Introduction

You may have had a passing interest in astronomy for years. Maybe you would thumb through different astronomy books and magazines at your local library or bookstore, just to keep up with goings-on in the rest of the universe. Whatever the reason, you started to think "gee, maybe I should buy a telescope." You've always wanted one ever since you were a kid, but just never decided that it was the right time . . . until now. Maybe it was a school assignment that your child brought home, or maybe even a story in the news. Whatever piqued your interest, you have decided that the universe is no longer a spectator sport. You want to participate!

But where to begin? Even a quick glance through this issue will show there are so many telescopes available that trying to determine which is best for you would appear to be impossible. Fortunately, that's not so at all. You just need a little help deciding on which telescope is right for you. That's where this article comes in.

Let's begin with the basics. As you may know, there are three broads categories of telescopes based on their optical design the refractor, the reflector, and the catadioptric. The refractors is easily identified by its long, thin tube that holds a large lens in front called the objective lens. Light passes through the objective, which refracts, or concentrates, the light to a focus toward the back end of the tube, where it passes through the eyepiece and out to the observer's eye. Although markedly improved over the past four centuries, this is the type of telescope that Galileo first used in 1609 to discover craters on the Moon, the phases of Venus, and the four major satellites in orbit about Jupiter.

Rather than using a single lens in front as Galileo's telescope did, most modern refractors actually have two lenses up front nested together to create what is called an achromatic objective. These combine to greatly reduce an optical imperfection called chromatic aberration, which causes bright objects to be encircled with fuzzy halos of vivid green, yellow, and purple.

Rather than rely on a lens in front, a reflector gathers and focuses light with a large primary mirror at the bottom of its tube. The primary, ever-so-slightly concave, reflects light back up to the front of the tube, where it draws to a focus. Over the years, several different reflecting telescopes have been devised to eliminate that problem, with the most popular coming from Isaac Newton in 1672. His Newtonian reflector inserts a small, flat mirror into the front of the telescope, diverting the light out through a hole in the side of the tube and into the eyepiece.

Finally, catadioptric telescopes are sometimes called compound telescopes since they combine some of the features of the refractor as well as the reflector. Light first passes through a large, clear lens, called the corrector plate, which tweaks it ever so slightly before reflecting off the primary mirror at the back of the tube. Bouncing off the primary, the light reflects toward the front of the tube, where a secondary mirror awaits to direct the light to the eyepiece.

Catadioptric telescopes can be further divided into two categories that differ by the curves of their corrector plates and mirrors: the Schmidt and the Maksutov. Of these, the most popular is the Schmidt-Cassegrain, sometimes abbreviated SCT. Maksutovs are also becoming very popular.

Regardless of their design, all telescopes share many common functions and terminology. For instance, we always refer to a telescope's size not by the length of its tube, but instead by its aperture, the diameter (usually expressed in inches, centimeters, or millimeters) of the instrument's main optic. The length of a telescope is determined by its focal length, the distance from the objective lens or primary mirror to the focal point, where the light rays come to a focus. As with aperture, focal length is commonly expressed in either inches, centimeters, or millimeters. Telescopes also have focal ratios, which is simply the number you get by dividing the focal length by the aperture. A 4-inch telescope with a focal length of 40 inches has a focal ratio, or f-number, of f/10, while a 6-inch telescope with a focal length of 48 inches has a focal ratio of f/8.

Optical quality should always be a top priority when selecting a telescope. After all, not all telescopes are created equally. Some are outfitted with flawless optics, while others use lenses and mirrors that are barely able to achieve focus. For a mirror or lens to perform properly, its curve(s) must be extremely accurate. Manufacturers often say that their telescope optics are diffraction limited. Diffraction limited means that the optics are so good that performance is limited only by the wave properties of light itself, and not by any flaws in optical accuracy. That is an important statement to look for, but you should also double check their return policy, just in case... All of the telescopes listed here are considered to have quality optics that will produce acceptable images.

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