Types of Telescopes

A quick guide to types of telescopes
With a little guidance, you can pick a high-quality telescope that can last a lifetime.
S&T: Craig Michael Utter
Here's a quick guide to help you make sense of all the types of telescope models available today. Armed with these few basics, you'll have a good idea what to look for (and what to avoid) when scouring the marketplace for your new scope. If you still have questions or need more details, check out our extended online telescope help guide.. Or if you want something that you can print out and read at your leisure, click here to download "What to Know Before You Buy" from SkyWatch 2010, our annual magazine, as a 2-megabyte PDF file.

Many (arguably most) good starter scopes cost $400 or more, but some superb choices are available for under $250. For some specific recommendations, read our review of Low-Cost Starter Scopes. But read this article first, so you'll understand the terminology in that review.

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 Dobsonian telescope
Dobsonian telescopes provide lots of aperture at relatively low cost.
S&T: Craig Michael Utter
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:

Types of telescopes
The three basic telescope designs use different optics to achieve the same result: making distant objects look bigger and brighter than they appear to your eye. See the text for links to animations that show how light passes through the optics of each design.
S&T: Gregg Dinderman

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. • 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.

Telescope mounts
For all their apparent diversity, telescope mounts boil down to two basic types. An alt-azimuth mount (left) permits the scope to move up-down and left-right. It’s quick to set up and intuitive to use. An equatorial mount (right) can track celestial objects by turning just one axis and can be more easily motorized — but to work properly it must be aligned with Polaris, the North Star.
S&T: Craig Michael Utter
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!

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