Even in these enlightened times, you can still find ads proclaiming telescopes that can magnify celestial objects "a million times" or some such ridiculous value. Swell. But does that mean it's any good? Nope. In fact, it probably means exactly the opposite.

Every telescope has either a primary lens or mirror that is used for collecting light. This is called the telescope's "objective" -- and the width of that objective's aperture is key. In the world of telescopes, size -- or at least proportion -- matters, because a telescope's light-gathering power is proportional to the objective's surface area, not its diameter.

So an 8-inch (20-centimeter) telescope has four times the light grasp of 4-inch (10-centimeter) telescope -- not two times. Putting it in practical terms, an 8-inch, given a dark, clear sky and good seeing conditions, should be able to detect stars as faint as magnitude 14 or better.

This is more than 380,000 times fainter than the faintest star seen by the naked eye, which is usually considered to be magnitude 6. A 4-inch telescope should be able to detect stars as faint as 12.5; a 3-inch, about 12.2. Even a small telescope allows the eye to see millions of stars that the naked eye cannot.

More light gathering area translates into a more detailed image, that is more information. That concept is referred to as "resolution" or resolving power. All things being equal, an 8-inch telescope should have twice the resolving capability of a 4-inch.

Now, bear with me while we let out a little more air from this term. A telescope's resolution is measured by how well it can separate two distant objects that are very close to each other. It's purely a theoretical value and also depends quite a bit on the quality of the optics, but it gives you a "ballpark" feel for how well your telescope should perform optically.

Keeping with our 8-inch and 4-inch comparison, a 4-inch telescope can theoretically resolve two stars that are 1.2 seconds of arc apart. One second of arc is the diameter of a quarter seen a little over 3 miles (4.8 kilometers) away

An 8-inch telescope can resolve an even closer pair -- 0.6 seconds of arc apart. I say "theoretically" because when you take into account Earth's dense, turbulent atmosphere, true resolution is almost always less than this.

About the best you can hope for under ideal seeing conditions is 1 second of arc, but that's pretty good.

A telescope's focal length is the distance light travels from a telescope's lens or mirror to the point inside the telescope where it is focused.

Focal lengths for commercial telescopes vary from 15.8 inches to 118 inches (400 millimeters to 3000 millimeters). The longer the focal length, the larger the image at the focal point. Think of it like the distance between a slide projector and the screen. Move the screen and slide projector further apart, and the image gets larger and dimmer.

"Focal ratio" is the ratio of the instrument's focal length to its aperture. It's found by dividing focal length by objective diameter. A telescope with a mirror of 8 inches across and a focal length of 48 inches has a focal ratio of f/6. (Notice that you can also find a telescope's focal length by multiplying focal ratio by aperture.)

As implied in our slide-projector analogy above, though a long focal-length telescope produces a large image at focus, it will also be fainter because the long focal path spreads out the light more.

Long focal lengths are considered to be in the f/9 or greater range. A telescope of a given diameter coupled with a short focal length, say a 3.5-inch (8.9-centimeter) f/5.6 (focal length 19.6 inches, or 49.8 centimeters), produces bright images but wide fields.

This is fine for observing large deep-sky objects and star fields, but if you also want to observe planets and double stars, you're going to want a slightly longer focal length.

Although a telescope is designed to gather light, how it accomplishes that task is a key factor in its design. It may deliver light in one of three ways: by bending light through a lens, reflecting it from a mirror or via a combination of both lenses and mirrors.

The lens-type is called a refractor. The mirror type is called a reflector. Telescopes that utilize both mirrors and lenses are called catadioptrics. Whichever type you prefer, they are all simply variations on a theme.

The basic design for refracting telescopes

Although the classic design of the refractor has undergone significant changes since Galileo's time (thank goodness), the principle is still the same.

A main lens composed of two or more different pieces of optically figured glass brings light to a focus at the opposite end of the tube. Refractors have the advantage of rendering sharp high-contrast images, large image scales (due to higher focal ratios) and excellent resolution.

Reflectors come in various designs, but we'll stick with the simplest, which is the Newtonian. Since its invention by Sir Isaac in 1668, the reflector has been very popular with amateur astronomers. It consists of a concave mirror positioned at the bottom of the tube that reflects and focuses starlight to a point just inside the tube's entrance. A flat secondary mirror positioned there redirects the light out the side of the tube and into a lens.

Newtonian reflectors provide accurate color rendition of celestial objects and are less expensive by the inch than refractors. You could purchase an 8-inch reflector for the cost of a modest 4-inch refractor.

The basic design for a Schmidt-Cassegrain telescope

Catadioptric telescopes employ the features of both refractors and reflectors. One of the most popular models today is the Schmidt-Cassegrain telescope, or SCT.

The SCT employs a spherical primary mirror at one end of the tube and a correcting lens at the other. The secondary mirror is mounted directly on to the correcting lens (or plate). This, in turn, redirects the light back down the tube and through a hole in the center of the main mirror, where the eyepiece is placed.

"Folding" the light path allows a manufacturer to produce a telescope with a focal length that is twice the length of the tube. Thus, SCTs are lightweight and portable, and produce excellent images.