All astronomical telescopes, large or small, share a common goal — to brighten and magnify your views of celestial bodies. Refractors, reflectors, and compound (catadioptric) 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 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. 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.

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° field of view — 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.

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 magazine. 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. (Find an astronomy club or star party using our Resources section.) 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 at least $200. 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.