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.

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.