The Mount: A Telescope's Most Under-Appreciated Asset

Most types of telescopes come supplied with one or two eyepieces you change the magnification by switching eyepieces with different focal lengths. For example, if a telescope has a focal length of 500 mm and a 25-mm eyepiece, then the magnification is 500/25, or 20×. This is simply the objective's focal length divided by that of the eyepiece, which you'll find on its barrel. The objective's focal length (F or FL) is the key to determining the telescope's magnification ('power'). Watch an animation of light passing through a compound telescope. Compound (or catadioptric) telescopes, which use a combination of lenses and mirrors, offer compact tubes and relatively light weight two popular designs you'll often see are called Schmidt-Cassegrains and Maksutov-Cassegrains.Watch an animation of light passing through a reflector. For a given aperture, these are generally the least expensive type, but you'll need to adjust the optical alignment every now and then - more often if you bump it around a lot - but that adjustment (called collimation) is straightforward. Reflectors gather light using a mirror at the rear of the main tube.Watch an animation of light passing through a refractor. In refractor lingo, an apochromat offers better optical quality (and is more expensive) than an achromat of the same size. While generally low maintenance, they quickly get expensive as the aperture increases. Refractors have a lens at the front of the tube - it's the type you're probably most familiar with.You'll encounter three basic 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. And even then, you'd need to wait for a night when the observing conditions are perfect. So you'd need a 12-inch-wide scope to get a decent image at 600×. For most purposes, a telescope's maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters). 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.Īvoid telescopes that are advertised by their magnification - especially implausibly high powers like 600×. 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. 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 a good small scope can still show you plenty - especially if you live far from city lights. Dobsonian telescopes, which are reflectors with a simple mount, provide lots of aperture at relatively low cost.Ī larger aperture lets you see fainter objects and finer detail than a smaller one can. As a rule of thumb, your telescope should have at least 2.8 inches (70 mm) aperture - and preferably more. The aperture's diameter (D) will be expressed either in millimeters or, less commonly, in inches (1 inch equals 25.4 mm). Look for the telescope's specifications near its focuser, at the front of the tube, or on the box. The most important characteristic of a telescope is its aperture - the diameter of its light-gathering lens or mirror, often called the objective. Aperture: A Telescope's Most Important Feature But don't overlook portability and convenience - the best scope for you is the one you'll actually use. And all other things being equal, big scopes show more and are easier to use than small ones, as we'll see below. The telescope you want has two essentials: high-quality optics and a steady, smoothly working mount. Armed with these few basic types of telescopes, you'll have a good idea what to look for (and what to avoid) when scouring the marketplace for your new scope. So here is a guide to help you make sense of the 'universe' of telescope models available today. Many (arguably most) good starter scopes cost $400 or more, though some superb choices are available for under $250. Instead, you should choose a telescope based on your observing interests, lifestyle, and budget. It can be a portal to the universe and provide a lifetime of enjoyment.īut there's no one 'perfect' telescope - just as there's no such thing as a perfect car. With a little guidance, you can pick a high-quality telescope that can last a lifetime.Ī telescope is a popular gift, especially so during the holidays.

Best telescope