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!
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° 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" telescopesfor 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.
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.
Eyepieces. By bringing light to a focus, a telescope forms an image — a little picture floating in the air inside the tube. But you need a way to view the image! That's what eyepieces are for. Think of them as like little magnifying glasses for looking at the image. Changing eyepieces lets you change a telescope's magnifying power (which equals the objective's focal length divided by the eyepiece's focal length). 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 very wide field of view, and they also can compensate somewhat for the aberrations that plague "fast" (low f/ratio) objectives. By contrast, many amateurs find that simpler, older designs like Kellners, Plössls, and Orthoscopics suffice for use on "slow" (high f/ratio) telescopes, such as 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 $40 to $250 on a good eyepiece.
A Barlow lens is also worth considering: it multiplies each eyepiece's power by two or three times, effectively doubling your eyepiece collection.
A tip: avoid buying a telescope that uses eyepieces with barrels that are 0.96 inch (24 mm) wide. This is generally a sign of poor quality. Just about all good eyepieces nowadays are made with 1¼-inch barrels or larger.
Finders. You've got a telescope set up with an eyepiece in place. Now what? Naturally, you'll want to point it to something! Simply sighting alongside the tube may enable you to find the Moon and a few bright stars or planets. . . maybe. But that's all. An astronomical telescope can't be put to good use without a finder of some kind.
The reason is that even with its lowest-power, widest-field eyepiece in place, a telescope shows you such a tiny piece of sky that you can't tell exactly where you're aiming.
A finder solves this problem. Three types, shown here, are commonly available. A few low-power, wide-field scopes come with simple peep sights; no optics involved. The next step up is the so-called "reflex" sight. This projects a glowing red dot or red circle on your naked-eye view of the sky; to set your telescope on a desired star or planet, you put the red marker on it. But you still have to be able to see your target with the naked eye.
Most telescopes are sold with a real finderscope: an actual little telescope that rides piggyback on the main scope. The finderscope's eyepiece has crosshairs that you set on your desired target.
A good finderscope has several advantages. It brightens and magnifies the view, allowing you to find things beyond the naked-eye limit. When properly aligned, a finderscope also allows you to point a telescope more precisely than do peep sights or reflex finders. This is especially important whenever you're aiming at a blank point in the sky where your charts tell you an interesting, faint object ought to be.
On the downside, most finderscopes turn the view upside down, and many entry-level finders cannot be used by eyeglass wearers.
In fact, all too many consumer-grade telescopes come with cheap finderscopes that are so poor they're useless. Beware of any that has a tube hardly thicker than your finger, or that gives a dim, fuzzy view in the daytime. A poor finder is a critical weak point that can kill the usefulness of the entire scope.
Star Charts. Once you warm up a new car and hit the road, you need a map to find your way — especially if you're in brand-new territory that you've never seen before! So it is with a telescope. In fact, even the most expert telescopic travelers use the biggest, best, most detailed sky maps they can get.
You may already own a planisphere, a rotating "star wheel" that helps identify constellations. Certainly you should be adept at using a wide-sky constellation map like this before embarking on telescopic astronomy (our companion article gets you started). However, a planisphere alone will no more get you to the Cat's Eye Nebula, say, than a map of the Earth will get you to the shoe store at the corner of Park and Elm. To mine the heavens' riches, you need a set of more detailed star charts.
Most astronomical atlases display all stars brighter than some specified magnitude, along with an assortment of nebulae, star clusters, and galaxies. An atlas that reaches 6th magnitude (the faintest you can see with the unaided eye under a dark, unpolluted sky) suffices for users of binoculars. But an 8th-magnitude atlas like our famous Sky Atlas 2000.0 (shown here) better serves a telescope user.
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 naked-eye stars. The view in binoculars is very much like the view in a good finderscope.
"Smart" or "Go To" Telescopes
You might think that with computers in everything from dishwashers to cars, someone would be putting computers in telescopes by now. You're 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 (if all goes well) automatically point to thousands of celestial objects.
Until recently such futuristic capabilities would set you back thousands of dollars. But a new generation of battery-powered "smart" or "Go To" scopes has come onto the market at affordable prices. 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 currently up, complete with a brief description of each on a digital readout. These scopes can literally give you a beginner's course in astronomy.
