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Starmaster 20", f4.3, Sky Commander & Sky Tracker Goto Drive



Let me start by summarizing my view on the Starmaster 20" f4.3 Telescope with a 1.6" thick mirror and the Sky Commander with Sky Tracker goto drive: It is the ultimate portable large aperture telescope. It is not the minimum required, nor is it even necessary to enjoy astronomy, but it is the best blend of compromises that must be dealt with in selecting a large-aperture telescope.

In selecting a large aperture telescope, a few parameters are worth considering:

1) Weight of smallest component
2) Height of eyepiece while observing
3) Amount of accessory equipment requiring transport (ladders, platforms)
4) Time to set up

Consideration of these parameters, along with the intended application of the telescope, demonstrates the superiority of the 20" Starmaster, proving it to have no competition.

My primary use of a large-aperture telescope is in sharing the heavens with friends, family, and at public star parties. For these uses, large aperture is desirable as it offers resolution of GCs and OCs into individual stars, and, given a dark sky site, makes the larger nebulae breathtaking to even an inexperienced observer. It also reveals star colors in clusters quite dramatically, yielding visual correlation to discussions of star types. For visual planetary observation, nothing exceeds the real-time visual information offered by large aperture, diffraction limited reflectors. Large aperture (greater than 10-15") refractors suffer from chromatic aberration. Small aperture refractors don't even come close. Period. One look through a diffraction-limited 20" reflector, even with the Paracorr "putting all that glass in the optical path" will make anyone willing to be honest agree that, among diffraction limited optics, aperture is everything. So what sets the Starmaster 20" apart from the other large aperture instruments commercially available? Let me show how the parameters I mentioned earlier bear this out.

First consider the weight of the smallest component. Mr. Singmaster's mirror cell design allows the mirror to be carried as a separate component, thereby minimizing the weight of the heaviest component. In this 20" scope with a 1.6" thick mirror, the heaviest component, the mirror and cell, weighs 59 pounds. Admittedly not "nothing", but this is the best compromise between weight and visual impact. Much of his weight savings comes from the thinness of his mirrors. This has a side-benefit of minimizing cool-down time. Most other large aperture scopes don't even offer 1.6" thick mirrors, perhaps because their mirror cells do not provide enough support for them. Starmaster's mirror cell is STEEL, not aluminum, yet its design minimizes its weight while taking advantage of the superior flexure-limiting characteristics of steel.

Next is eyepiece height. This is a factor of aperture and focal length. The shorter the focal length, the shorter the eyepiece height, minimizing the need for heavy, bulky ladders and the attendant time, inconvenience, and comfort hurdles they impose. According to the two premier mirror makers in the US, there is a limit to how short focal length can be without adversely affecting planetary views (assuming a Paracorr will be used). They further agree that this limit is f4.3. Taking this into account, the largest aperture f4.3 scope that allows safe observing without a bona-fide 6' ladder is 20. That assumes no additional height is gained from a tracking platform. As you may have experienced yourself, visual gains beyond 18 inches of aperture, at which details of many galaxies become directly visible, begin to follow the law of diminishing returns. Especially when the need for a ladder and its attendant risks (falling) and transportation requirements (often must be tied to the roof) and the fact that a ladder typically requires an additional trip during setup and breakdown are taken into account.

This brings us to the next parameter, namely additional accessories required for observing with the scope. Typically large aperture observing requires a 6' ladder; and public sky shows are dramatically less tedious when tracking and electronic object locating are employed. With Mr. Singmaster's StarStep chair, which takes up less width space than any observing chair I have ever seen, the Starmaster 20" has spared me the misery of a ladder, serving me perfectly while sharing the sky's wonders with children and adults. Because it is so lightweight and carry-friendly, I typically carry the StarStep Chair and my eyepiece case in the same trip. Packing the StarStep in the car only requires about 3 inches of width space. Now let's address object location and tracking. In my experience, electronic object finding and tracking are two essential features for sharing the sky with more than one person in an efficient fashion. While I can find the more impressive deep sky objects with respectable speed in a dark sky location, light pollution (where it is most convenient to observe with a group), can turn a hunt for even the Ring Nebula into a 10-minute fiasco. While I believe that a major aspect of my enjoyment of astronomy comes from my knowledge of the sky and my ability to visit my favorite deep sky objects from memory with a pair of binoculars, I find that participant interest is maintained much more easily when a few minutes of down time are not endured by the crowd between each object during a sky show. Often, while the scope is slewing, I find myself answering questions that I would either be half-answering or might appear to be rudely ignoring if I were trying to answer them while star hopping. While admittedly a side-line issue, watching a nine-foot (with dew shield) telescope slew effortlessly across the sky, coming to rest precisely on the next object can be quite an impressive sight. It's almost as entertaining as the sky show! The Sky Tracker goto system begins tracking as soon as the object arrives in the field of view. As anyone who has shared high-powered views of astronomical objects with those new to astronomy knows, tracking is fairly mandatory. My group observing sessions are directed toward maximizing people's interest in astronomy. High-powered views (when the atmosphere will allow them) are very helpful at feeding that interest. While I like my equatorial platform tremendously for my 10" Teleport telescope, my refusal to incorporate a ladder and all of its baggage precludes use of a platform with a 20" f4.3 scope. It would simply raise the scope out of reach.

The last parameter I will address is time of setup and breakdown. Actually, time of setup is only important from a hassle standpoint, because the time it takes the mirror to acceptably cool down (30 minutes to an hour) is greater than the setup time with most, if not all, of the commercially available large aperture Dobs. Mr. Singmaster has, once again, made it maximally efficient. Pairing the truss tubes into 4 permanently bound sets not only cuts assembly/breakdown time by over 80%, but it minimizes variation of the optical axis from one assembly to the next, resulting in minimal adjustments to collimation, adding to the efficiency and further minimizing the time of setup. In my experience, breakdown time is where one comes to appreciate the elegant simplicity of design of this scope. When one is tired and not thinking with the greatest of clarity because of sleep deprivation and exhaustion in the wee hours of the morning, this scope breaks down in 13 minutes. In that time I can consistently go from thinking about quitting to being behind the steering wheel, driving away with the scope completely disassembled and neatly stowed with all the accessories.



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