Some background now: About ten years ago, I was looking for refractor-like planetary performance on a reflector budget. It occurred to me that a really conscientious optician could make an excellent long-focus mirror in approx. 16" size. I had the opportunity to purchase a 15" F/4.9 mirror that had good RTA and wavefront numbers, and although I was skeptical because of its short f/ratio, I decided to go for it because the price was right, there was much to be gained if it worked, and not too much to lose even if it didn't fully satisfy my expectations. Plus, the seller said he would stand by the warranty if I was disappointed at all. At worse, it'd be a decent scope for deep-sky work, I reasoned, and a big step up from my old Coulter 13.1" scope.

Lo and behold! - I was completely blown away by the clarity of definition through that mirror in the scope I made for it, and its images compared very favorably with a well-known 16" F/6 telescope in use by our Kansas City club members. That particular mirror was made by the Naval Observatory and is a good benchmark of optical quality. But that 15", with 1.5" thickness, was nearly its equal, especially considering the 14% difference in light grasp. It virtually seemed to consume planets and had an insatiable optical appetite for this type of object! I knew immediately I had been suffering from "paradigm paralysis."

Since that time, I've gone into business producing optical systems under the Starmaster label. This work has included star testing literally hundreds of mirrors that passed through my shop and observing site, from F/4 through F/6, in sizes from 7" through 24". The results: The short-focus, large-aperture optics disclose detail on planetary disks that are unrivalled by smaller aperture refractors with apochromatic design, and are equal to longer-focus reflectors of F/5 or longer F/ratio, given the same aperture. Conclusions: When it comes to large aperture reflectors, There is no reason whatever to use an F/ratio greater than F/4.1 to achieve the finest possible planetary performance, PROVIDED that the system uses a coma-corrector to create a wide diffraction-limited field.

So why does this paradigm (that large, fast reflectors are bad for planets) continue to prevail among many amateurs? If this is so, how come the word hasn't gotten around? Well, for one thing, there are additional criteria that need to be satisfied to make a short-focus primary realize its potential, and most frequently they are not so optimized.

These include:

1. Cell Design
1. Edge supports seem to provide a far better method of preventing deformation than the traditional sling support. It appears that the sling tends to deform the lower edge of the primary by pulling it against the lower support points at different elevations. Although I haven't done any simulations in software to prove this point, the empirical results seem to indicate solid edge supports work better. There is no image shift at any elevation, and no astigmatism when the telescope is near-horizontal. I can't say the same thing for telescopes through which I've observed that use a sling mount for the primary.
    
2. Stability and freedom of movement of the support points. The supports must be constrained to prevent rotation of the triangles, yet allow equal freedom of accommodation in all axes.
    
3. The cell design we use with "equal loading" four-point lower edge supports is fashioned after the test setup used by both Pegasus Optics and Zambuto Optical Company to test their mirrors. Neither Pegasus nor Zambuto uses a sling to support their optics during testing.
    
2. Optical Quality.
1. Figuring. A properly figured primary mirror, whether F/4 or F/8 or in-between, will stand up to high magnification, seeing conditions permitting. Short F/ratio primaries require more precise figuring because the figuring tolerance is inversely and proportionally tighter in shorter f/ratios.
    
2. Primary thickness. The thinner the primary, the better it equilibrates. I've found by side-by-side comparison of 1.6" and 2" telescopes, that a 1.6" thick primary equilibrates about 35 - 50% faster than a 2" thick one, depending on the temperature delta during the observing session, particularly in the earlier part of the session. It should be noted that figuring a thin primary is considered a more challenging task than for a thicker one because of the tendency to flex during figuring.  However, Pegasus and Zambuto have proven to be up to the task.
    
3. Secondary quality. Before I acquired a steady source of reliable secondaries, I had to spend a great deal of shakedown time on completed telescopes assuring that the secondary introduced no error into the system. Many secondaries from previous sources had to be returned, usually because of astigmatism along the major axis. My  present supplier individually tests each mirror before having it coated and I have yet to find a problem with them. Moral to the story: Spending a few extra bucks on a known high-quality secondary that are thoroughly tested, is well worth it. Anything less than the best quality secondary jeopardizes the quality and performance of even the best primary mirror.
    
4. Coatings. It is my belief, and that of many other knowledgeable observers, that a "standard," single-layer coating has less potential for variation and roughness than a multi-layer metallic coating.  My experience bears this out. I've had excellent experience with an ion-deposition process with quartz overcoat on the primary, and a dielectric-overcoated enhanced coating on the secondary.
    
3. Drive System.
   A drive system that keeps the object centered in the "sweet spot," along with a coma corrector that makes the sweet spot a larger and therefore easier target, will assure that the optical rays are concentrated to a size comparable to that of the diffraction spot diameter in the critical part of the image where resolution is most needed. Part of the problem associated with an undriven system is the need to shove the scope from the top end until the object is in the ideal "drift" position, let its vibration die down, and hopefully by the time the image has stabilized, it crosses the approximate center of the field. Most of the time, during undriven viewing, the object will be out of the best part of the field, especially if no coma corrector is in use to extend the diameter of the useful field width.
   
   With the Sky Tracker drive system, the incremental steps applied to the base of the telescope create no observable backlash or instability when the altitude or azimuth motors engage, and the movement of the telescope is imperceptible. Even at powers over 1000X, the object stays in view with an occasional nudge of the buttons on the hand-held control paddle. Because the object stays virtually centered, 100% of the observer's time and attention is spent concentrating on the object, instead of diverting 75% of his energy (and time) to pushing the top end.

I hope these comments don't cause refractor fanciers to take umbrage, and realize there's a qualitative difference in large reflector images compared to, for example, larger apochromatic refractors, which typically are about 8" aperture. The images through such a refractor are pristine, subtle, and are much to be appreciated for their optical quality, not to mention the value of such an instrument. However, the 2.7X difference in resolution and 7X greater light grasp provided by (for example) a 22" scope can provide commensurately more visual information, detail and brightness. From what I have seen, this is all too obvious when observing conditions permit this kind of comparison. I believe it is true that all would agree that the planetary images through such a large-aperture system are absolutely stunning, by any standard, when you see them for the first time or even the fiftieth time.

High performance planetary images is not where these telescopes end. As deep sky instruments they excel just as well. Recently, Joe Porter, who owns an excellent 12.5" F/5.6 telescope and is an accomplished observer, was observing M2 with me through a 24" F/4.2 telescope with a 1.6" thick primary. Joe commented, "It goes through this side, then through the center, and then right out the other side!" and he meant it was completely resolved like he had never seen it before, which he commented on at length. And, at a recent star party in my area where there were two 22" F-4.1 telescopes with identical configuration as well as several other large aperture fast F-ratio scopes, ranging from an 18" F-3.75 to the previously mentioned 22"ers. The incredible seeing at the time allowed the use of over 1100X on Saturn. More was possible if I had not been too tired to try it, being near death's door from sleep deprivation by 4:00 a.m. Numerous comments from various "old hands" as well as newbies to the hobby at this event, in response to what they observed in these scopes, boiled down to an unspoken conclusion that the old paradigm is not yet dead, but it's about time to hold an appropriate ceremony to bury it. It logically follows that the new paradigm shift should be to the concept that large reflectors, when properly configured, do an excellent job on planetary and extended object detail.

Rick Singmaster