Choosing and Using Your First Telescope

ADMIT IT: You want a telescope! You’ve already put in your time as a novice skywatcher — you can recognize a few constellations, have binoculars, know to be leery of cheap telescopes sold in department stores, and maybe you’ve even attended a local star party or two. In short, you’ve laid the foundations for a working knowledge of the sky, and now you’re ready to take the plunge. Scanning the ads in Night Sky and elsewhere makes your heart go pitter-patter. You are going to be a telescope owner!

But there’s such a wide variety of equipment out there today, it’s hard to know what to buy. Should you get a reflector, a refractor, or a hybrid that has both a lens and mirrors? Are you content to push your scope around the sky, or do you want one with a motor drive? How big a telescope should you buy? Is photography in your future? Are computerized telescopes for you?

While it’s tempting to whip out your credit card and call one of the dealers listed in Night Sky, you should spend a few minutes with this article to help you narrow your choices. You’ll find that buying a telescope isn’t much different from buying a car, a TV, or a camera. There are probably many models that would make you happy. The trick is to define your needs and concentrate on a small subset of candidates.

What Exactly Is a Telescope, Anyway?

A telescope’s job is to gather light from distant objects and magnify the view. While different telescope designs concentrate that light in different ways, the goal remains consistent. Light is collected by a primary mirror or lens and is focused to an image, which you view through a magnifying eyepiece.

The primary function of a telescope is to gather light. Most beginners mistakenly place too much emphasis on a telescope’s magnification abilities. Think of it this way: if you’re looking for something small in a dark room, what you need most is light, not a magnifying glass. All other characteristics of the telescope (resolving power, maximum useful magnification, and so on) are directly related to the scope’s aperture. So, all other things being equal, you should buy a scope with the largest main mirror or lens that’s practical for you. For many beginners this means a telescope with an aperture of 3 to 6 inches, but this is not a hard and fast rule.

Nevertheless, beware of going too large. Why? Even though big scopes can gather a lot of light, they can be a hassle to drag outside — and you may lose your motivation to take it out night after night.

But Magnification Is Important, Right?

Yes. Although many of the objects at the top of a beginner’s observing list are quite large (the Andromeda Galaxy is roughly eight times the apparent diameter of the moon), sometimes you’ll want to zoom in "close" to see detail on planets and the moon or to resolve a tightly paired double star.

You can figure out a telescope’s magnifying power easily. First, find the eyepiece’s focal length. It’s shown right on the barrel. Next, find the telescope’s focal length — look for “F” or “F.L.” printed near the front lens, on a specification sticker near the eyepiece, or in the user manual. Both these numbers should be given in millimeters, and magnification (or "power") is just the telescope’s focal length divided by the eyepiece’s. For example, a telescope with a focal length of 1,000 millimeters, coupled with an eyepiece of 25-mm focal length, gives a magnification of 40× (1,000/25). To change the magnification, select an eyepiece marked with a different number: Switch to a 10-mm eyepiece and you’ll crank up the power to 100× (1,000/10).

Resist the temptation to push the power too high in the beginning. Raising the magnification makes objects look larger in the gather light. Most beginners mistakenly place too much emphasis on a telescope’s magnification abilities. Think of it this way: if you’re looking for something small in a dark room, what you need most is light, not a magnifying glass. All other characteristics of the telescope (resolving power, maximum useful magnification, and so on) are directly related to the scope’s aperture. So, all other things being equal, you should buy a scope with the largest main mirror or lens that’s practical for you. For many beginners this means a telescope with an aperture of 3 to 6 inches, but this is not a hard and fast rule. Nevertheless, beware of going too large. Why? Even though big scopes can gather a lot of light, they can be a hassle to drag outside — and you may lose your motivation to take it out night after night. eyepiece, but it also magnifies undesirable things as well, such as shakiness in the telescope’s support and blurring caused by the atmosphere. The trick is to use the proper magnification for a particular object on a particular night. There’s a rule of thumb that says a telescope should be good for a maximum power of about 50× per inch of aperture. As such, a 3-inch telescope shouldn’t be pushed beyond about 150× and an 8-inch scope beyond about 400×. Keep in mind that the maximum useful magnification will vary, depending upon local observing conditions and the quality of the telescope’s optics. But it’s a rare telescope that can give satisfactory views at 675×, no matter what it says on the box!

Do You Need a Computerized Telescope?

Today’s electronic telescopes can do some amazing things. Key in some basic information about the time, date, and your observing location, and at the touch of a button the scope will point itself at any of the thousands of celestial objects stored in its memory. Hit another button and interesting facts about the object that you’re viewing will scroll across a small display built into the scope’s hand control. Most computerized scopes also have a tour mode that suggests interesting objects for you to view, tailored to the night you are observing. Finally, you can even purchase voice-recognition software that allows you to talk to your telescope — and, yes, it talks back!

Some variants of Go To telescopes have computerized electronics but no motors. To find a celestial object with these so-called "Push To" telescopes, you move the tube by hand, aided by information shown on the scope’s electronic display.

If all this sounds fun, convenient, and timesaving, that’s because it is. What’s more, computerized telescopes were once the territory of well-heeled or very ambitious astronomers, but they have now come down in price to the point where almost anybody can afford one. While today’s computerized scopes are certainly smart, you still need some basic knowledge of the night sky to operate them. Even the most sophisticated Go To scopes require you to verify the identity of some bright stars in order to get their internal computers synchronized with the sky.

How Is a Telescope Mounted?

Broadly speaking, telescope mounts fall into two categories: altazimuth (“alt-az”) and equatorial. Altazimuth is a fancy term denoting a simple up-down, left-right type of mount. To keep an object centered in the eyepiece as the sky rotates overhead, the telescope moves on both axes simultaneously (either by motor drives or by you pushing the scope by hand).

One variation of an altazimuth mount that you’ll see often on these pages but rarely on a showroom floor is called a Dobsonian. These mounts, made of simple wooden materials, sit low to the ground and cradle a reflector in a kind of saddle. Don’t worry about the absence of knobs or dials — "Dobs" are economical, very sturdy alternatives to the traditional tripod approach.

Equatorial mounts also have two axes, but they’re designed to have one of them pointed at the North Star. This allows them to track the heavens by turning around just that one axis. Equatorial mounts were common in the days before the advent of microprocessors to control the complex motorized motions needed to have an altazimuth mount track the sky, and they’re still convenient for viewing (especially at high power) when you have to move a scope manually. But for a given size telescope, equatorial mounts are larger, heavier, more expensive, and take more time to set up properly each night than comparable altazimuth mounts.

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© 2005 Reprinted with permission from Sky Publishing Corp.