A Sky & Telescope press release
Sean Walker, Equipment Editor, Sky & Telescope
617-864-7360 x22105, swalker@SkyandTelescope.com
Kelly Beatty, Senior Editor, Sky & Telescope
617-864-7360 x22168, kbeatty@SkyandTelescope.com
|Note to Editors / Producers: This release is accompanied by high-quality illustrations and animations.|
A telescope is a popular gift, especially so every December. It can be a portal to the universe and provide a lifetime of enjoyment. But there's no one "perfect" telescope — just as there's no such thing as a perfect car. Instead, choose a telescope based on your observing interests, lifestyle, and budget. And buyer beware: a telescope should not be bought on impulse.
"Don't expect a lot from the majority of telescopes costing less than $200, and certainly be wary of anything sold in a toy shop or department store," says Sean Walker, Equipment Editor of Sky & Telescope magazine. "Do some research before buying, and then go to a reputable store or online dealer that specializes in telescopes or related products, such as cameras or consumer electronics."
Here's expert advice from the editors of Sky & Telescope for anyone searching for a first-ever telescope.
Telescopes come in many shapes, sizes, and prices. Yet all of them fall into one of three general classes: refractors (those that collect light using lenses), reflectors (those with mirrors), and compound telescopes (hybrids of the two). Each has its strengths and weaknesses, but all share the same function: to gather light from a distant object and form a sharp image that can be scrutinized by eye or camera.
• Refractors have a lens at the front of the tube — it's the type most people are familiar with. While generally low maintenance, refractors quickly become expensive as the diameter of the main lens increases. In refractor lingo, an apochromat offers better optical quality (and is more expensive) than an achromat of the same size.
• Reflectors gather light using a precisely-shaped curved mirror at the rear of the main tube. For a given diameter, 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 — look for these phrases in ads or on the telescope itself.
"Whatever design you choose, optical quality should be your top priority," notes S&T Senior Editor Kelly Beatty. "It's the key to seeing the night sky at its best." Running a close second is a solid, steady mount with smooth, dependable motions.
If at all possible, try before you buy — visit a local astronomy club and look through members' scopes to see which ones you like. If you purchase a unit online, make sure there is a good return policy. Avoid used-equipment offers unless you're certain about what you're buying.
What to Look For
Here are important characteristics to look for in any telescope, regardless of type:
The aperture (diameter) of the primary lens or mirror in your telescope determines two things: light-gathering power and resolving power (the ability to see fine detail). The larger the aperture, the more light your scope collects and the fainter the objects you can see. With increased aperture also comes increased resolution — a larger-aperture telescope will reveal smaller features on the Moon and in distant nebulae and galaxies.
Focal Length and Magnification
The distance from the primary lens or mirror to the point where the image of a distant object comes into focus is called the focal length. The magnification, or power, of any telescope-eyepiece combination is easy to calculate: divide the focal length of the scope by that of the eyepiece. So a 25-mm eyepiece used with a refractor having a focal length of 900 mm gives 36 power (900 / 25 = 36), usually written as 36×. As a general rule, twice the aperture in millimeters (or 50 times the aperture in inches) is the maximum usable magnification. Beyond that, the image gets so faint and fuzzy that it seems forever out of focus.
Beginners are frequently surprised at how small a window on the sky their scope presents when used at medium to high power. So all telescopes — regardless of type or design — should be equipped with a high-quality finder, an observing aid that assists in locating celestial objects. Very common these days are "red-dot" finders, which use an LED to project a red dot or centering pattern on the search area but don't magnify the view.
A telescope with the finest optics will be rendered useless without a suitable mount. A good mount (1) holds the instrument firmly with little vibration, (2) allows the tube to be pointed to any part of the heavens quickly and accurately, and (3) permits smooth and precise tracking of a celestial object as Earth's rotation carries it from east to west across the sky. Two basic types of mounts accomplish these tasks: altazimuth and equatorial.
Alt-azimuth ("alt-az") mounts, which move up-and-down and side-to-side, require simultaneous manual corrections for two axes to keep celestial objects in view. Unless you have a motor-driven altazimuth mount, for high-magnification visual observations — and especially for faint-object astrophotography — you'll probably want an equatorial mount.
An equatorial mount also uses two axes, but one of them is aligned parallel to Earth's axis of rotation by being pointed at the north celestial pole, near Polaris, when viewing from the Northern Hemisphere. Then, once a celestial object has been found, you only have to pivot the scope around its "polar" axis to keep the object in view.
Many telescopes use a built-in computer to drive the mount's motors. Once properly initialized, the computer takes over and can automatically aim the telescope at any desired object and track it as it moves across the sky. This is the essence of a "Go To" telescope. Depending on the sophistication of the system, you might need to enter your viewing location, date, and time at the beginning of an observing session. You might also need to point the scope at two or three bright stars or planets in order to synchronize the instrument's coordinate system with that of the sky.
Go To scopes aren't for everyone — the setup process might be confusing if you don't know how to identify bright alignment stars in the sky. And lower-priced Go To models come with smaller-aperture telescopes than similarly priced, entry-level scopes that have no electronics.
Much more information about buying and using telescopes for all interest levels in Sky & Telescope's extensive online list of freely accessible articles.
S&T also maintains a searchable directory of astronomy clubs worldwide.
For skywatching information and astronomy news, visit SkyandTelescope.com or pick up Sky & Telescope, the essential guide to astronomy since 1941, with subscribers in more than 100 nations. Sky & Telescope and SkyandTelescope.com are divisions of F+W, a content and ecommerce company. F+W also publishes SkyWatch (an annual guide to the night sky) as well as books, star atlases, posters, prints, globes, apps, and other fine astronomy products.
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