Choosing Your First Telescope

So you've decided to take the plunge and get a telescope. Congratulations! That alone is a big step. But what comes next? Not an impulsive shopping trip to the nearest mall! Buying a telescope is very different than buying a television, and department-store salespeople rarely understand what amateur astronomers need.

Rule Number One: shun the flimsy, semi-toy, "500 power!" department-store scopes that may have caught your eye. The telescope you want has two essentials: high-quality optics and a steady, smoothly working mount. You may also want the telescope to be nice and large, but don't forget portability and convenience. Your first telescope shouldn't be so awkward heavy that you can't tote it outdoors, set it up, and take it down reasonably easily.

Those are the basics. But to choose a telescope that meets your needs, you need to ask some questions—of yourself, of other amateur astronomers, and finally, of the people who make and sell telescopes for a living.

Aperture: A Telescope's Heart

All astronomical telescopes, large or small, are designed to do two things: to brighten and magnify your views of celestial objects. Refractors, reflectors, and compound (catadioptric) telescopes do this in different ways, each with its benefits and drawbacks.

Whatever the telescope, its most important spec is its aperture: the diameter of its main, light-gathering lens or mirror. (This lens or mirror is called the telescope's objective.) The bigger the aperture, the sharper and brighter the view will be.

Therefore, a bigger aperture allows you to use more magnification. You can actually make any telescope provide any magnification at all (just by changing eyepieces), but without large aperture, high magnification is worthless—it just shows a blurry, dim mess. A telescope that can only be pushed to 50x (50 times magnification) before the view goes blurry will reveal Jupiter's moons, Saturn's rings, and some detail in the brightest star clusters, nebulae, and galaxies. But to discern Martian surface features or to see both members of a tight double star, you really would like to have sharp views at 150x or more.

Depending on optical quality and observing conditions, you can expect to get anywhere from 20x to 50x of useful magnification per inch of aperture. In other words, a 4-inch scope tops out at 200x under ideal conditions, but a 6-inch scope can work well as high as 300x under ideal conditions.

But that's the maximum; most of the time, you'll find that the best views are actually had at the telescope's lowest power. If the advertising on the box hypes super-high power, the manufacturer is trying to fool you.

Perhaps more importantly, big aperture also lets you see fainter objects. This is different from providing magnification. In fact, the problem with most astronomical objects is not that they're too small and need more magnifying, it's that that they're too faint and need more light—in other words, more aperture.

For example, several dozen galaxies beyond our Milky Way can be discerned through my 4½-inch (105-mm) reflector. Some are more than 50 million light-years away. Not bad for a telescope I can tuck under my arm and carry on a plane! But with my 12½-inch Dobsonian, hundreds of galaxies are within reach. Even if I use the same magnification on both scopes!

Focal Length

If a telescope's aperture is its most important spec, its focal length comes next. Say you have two telescopes with the same aperture but different focal lengths. The one with the longer focus (that is, a higher-numbered f/ratio) will generally lend itself better to high-magnification viewing. (The f/ratio is just the focal length divided by the aperture.) One reason: you can stick with longer-focus eyepieces, which are easier to use, especially for eyeglass wearers. Another reason: "fast" objectives, those with low f/ratios, are harder to manufacture well, and thus they tend to make fuzzier images unless you've paid a premium for top-quality optics.

"So it's simple: I should go for the largest, longest telescope I can afford." Maybe; maybe not! A long focal length is preferable if your primary targets are high-power objects like the Moon, planets, or double stars. And a large objective is a necessity if you dream of viewing numerous galaxies. But if you want to take in large swaths of the Milky Way or sparkling showpieces like the Pleiades in a wide view, then a short, small, scope is called for—one that works nicely at low power.

"Why's that?" Because high power only let you see a small patch of sky at once. With standard eyepieces (those with 1¼-inch-wide barrels), a focal length of 20 inches (500 mm) can provide a 3-degree field of view—enough to take in all of Orion's Sword. A scope with a focal length of 80 inches (2000 mm), by contrast, barely lets you encompass M42, the Orion Nebula in the Sword's center.

"What if I want to do a bit of everything?" Don't worry, there are plenty of midway compromises. Many astronomers think of the 6-inch reflector as an ideal "do-it-all" instrument. But even with that aperture, you still face a tradeoff between a wide-field performance (f/5 or thereabouts) and high-power performance (optimal at f/8 and up). And remember that the long-focus unit will be bigger and heavier and so will require a beefier mount—making it harder to carry, set up, and store. Everything's a tradeoff.

A Telescope's Other Half

Just as a car's engine is useless without a chassis and wheels, the optical tube assembly is only half a telescope. The other half is the mount. It is just as important as the optics if not more so. It has to be steady, sturdy, and smoothly working.

Telescope mounts come in two basic kinds. An equatorial mount allows the telescope to move in the directions of celestial north-south and east-west. This can be a big help. If you align one axis of an equatorial mount on Polaris, you can track celestial targets as the Earth turns by moving the telescope around just this one axis. Many equatorial mounts come with an electric motor to do this for you. Motor tracking is especially useful for high-magnification viewing and for showing celestial objects to groups of people. It's also a prerequisite for most through-the-telescope photography.

An altazimuth (altitude-azimuth) mount, by contrast, moves up-down (in altitude) and right-left (azimuth). A photo tripod is an example of an altazimuth mount. Another is the popular Dobsonian mount, shown below.

Altazimuth mounts are generally lighter than equatorials, in part because they don't require counterweights to balance the telescope. (I hasten to note, however, that the equatorial "fork" mounts sold with many compound telescopes are relatively lightweight, too; the photo above shows one example.) Dobsonian mounts, in particular, can be very stable and low-cost.

But altazimuths do not readily lend themselves to motorized operation, and you have to move the telescope in two directions simultaneously to track celestial objects as the Earth turns. While this becomes second nature to many observers, others find it maddening. (See the section below on "smart" telescopes for a high-tech way around this problem.)

Your own personality should play a part in choosing a mount. Are you comfortable with instruments that require tools and a head for numbers to set up and use? Or are you looking for the astronomical equivalent of a point-and-shoot camera? A Dobsonian can be set up in the time it took to read this paragraph. An equatorial mount can take a bit longer if you want to get the most out of its features. Computerized "smart scopes," which promise easy object-finding, are actually the most complicated to deploy.

Other Essentials

We've already covered a lot of ground, and hopefully the tech talk you may get from a salesperson or stargazer will now make more sense. But a few topics remain before we can set you loose on your hunt. Most of us picture the big things when we think of a telescope, and those stand out in catalogs and ads. But just as you can't drive a car off the lot without the keys, there are little essentials you'll need to use a telescope to journey among the stars.

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© 2005 Reprinted with permission from Sky Publishing Corp.