Newtonian tube should have a radius a good few centimeters wider than the mirror radius. However, we have seen that as a tube becomes more "open," dew is more likely to form on the primary and secondary mirrors. This can easily be removed by a hair-dryer, but preventing it from forming in the first place is better. A dew heater band will cause permanent heat turbulence in the light path whereas the momentary blast from a hair-dryer will dissipate away. Of course, as soon as the primary or secondary have cooled, dew begins to form again! While discussing custom-built planetary Newtonians I have to mention the excellent long-focus planetary Newtonian of York amateur Mike Brown. This is shown in Figure 3.19, along with Mike's exquisite mirror cell in Figure 3.20. Such a telescope will stay collimated for life because the diffraction-limited sweet spot is so large.

Because the planetary webcam user's telescope does not need to be large, a simple user-friendly observatory is well within the abilities of even the most hopeless DIY enthusiast to construct. For my 250-mm Newtonian I thought long and hard about what the best, most user-friendly, and least obtrusive telescope shelter might be. I came up with the design shown earlier in Figure 3.17.

Because precise polar alignment is not essential for planetary observing, the telescope glides out on rails from a kennel-like structure attached to the southeast-facing wall of the house. Everything can be up and running in a matter of minutes and the telescope is the heaviest structure that needs to be moved.

Before I finish this chapter, I would just like to say a few words about bad optics. Figure 3.21 shows a Zygo interferometer plot of a 40-cm mirror that was advertised as being accurate to 1/10th wave PV but was actually only accurate to half a wave! This mirror was made by a backstreet mirror-maker who claimed his mirrors were far superior to mass-produced ones. In fact, they were far inferior. Fortunately, such con men do not survive for long as they are soon found out with the power of a Zygo interferometer.

250mm Newtonian
Figure 3.19. The 250-mm f/9.3 Newtonian of Mike Brown from York, U.K. The instrument has a diffraction limited field of almost 18 mm diameter at the Newtonian focus. Image: Mike Brown.
250mm Newtonian

Figure 3.20. A well-designed mirror cell, made by Mike Brown from York, U.K., for his 250-mm Newtonian. Note the three-triangle/nine-point suspension system and the open frame allowing air circulation. Considerable attention has been applied to making the design concentric with the tube mounting holes. Image: Mike Brown.

Figure 3.20. A well-designed mirror cell, made by Mike Brown from York, U.K., for his 250-mm Newtonian. Note the three-triangle/nine-point suspension system and the open frame allowing air circulation. Considerable attention has been applied to making the design concentric with the tube mounting holes. Image: Mike Brown.

Light Source Drawing
Figure 3.21. A competitors mirror advertised as 1/10th wave PV actually turned out to be 0.442 wave, 4.4 times worse than advertised. Star images in the instrument were noticeably soft and planetary views very disappointing for a 350-mm instrument. Image: Orion Optics.

Finally, before we leave the chapter on high resolution, I would like you to have a look at Figure 3.22. This is an image, taken by Damian Peach, of the intra- and extrafocal diffraction patterns seen in a badly designed 25 cm f/12.5 Maksutov. The optical tube, despite costing $12,000, was so badly ventilated that it could never deliver quality results. Indeed, side-by-side with a 23.5-cm Celestron SCT, costing one-eighth (!!) the price, its performance was pitiful. Price does not necessarily mean performance!

Comparison Aperture Astrophotography
Figure 3.22. Thermal problems in a $12,000 Maksutov-Cassegrain are obvious when one examines the intra- and extrafocal diffraction patterns. The distorted, fluted pattern is characteristic of heat seeking a way out. Image: Damian Peach.


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