You'll get lots of hype when trying to pick a telescope. But by knowing just a few basics, described here, you'll be able to choose the one that's right for your observing interests, lifestyle, and budget.
If you still have questions or need more details, check out these additional resources:
- Telescope buyer's guide (online article)
- "What to Know Before You Buy" (PDF) from SkyWatch 2010
- Guide to Buying Your First Telescope (video) from Sky & Telescope's Skywatching Video Series
Many (arguably most) good starter scopes cost $400 or more, though some superb choices are available for under $250. But read this article first, so you'll understand the terminology and what type of telescope will be best for you.
The telescope you want has two essentials: high-quality optics and a steady, smoothly working mount. And all other things being equal, big scopes show more and are easier to use than small ones, as we'll see below. But don't overlook portability and convenience — the best scope for you is the one you'll actually use.
Aperture: A Telescope's Most Important Feature
The most important characteristic of a telescope is its aperture — the diameter of its light-gathering lens or mirror, often called the objective. Look for the telescope's specifications near its focuser, at the front of the tube, or on the box. The aperture's diameter (D) will be expressed either in millimeters or, less commonly, in inches (1 inch equals 25.4 mm). As a rule of thumb, your telescope should have at least 2.8 inches (70 mm) aperture — and preferably more.A larger aperture lets you see fainter objects and finer detail than a smaller one can. But a good small scope can still show you plenty — especially if you live far from city lights. For example, you can spot dozens of galaxies beyond our own Milky Way through a scope with an aperture of 80 mm (3.1 inches) from a dark location. But you'd probably need a 6- or 8-inch telescope like the one shown at right to see those same galaxies from a typical suburban backyard. And regardless of how bright or dark your skies are, the view through a telescope with plenty of aperture is more spectacular than the view of the same object through a smaller scope.
Avoid telescopes that are advertised by their magnification — especially implausibly high powers like 600×. For most purposes, a telescope's maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters) . So you'd need a 12-inch scope to get a decent image at 600×. And even then, you'd need to wait for a night when the observing conditions are perfect.
Types of Telescopes
You'll encounter three basic types of telescopes:
• Refractors have a lens at the front of the tube — it's the type you're probably most familiar with. While generally low maintenance, they quickly get expensive as the aperture increases. Watch an animation of light passing through a refractor.
• Reflectors gather light using a mirror at the rear of the main tube. For a given aperture, these are generally the least expensive type, but you'll need to adjust the optical alignment periodically — especially if you bump it around a lot.Watch an animation of light passing through a reflector.
• Compound (or catadioptric) telescopes, which use a combination of lenses and mirrors, offer compact tubes and relatively light weight; two popular designs are called Schmidt-Cassegrains and Maksutov-Cassegrains. Watch an animation of light passing through a compound telescope.
The objective's focal length (F or FL) is the key to determining the telescope's magnification ("power"). This is simply the objective's focal length divided by that of the eyepiece, which you'll find on its barrel. For example, if a telescope has a focal length of 500 mm and a 25-mm eyepiece, the magnification is 500/25, or 20x. Most types of telescopes come supplied with one or two eyepieces; you change the magnification by switching eyepieces with different focal lengths.
The Mount: A Telescope's Most Under-Appreciated Asset
Your telescope will need something sturdy to support it. Many telescopes come conveniently packaged with tripods or mounts, though the tubes of smaller scopes often just have a mounting block that allows them to be attached to a standard photo tripod with a single screw. (Caution: A tripod that's good enough for taking your family snapshots may not be steady enough for astronomy.) Mounts designed specifically for telescopes usually forgo the single-screw attachment blocks in favor of larger, more robust rings or plates.On some mounts the scope swings left and right, up and down, just as it would on a photo tripod; these are known as altitude-azimuth (or simply alt-az) mounts. Many reflectors come on an elegantly simple wooden platform, known as a Dobsonian, that's a variation of the alt-az mount. A more involved mechanism, designed to track the motion of the stars by turning on a single axis, is termed an equatorial mount. These tend to be larger and heavier than alt-az designs; to use an equatorial mount properly you'll also need to align it to Polaris, the North Star.
Some telescopes come with small motors to move them around the sky with the push of a keypad button. In the more advanced models of this type, often called "Go To" telescopes, a small computer is built into the hand control. Once you've entered the current date, time, and your location, the scope can point itself to, and track, thousands of celestial objects. Some "Go To"s let you choose a guided tour of the best celestial showpieces, complete with a digital readout describing what's known about each object.
But Go To scopes aren't for everyone — the setup process may be confusing if you don't know how to find the bright alignment stars in the sky. And lower-priced Go To models come with smaller apertures than similarly priced, entry-level scopes that have no electronics.
Remember . . .
Any telescope can literally open your eyes to a universe of celestial delights. With a little care in selecting the right type of telescope for you, you'll be ready for a lifetime of exploring the night sky!