First Time Buyers Guide

Aperture: A Telescopes 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, from a dark location you can spot dozens of galaxies beyond our own Milky Way through a scope with an aperture of 80 mm (3.1 inches). But you’d probably need a 6- or 8-inch telescope 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 impressive than the view of the same object through a smaller scope.

Mounts: Supporting Your Telescope

Astronomers consider their telescope’s mount to be equally as important as the telescope/optical tube – and sometimes spend more money on the mount than what is sitting on top of it.

A mount’s job is to:

• Keep the telescope from shaking – at high power, every little wobble or vibration is magnified so much that the whole image can become un-viewable.
• Allow the telescope to move extremely smoothly. Celestial objects don’t stand still for you to look at. Or, more precisely, they do stand still, but they appear to move across the sky as Earth rotates underneath you.

Your telescope will need something sturdy to support it. Many telescopes come conveniently packaged with tripods or mounts (see our Telescope Kits). Be aware that 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 favour 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 (and many newer models don’t even require you to do that), 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.