Still, these scopes aren't for everyone. For one thing, the affordable models have much smaller apertures than similarly priced entry-level scopes with no electronics. Second, a computerized scope can require a lot of careful setup that has to be done correctly at the start of each observing session (in the dark!) to make it work right. Third, these telescopes consume lots of electricity; some will exhaust a set of eight batteries in one night's use. Finally, when your "smart scope" fails to show a particular object, you may have trouble figuring out whether your eye or the scope's pointing is at fault — unless you already know the sky and your charts well enough to confirm that the instrument is indeed pointed to exactly the right spot.
Be An Informed Buyer
Now that you're up to speed on some of the most important concepts and terms, take the time to peruse the ads and product reviews in recent issues of Sky & Telescope magazine. Search for additional reviews and opinions on amateur astronomy Web sites and newsgroups. Then go ahead and call or write to makers of instruments you might be interested in. Their brochures and catalogs should tell you much of what you want to know; if not, call and ask.
However, nothing substitutes for firsthand experience. The best way to acquaint yourself with the wide world of telescopes is to ask to join a local astronomy club's nighttime get-togethers — or perhaps attend an even bigger "star party." There, you can try out and ask about a wide variety of telescopes. (Find an astronomy club or star party in our directory.)
You may also be able to buy a used telescope from someone in an astronomy club. Used telescopes carry risks, including undisclosed damage and no warranty coverage, but they can also be spectacular bargains. You can also find used telescopes on the Internet. (Be sure to take reasonable precautions if buying from a private party online.)
Of course, many buyers will want a new instrument. This should be bought from a source specializing in astronomical telescopes. Many camera stores are excellent sources of astronomical products as well.
If you're set on buying new, be prepared to spend at least $200 to $400, and even then you have to be careful to avoid junk. If this is beyond your means, your astronomical aspirations will probably be best served by buying a decent pair of binoculars and a lawn chair. Remember that whatever investment you make should be of good enough quality to serve you well for decades.
Kicking the Tires
You can't test a telescope's optical performance properly in a store, and many of the best telescopes are sold by mail order anyway. So you should ask the 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 refund. (If you buy the telescope mail order, expect to pay for shipping if you return it. In addition, being asked to pay a "restocking fee" is only fair if you want the privilege of being able to test-drive and return a telescope that's not actually broken.)
Once you get the telescope, new or used, scrutinize everything about it in daylight. The mount should be stable enough to remain standing even if someone bumps into it in the dark. Give the scope a gentle tap while viewing some distant target. Does the view jump around only a little, then stabilize? Good! But if it hops around for several seconds or more — or if the view moves so much when you hold the focus knob that you can't focus well — it'll be endlessly frustrating. Finally, you should be able to move the telescope easily and smoothly (whether by pushing the telescope's tube, turning a knob, or switching on an electric motor) without jerkiness or a backlash problem.
Assessing the telescope's optical performance is harder for a newcomer to visual astronomy. But even an inexpensive telescope should pass the following nighttime tests.
Point the scope at a starry region in or near the Milky Way. Use a fairly low-power eyepiece. Stars at the center of the view should focus to points without any flares or colored halos. (Flares or halos may appear at the edge of the field of view, but they shouldn't be prominent until at least halfway out.)
Now get aimed at a fairly bright star and switch to high power. Focus the star, then turn the focus knob just a little one way, then the other way. The out-of-focus images that you see when you turn the knob one way, then the other, should be nearly alike. This is a strict test; rare is the telescope that passes it perfectly. But if the either-side-of-focus images are quite obviously different from each other, the optics are poor and the views will never be as sharp as they should be. (Eyeglass wearers with astigmatism should keep their glasses on for these tests.)
Finally, examine the Moon: it should look crisp, not hazy, and it shouldn't produce distracting ghost images (the result of inadequate coatings somewhere in the optics).
Keep in mind that perfection is expensive, and that a lot can be seen with less-than-perfect equipment. (It's the only kind I've ever owned!) Be patient with your new telescope and with yourself. At the same time, don't be afraid to ask for help! As time goes on, the wonders of the heavens will become familiar friends.