Choosing Your First Telescope  By Joshua Roth (Courtesy Sky & telescope)    

So you've decided to take the plunge and get yourself a telescope. Congratulations! That alone is a big step. But what comes next? Not an impulsive shopping spree at the nearest mall! Buying a telescope is very different than buying a television, and department-store salespeople are rarely familiar with the needs of amateur astronomers. For starters, shun the flimsy, semi-toy, "600 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 think you need a large instrument, but don't forget portability and convenience. Your first telescope shouldn't be so heavy that you can't tote it outdoors, set it up, and take it down reasonably easily. Those are the basics. But to actually choose a telescope that meets your needs, you need to ask some questions ¡X of yourself; of practicing amateur astronomers; and, finally, of the people who make and sell telescopes for a living.

Reflector telescope All astronomical telescopes, large or small, share a common goal ¡X to brighten and magnify your views of celestial bodies. Refractors, reflectors, and compound (catadioptric) telescopes do their jobs in different ways ¡X each with its own benefits and drawbacks.  Yet many fundamentals apply to any telescope. Of primary importance is a telescope's aperture: the diameter of its light-gathering lens or mirror. (That lens or mirror is often referred to as a telescope's objective.)Aperture makes a big difference in the level of detail you can see. A telescope that can only be pushed as high as 50x (50 times magnification) will reveal Jupiter's moons, Saturn's rings, and some degree of detail in the brightest star clusters, nebulae, and galaxies.

Refractor telescope  But to discern Martian surface features or to see both members of a tight double star, you really would like to be able to use at least 150x. Depending on optical quality and observing conditions, you can expect to get anywhere from 20x (mediocre) to 50x (excellent) per inch of telescope aperture.  Aperture also enables you to see fainter objects. For example, several dozen galaxies beyond our own 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.

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

"So it seems clear: I should go after the largest, longest telescope I can afford." Maybe; maybe not! A long focal length is preferable if your primary targets are the Moon, the planets, or double stars. And a large objective is a necessity if you dream of viewing numerous distant galaxies. But if you want to take in large swaths of the Milky Way or sparkling showpieces like the Pleiades, a short, small scope is called for. "Why's that?" Because a long focal length only lets you see a small patch of sky at one time. With standard eyepieces (those that have 1¼-inch-wide barrels), a focal length of 20 inches (500 mm) can provide a 3¢X field of view ¡X enough to take in all of Orion's Sword. A focal length of 80 inches (2000 mm), by contrast, barely lets you encircle M42, the famous Orion Nebula in the Sword's center. "What if I want to do a bit of everything?" Don't worry. There are plenty of acceptable compromises. Many astronomers take the 6-inch (152-mm) reflector to be an ideal "do-it-all" instrument. But keep in mind that even with that aperture, you still face a tradeoff between a wide field of view (f/5 or thereabouts) and high-power performance (optimal at f/8 and up). The long-focus unit will also be heavier and require a beefier mount.

photo courtesy Akira Fujii.

	A Telescope's Other Half Just as a car's engine is useless without a chassis, an optical-tube assembly is only half a telescope. Even stargazing with binoculars only really works well if you're leaning on a windowsill or reclining in a lawn chair with arm supports. And binoculars typically magnify things only 7 to 10 times. By contrast, even the smallest telescopes typically work at 30x and up. Finding objects without a mount is next to impossible at such magnifications, as are steady views.  An equatorial mount allows motion in two directions: north-south and east-west. Because one axis is aligned with the Earth's, an equatorial mount allows you to track celestial targets with a single push or knob. Furthermore, many equatorial mounts have electric motors for hands-off tracking. This is especially useful for high-magnification viewing and for showing celestial objects to groups of people. It's also a prerequisite for long-exposure photography. An altazimuth (altitude-azimuth) mount, by contrast, permits up-down (altitude) and right-left (azimuth) motions. A video tripod is a familiar example of an altazimuth mount (and indeed, a sturdy one suffices for a lightweight, low-power scope). Another now-universal variation is the Dobsonian mount, shown below.  Altazimuth mounts are generally lighter than their equatorial counterparts, in part because they don't require counterweights to balance the telescope.		(I hasten to note, however, that the equatorial "fork" mounts supplied with many compound telescopes are relatively lightweight, too; the photo above shows one example.) Dobsonian mounts, in particular, can be extremely stable and economical. But altazimuths do not readily lend themselves to motorized operation, and you have to move the telescope in two directions simultaneously to track celestial targets. While this becomes second nature to many observers, others find it maddening. (See the section below on "smart" telescopes for a novel, 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? Equatorial mounts generally require several minutes of assembly, careful balancing, and polar alignment, while some Dobsonians can be set up in the time it took to read this paragraph, and some "smart scopes" on "alt-az" mounts can be deployed nearly as rapidly.

