Power Isn't Everything

It may surprise you, but a telescope's aperture is not what determines its magnification ("power"). When seeing a telescope for the first time, a novice will usually ask, "How much does it magnify?" The answer is, "Any amount you want." Any telescope can provide an almost infinite range of magnifications, depending on the eyepiece you put into its eye end.

But don't get the idea that super-high powers will do you any good. Two main factors limit the power that shows a decent view with a given instrument: aperture (again) and the atmospheric conditions.

Only so much detail exists in the image created by a telescope's main mirror or lens, so you must find the optimum magnification to see this detail — without spreading out the target's precious light too much, making a dim object too dim to see or turning a bright object into just a big blur.

This is why observers generally use low powers for looking at faint things like galaxies and nebulae, and no more than medium-high powers for bright things like the Moon and planets. Just as enlarging a photograph too much will simply show you the grain in the film or the pixels on the chip, so too will excess magnification just make your target blurry.

How much power is too much? There's a simple rule to find the top useful magnification: 50 times your telescope's aperture in inches, or twice its aperture in millimeters. And that's if the scope has perfect optics and the night air happens to be unusually steady.

This means that a high-quality 4-inch (100-mm) scope should not be pushed beyond about 200x. To put this in perspective, even a small instrument that has good optics will show you Saturn's rings or the principal cloud belts on Jupiter, since these can be seen at a magnification of 75x. On the other hand, if you see a small, 60-mm department-store telescope scope labeled as delivering "300 power!!!", you'll know it's advertising hype and you should wisely look elsewhere.

Calculating Magnification

Now you know the maximum practical power for any given instrument. But how do you get it? What do those little numbers on the eyepieces tell you about the magnification they give?

Every scope has a focal length, which is effectively the distance from the primary lens or mirror to the image it forms. (This is not always the same as the length of the tube, since, as we'll see later, some telescopes optically "fold" the light path internally.) Focal length is the large number you'll often see printed or engraved on the front or back of the scope, usually between about 400 and 3,000 millimeters depending on the scope's aperture and type.

Eyepieces have focal lengths too — 25mm or 10mm, for example. Simply divide the focal length of the scope by that of the eyepiece; that's the magnification. For instance a 1,000-mm focal length scope, used with a 25-mm eyepiece, delivers 1,000 / 25 = 40 power (or 40x).

Here's a simple magnification calculator:

Telescope focal length =
Eyepiece focal length =
Magnification =

Why Does the Moon Look Fuzzy?

Even with the best telescope, you'll notice that you can discern finer lunar or planetary detail on some nights than on others. Sometimes the sharpness of the view even changes from one second to the next. At high power, you'll see that planets and stars shimmer and blur on most nights. The fault lies not with the scope but with Earth's turbulent atmosphere, and sometimes with very local conditions such as warm air rising from a nearby asphalt driveway that soaked up solar heat all day. Astronomers refer to turbulent nights as having bad "seeing."

Large apertures allow observers to pick out faint objects and fine detail on the Moon and planets, but regardless of aperture, the better the seeing, the more you can see. Since steady air is so important, large telescopes — even those in the 10-inch-plus category — are often limited to 250x or 300x on all but the very steadiest nights.

Any experienced observer will tell you that with practice, you'll see more detail in an image — not only because your eye gets better trained, but because the longer you look, the better your chance of catching a few moments of unusually steady atmospheric seeing.

Is Bigger Always Better?

So why go for a telescope larger than 10-inch aperture if the sky conditions will limit you? Large apertures are most often chosen by observers who want to gather as much light as possible for viewing dim things: galaxies, nebulae, and star clusters. These so-called "deep-sky" objects are generally viewed at much lower powers than the Moon or planets, so the quality of the atmospheric seeing is less of an issue. Also, larger aperture generally leads to shorter exposure times for those interested in astrophotography, especially when combined with a short focal length.

But even if a large instrument is within your budget, there's the question of portability. A really large amateur scope requires either a permanent observatory so you never have to move it, or willing buddies to help you lift and assemble it for each observing session, then take it down afterward. Clearly, there's a tradeoff between convenience and performance &$151; and everyone will have his or her own definition of what is "portable." It's easy to succumb to "aperture fever," in which you're seized by a compulsion to buy the largest telescope you can. The sad fact is that the leviathan is all too often consigned to the basement or closet, being too heavy and unwieldy for regular use. Remember, the telescope that you use most often is the one that will actually show you the most.

Pay close attention to the weight of the scope you're considering buying, usually listed in the small print. Get a barbell or a log that weighs this much on your bathroom scale. Carry the log around with you. Carry it back and forth from where you'll store the telescope to where you'll use it. Are there stairs along the way? How often will you want to do this at the end of a long day?

Scopes of Every Size and Shape

Having gained an appreciation of a few important optical principles governing a telescope's performance, and the tradeoff between performance and portability, we can now explore the different types of scopes available.

You'll be forgiven for thinking there's an infinite variety from the ads in the astronomical press. Yet for all their varied shapes and sizes, telescopes can be divided into three classes: refractors, reflectors, and catadioptrics.

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