Sky Publishing Corporation


By Joshua Roth
Adapted from SkyWatch

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: a solid, steady, smoothly working mount and high-quality optics. 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 -- of yourself; of practicing amateur astronomers; and, finally, of the people who make and sell telescopes for a living.

A Telescope's Heart

Compound Telescope

Reflector Telescope

Refractor Telescope

(Click on images
for larger views)

All astronomical telescopes, large or small, share a common goal--to magnify and brighten your views of celestial bodies. Refractors, reflectors, and compound telescopes do their jobs in different ways -- 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 50× (50 times magnification) will reveal Jupiter's moons, Saturn's rings, and some degree of 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 be able to use at least 150×. Depending on optical quality and design, you can expect to get anywhere from 20× (mediocre) to 50× (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 4-1/8-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 12-inch (305-mm) Dobsonian, hundreds of galaxies are within reach.

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. One reason: you can stick with longer-focus eyepieces, which are easier to use, especially for eyeglass wearers. Another reason: "fast" objectives (those with a small f/ratio) 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 stalks), a focal length of 20 inches (508 mm) can provide a 3° field of view -- enough to take in all of Orion's Sword. A focal length of 80 inches (2,032 mm), by contrast, barely lets you encircle M42, the famous nebula in the Sword's center.

Left to right: 1. 3½" Maksutov-Cassegrain on tabletop fork mount; 10 pounds and carry-on luggage. 2. 6" reflector on Dobsonian mount; 35 pounds. Fits in back seat or car trunk. 3. 3" refractor on photo/video tripod. 10 pounds. Transports as carryo-on luggage. 4. You, your lifestyle, your home, and your budget. 5. 5" Schmidt-Cassegrain on equatorial mount and field tripod; 30 pounds; Back seat or car trunk.

"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.

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 only magnify things 7 to 10 times. By contrast, even the smallest telescopes typically work at 30× and up. Finding objects without a mount is next to impossible at such magnifications, as are steady views.

An equatorial mount (right) allows motion in two directions: north-south and east-west. (Click on image for larger view.) 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 (left, click on image for larger view), 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 (facing page, bottom). 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.) Dobsonian mounts, in particular, can be extremely stable and economical as well.

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 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.

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 you can't drive a new car off the lot without the keys! Likewise, there are a couple of 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. So 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 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."

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 $100 on a good beginner's 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 that are 0.96 inch (24 mm) wide. The better designs are generally not available in this size.

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.

Peep sight

Reflex sight


Three types 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 very 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.

A finderscope has several advantages. Because its objective is larger than your eye's pupil, it brightens stars (and the larger models can actually show you some star clusters and nebulae directly). And, when properly aligned, a finderscope allows you to point a telescope more precisely than do peep sights or reflex finders. This is especially important for long-focus telescopes that have narrow fields of view.

On the downside, most finderscopes turn the stars upside down, and many entry-level finders cannot be used by eyeglass wearers. In any case, you'll want to avoid (or replace) any right-angle finder, or one that doesn't give sharp images.

"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. One system even responds to voice commands!

This "smart" 3½ inch Maksutov-Cassegrain telescope includes a computer (in the hand controller) that automatically points it at thousands of objects. Click on image for larger view.

Until recently such futuristic capabilities would set you back many thousands of dollars. But a new generation of battery-powered smart scopes is now coming onto the market at 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.

Too many new owners of telescopes -- even with finders -- can't locate anything but the Moon and a few bright stars or planets with them. Eventually they throw their hands up in frustration and their telescopes into a closet. This problem could become a thing of the past in an era when a budding astronomer can point his or her telescope at virtually any object with the push of a button.

Still, these new scopes aren't for everyone. For one thing, the affordable models have small apertures. Second, in an altazimuth telescope the image in the eyepiece rotates as the instrument tracks objects rising and setting. If you want to pursue deep-sky (long-exposure) photography, you need a sturdy and precisely aligned equatorial mount instead. Finally, the loud whir of electric motors can spoil the mood of an otherwise calm and quiet night under the starry canopy.

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 or SkyWatch. Then go ahead and call or write to anyone who manufactures instruments you might be interested in. Their brochures and catalogs should tell you much of what you want to know; if not, call the manufacturers or their dealers and ask away.

However, nothing substitutes for firsthand experience. By far the best way to acquaint yourself with the wide world of telescopes is to participate in an astronomy club's nighttime observing session, or "star party." There, you can try out and ask about a wide variety of telescopes. (An exhaustive list of astronomy clubs can be found in our astro directory pages.)

You may also be able to buy a used telescope from someone in an astronomy club. Used telescopes carry some risks, including undisclosed damage by the previous owner and a lack of warranty coverage. However, 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 find that a new instrument best suits them. 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 a new instrument, be prepared to spend several hundred dollars. If this is beyond your means, your astronomical aspirations will probably be best served by buying a decent pair of binoculars and a sturdy lawn chair. At the same time, realize that many excellent beginners' telescopes are available for well under $1,000. Remember that whatever investment you make may serve you well for several decades.

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, 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.

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 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 useless. Finally, you should be able to move your telescope easily and smoothly, whether by pushing the telescope's tube, by turning a knob, or by 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. Point the telescope at a starry region in or near the Milky Way. Use your telescope's lowest power. Stars at the center of your telescope's field of view should focus to points of light without any distracting 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 set your sights on a fairly bright star and switch to high power. Focus the star, then turn the knobs one way, then the other. The two out-of-focus images should resemble one another and be more or less circular. Examine the Moon: it should look crisp, not hazy, and it shouldn't produce numerous ghost images (the consequences 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.

Joshua Roth is technical editor of Sky & Telescope magazine and an avid backyard astronomer.

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