Those new to the hobby of astronomy who are considering a telescope purchase typically wonder just which type and what size are the best choices. This month we provide no-nonsense succinct guidelines to help answer these questions.
There are three basic types of optical telescopes used for viewing the heavens. Best-known is the refractor or lens-type, which employs a compound objective having two or more elements to collect light and bring it to a focus. It’s what most people think of when they hear the word “telescope”—someone looking through the end of a long tube at the sky. Another form is the reflector or mirror-type, which uses a precision concave primary mirror to do the same. The third type is a combination of these two. Called a catadioptric or compound telescope, it uses a system of both mirrors and lenses. They come in two versions—the Maksutov and the very popular Schmidt-Cassegrain (or SCT).
Generally speaking, refractors with their unobstructed light paths (no secondary mirrors) and closed tubes give the sharpest images and rarely require any adjustment (collimation). Reflectors offer the most aperture for the dollar, but their open tubes can result in image-degrading tube currents (with the exception of Edmund’s unique Astroscan, which has an optical window to seal the tube). Due to their folded light-paths, catadioptrics are the most compact for their size but are typically the most costly of the three types. Both SCT’s and reflectors require collimation on occasion (an easily-mastered process).An exception here is again the Astroscan, which is permanently collimated at the factory.” Volumes have been written about the advantages of one type of telescope over the others. Suffice it to say that if optically sound, each of them performs equally well and can provide a lifetime of viewing pleasure. The final choice often comes down to a matter of affordability. In any case, purchasing a telescope is a good investment—for unlike many other technical devices they typically appreciate in value over time rather than depreciate!
The range of telescope sizes in use by stargazers today is astounding—from hand-held or table-top-mounted small scopes to large observatory-class instruments. Often seen at star parties are the popular “Dobsonian” reflectors, some of which are so huge that they are moved around in trucks and require stepladders to reach their eyepieces! Named for their simplified mountings invented by famed telescope-maker John Dobson, they are ultra-easy to use and in the smaller sizes are very affordable. There’s also a wide variety of more complex (and typically heavier and costlier) mountings, including computer-driven/controlled ones able to locate thousands of targets automatically from a keypad. However, most of these so-called “Go-To” systems have a “learning curve” in terms of setup and actual use under the stars at night. (And to us purists, they take much of the fun out of leasurely stargazing using a good star map or atlas to “star hop” our way to celestial treasures!)
As to the best size, a valuable rule-of-thumb here is that the smaller (i.e., more portable) the telescope, the more often it’s likely to be used. Lugging a 100-pound instrument outside and setting it up in the dark is enough to dissuade many observers, especially on frigid winter nights. Professionally, I’ve been privileged to have used some of the largest telescopes in the world. But my personal instruments are a 3-inch refractor, a 4-inch reflector, and a 5-inch catadioptric—any one of which I can easily pick up and carry about with one hand!
Edmund Scientific is pleased to offer a selection of all three types of telescopes in a range of sizes, mountings and prices. To see them, simply go to www.scientificsonline.com and click on the “Telescopes” icon.
Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.