Other Essentials We've already covered a lot of ground, and hopefully the tech talk you may get from a salesperson or stargazer will make more sense now. 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 new car off the lot without the keys, there are a few little things you'll need to use a telescope to journey among the stars. Eyepieces. By bringing light from distant objects to a focus, a telescope forms an image. Now you need a way to view that image. That's what eyepieces are for. Swapping eyepieces lets you change a telescope's magnifying power, which simply equals the objective's focal length divided by that of the eyepiece. Every telescope owner should have several. Eyepieces come in a bewildering variety of designs with exotic names. Generally speaking, the more expensive an eyepiece, the more lens elements it has. Complex multi-element designs can give a wider field of view, and they also can compensate to a degree for the aberrations that plague "fast" (low f/ratio) objectives. By contrast, many amateurs find that simpler designs like Kellners, Plössls, and Orthoscopics suffice for use on "slow" (high f/ratio) telescopes like the once-universal 6-inch f/8 Newtonian reflector. Most telescopes come supplied with one or two eyepieces. Ideally, you'd like to have a set that spans a range of magnifications. You can expect to spend anywhere from $25 to $250 on a good eyepiece. A Barlow lens is worth considering, too: it will double or triple each eyepiece's power, effectively doubling the size of your eyepiece collection. One useful hint: try to avoid buying a telescope that uses eyepieces with stalks or barrels that are 0.96 inch (24 mm) wide. The better designs are generally not available in this size.

Three ways to take aim at the sky. Left: Lensless peep sights suffice for small telescopes with wide fields of view. Center: Reflex sights project a dim red dot or circle on the sky, improving precision. Right: Finderscopes make more targets visible and enable the most precise pointing. But many models turn the sky upside down or are hard to use with eyeglasses on.

Finders. You've got a telescope mounted with an eyepiece in place. Now what? Naturally, you'll want to point it toward celestial targets! Sighting alongside the tube may enable you to find the Moon and a few bright stars or planets with a small, wide-field scope. But, just as a hunter won't get far without a gun sight, a telescope can't be put to good use without a finder of some kind. Three types, shown here, are commonly available. A few wide-field scopes come with lensless peep sights that encircle a patch of sky without magnifying or brightening it. The next step up is the so-called "reflex" sight. This device projects a red dot or circle on your naked-eye view of the sky; to set your telescope on a desired star or planet, you simply line that object up with the red dot or circle. Note that few telescopes are supplied with a reflex sight, you generally have to buy one separately. Most commercially available telescopes are sold with a real finderscope, a small refractor that rides piggyback upon the main telescope. The finderscope's eyepiece has cross hairs that you set on your desired target as you look through the device.

Star Charts. Once you warm up a new car and hit the highway, you need a map to find your way. So it is with a new telescope as well. You may already own a planisphere, the rotating "star wheel" that helps you identify constellations. Certainly you should be adept at using one before embarking on telescopic astronomy (our companion article gets you started). However, a planisphere alone will no more help you find the Cat's Eye Nebula, say, than a freeway map will help you find the shoe store at the corner of Park Avenue and Elm Street. To mine the heavens' riches, you need a set of astronomical star charts.  Most astronomical atlases display all the stars above some specified brightness level, along with an assortment of nebulae, star clusters, and galaxies. An atlas that reaches 6th magnitude (roughly the brightness limit of the unaided eye) suffices for users of binoculars or small telescopes, while an 8th-magnitude atlas like our famous Sky Atlas 2000.0 (shown here) will better serve the user of a large-aperture, narrow-field telescope. If you haven't used star charts before, there's no better way to get started than with binoculars (see our primer on binocular astronomy). Stargazing with binoculars offers two bonuses: views are right-side-up, and the field of view is wide enough to take in recognizable formations of stars.

"Smart" Telescopes You might think that with computers in everything from appliances to automobiles, someone would have put a computer in a telescope by now. And you'd be right! Actually the computer doesn't go in the scope itself, but in the mount, along with electric motors on both axes. A motorized telescope on a "smart" altazimuth mount can track celestial objects as accurately as one on a more bulky and complicated equatorial mount. Even better, once you set up the scope and initialize the computer with the current date, time, and location, it can automatically point to thousands of celestial objects. Until recently such futuristic capabilities would set you back many thousands of dollars. But a new generation of battery-powered "smart scopes" has come onto the market at very affordable prices. These instruments can do things no commercial telescope has ever done before. A keypress or two gives the times of sunrise and sunset, moonrise and moonset, and the dates of meteor showers, solstices, equinoxes, and eclipses. Or choose a guided tour of the best celestial showpieces, complete with a digital readout describing what we know about each object. These scopes literally give you a beginner's course in astronomy.

Kicking the Tires You generally can't test a telescope's optical performance in a store, and many telescopes are sold by mail order. As a result, you should ask any vendor to spell out return policies (preferably in writing). Make sure you'll be given enough time to try a scope out under the stars, and the opportunity to return it for a full refund if it doesn't meet your needs. (One more caveat: if you bought the telescope by mail order, be sure to determine whether you or the vendor will pay for shipping in the event you wish to return the telescope for a refund. You should be prepared to pick up at least part of the shipping tab in exchange for the privilege of being able to "test-drive" the telescope.) Once you obtain a telescope, new or used, you can immediately scrutinize its mechanical features and its mount, even in daylight. Any telescope mount, be it a camera/video tripod or a computer-controlled equatorial, should be stable enough to remain standing even if someone bumps into it in the dark. Give the mounted scope a gentle tap while viewing some distant target. Does the view jump around briefly, then stabilize? Good! But if it hops around for several seconds or more, it'll be endlessly frustrating. Finally, you should be able to move your telescope easily and smoothly, whether by pushing the telescope's tube, turning a knob, or switching on an electric motor. It's harder for a newcomer to visual astronomy to critically assess a telescope's optical performance. But even an inexpensive telescope should pass the following nighttime tests.