Choosing Your First Telescope
Every year millions of people buy a telescope, but few know what to look for when making their purchase.
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.
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.
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.
You'll encounter three basic telescope designs.
• 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.
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 telescopes come supplied with one or two eyepieces; you change the magnification by switching eyepieces with different focal lengths.
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.
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.
A telescope can literally open your eyes to a universe of celestial delights. With a little care in selecting the right one, you'll be ready for a lifetime of exploring the night sky!