Why a CAT

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Since you're reading this, I'm guessing you have made an exciting decision: You want a telescope. Specifically, you want a telescope for looking at the sky, a telescope that will open the depths of space to your gaze and allow you to visit the Moon, the planets, and all the strange and distant wonders of our magnificent universe. And you are not looking for just any telescope, either, but for a Schmidt Cassegrain telescope (SCT), whose full-color advertisements fill the pages of astronomy magazines.

In our consumer culture, most of us have become wary of high-pressure ads from manufacturers who promise the Moon and deliver little. Luckily, that is not the case when it comes to SCTs. Sometimes, the advertising does contain hyperbole, but Schmidt Cassegrains really can deliver the Moon—and the stars, too.

SCTs, like anything else, are not perfect, but when all is said and done, the Schmidt Cassegrain may be the most versatile, technologically advanced, and easy-to-use telescope ever sold to amateur astronomers. Since SCTs were first offered at prices the average person could afford way back in 1970, they have dominated the amateur astronomy telescope market. Don't believe that? Take a stroll around the observing field of a local astronomy club during the next star party. Chances are a majority of the telescopes there will be SCTs. Fancy advertisements alone simply could not account for the enduring popularity of Schmidt Cassegrains. Something good is going on.

Not that an SCT (Plate 1) Iooks much like a telescope of any kind to novice astronomers. Catadioptric telescopes (CATs, for short), which are telescopes that use both lenses and mirrors, do not much resemble the telescopes we are used to seeing in the movies or on television. The eyepiece is where it "ought" to be, at the end of the tube, and that tube is perched on a tripod, but that is where the similarity ends. The tube is short and fat, looking more like a beer keg than a respectable

R. Mollise, Choosing and Using a New CAT,

DOI: 10.1007/978-0-387-09772-5_1, © Springer Science + Business Media, LLC 2009

Plate 1. (SCT) An 8-inch Schmidt Cassegrain telescope set up at a dark site and ready for an evening of deep space voyaging." Credit: Author telescope. It is not just attached to a tripod, either. It is sitting on a complicated-looking "mount" festooned with myriad lights and switches.

The SCT looks different enough in beginners' eyes to be positively frightening, maybe scary enough to make a new astronomer who just wants a good look at the craters of the Moon turn tail and run. Appearances can deceive, however. The SCT is at heart an uncomplicated telescope. Despite its looks, its basic operation is easy to understand, and it is actually one of the most user-friendly scopes ever made.

And, it is not just user friendly. A beginning amateur astronomer may start out just wanting a look at the good old Moon but will soon find the faithful SCT can take even a novice observer way beyond our cosmic neighborhood—maybe even as far as the daunting depths of the universe inhabited by the mysterious quasars. Although nothing in the design of the SCT is astoundingly innovative, its basic layout is extremely sound and features good optics in sizes sufficient to take even a tyro a long, long way from home.

Capability is just the beginning of the SCT story, though. What also sets these CATs apart is their versatility. Other telescope types—Dobsonian reflectors and apochromatic refractors, for example—may do some things better than the SCT, but no telescope is as capable of doing so many things as well as the Schmidt Cassegrain. One of the reasons is that, like the personal computer, the SCT is a system . Much as the personal computer (PC) industry has done, the world's two SCT makers, Meade and Celestron, have standardized their products. A camera adapter sold by Meade will usually work just as well on a Celestron. Also, as in the computer industry, there are numerous third-party manufacturers making accessories for the telescopes. Actually, some of the best accessories for Meade and Celestron SCTs do not come from either company but from the hordes of aftermarket vendors large and small. SCTs have been in production and basically unchanged for nearly 40 years, and that means any accessory imaginable—focus motors, digital setting circle computers, electronic cameras, spectrographs, and much more—has probably already been made by somebody and will work on any Schmidt Cassegrain, old or new. As astronomy interests change over the years, an SCT can also change.

Does the SCT's ability to do so many things in astronomy have a downside? An old aphorism that is often all too true is "jack of all trades, master of none." In some ways, that is the case when it comes to CATs. As good as an 8-inch SCT is for planetary observing, for example, it will never be able to do quite as well as a high-priced apochromatic refracting (lens-type) telescope. As far as it may be able to voyage out into deep space, it will never show as many objects as a Dobsonian reflecting telescope with a 20-inch diameter mirror.

The SCT really does not fall far behind any other telescope in doing anything however. The differences in the planetary images of an SCT and a refractor are small and subtle. New observers may not be able to detect this difference for years. When observing deep space objects, the SCT has some features that help it keep up with the largest Dobsonians. Following is a discussion of a few of the many things a Schmidt Cassegrain can do well.

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