Refractors

A refractor is the stereotype of how a telescope is supposed to look — a long, gleaming tube with a large lens in front and an eyepiece at the back. The front lens (the objective) focuses light to form an image in the back. The eyepiece is a little magnifying glass with which you look at the image.

High-quality refractors are often sought out by lunar and planetary observers who value their crisp, high-contrast images that can take high magnification. In fact, when well made a refractor can provide the finest images attainable with a given aperture.

Another advantage of the refractor is that it's generally more rugged than other types of scopes, because its lenses are less likely to come out of alignment. For this reason refractors are well suited to those who wish to have a "pick up and go" instrument or who have no desire to tinker with the optics.

But these nice features come at a price. A really fine large objective lens is a work of art that requires special glass and individual hand-crafting. For this reason, refractors are the most expensive instruments of any given aperture. Also, in their commonly encountered forms, refractor tube lengths can be unwieldy. A 4-inch refractor can be 4 feet or more long. And since the eyepiece is at the lower end of the tube, a tall tripod is required if you expect to observe objects overhead. Such a tripod has to be very solidly built to prevent wobbles at high powers, so it may be heavy or unwieldy, not to mention expensive. For deep-sky observers a refractor may not have enough light grasp for viewing faint objects, and the fields of view may be narrow. Modern optical design has led to shorter, more manageable refractors, but at a correspondingly higher cost.

It's Done With Mirrors

The second type of telescope, the reflector, uses a mirror to gather and focus light. Its most common form is the Newtonian reflector (invented by Isaac Newton), with a specially curved concave (dish-shaped) primary mirror in the bottom end of the telescope. Near the top a small, diagonal secondary mirror directs the light from the primary to the side of the tube, where it's met by a conveniently placed eyepiece.

If you want the most aperture for your money, the reflector is the scope for you. When well made and maintained, a reflector can provide sharp, contrasty images of all manner of celestial objects at a small fraction of the cost of an equal-aperture refractor.

The tube of a Newtonian is considerably more manageable, too. Its length is rarely more than eight times the diameter of the primary mirror, and frequently less. This means an 8-inch Newtonian can be housed in a tube hardly over 4 feet long, fitting in the back seat of a small car for transportation to dark, rural skies. Combine this with the Newtonian's generally low center of gravity well below the eyepiece, and you end up with an instrument on a compact, stable mounting that presents the eyepiece at a convenient height for just about any sky orientation.

And there's another benefit. A reflector is, by and large, the only type of telescope that shows you a "correct-reading" image rather than a mirror image. This is especially important when you're trying to compare what you see in the eyepiece to what's on a star map.

For the best value of all, much consideration should be given to a particular type of reflector known as the Dobsonian. This is a Newtonian on a very simple, very rugged mount. These extremely popular instruments are available in apertures from 4 inches to more than 30 inches and represent the ultimate in observer convenience for casual viewing.

Like all reflectors (there are other types, but we'll skip them because they're rarely encountered in amateur hands), a Newtonian will require occasional maintenance. Unlike a refractor's solidly mounted lens, a reflector's mirrors can get out of alignment and hence will need periodic collimation (adjustment) to ensure peak performance, particularly if the telescope is moved frequently. This is no big deal once you get the hang of it, and the mirrors of the average Newtonian may not require tweaking for months at a time. But for those not mechanically inclined, having to collimate a reflector even occasionally may be frustrating.

The reflector's open tube means that dust and dirt are more likely to accumulate on the optical surfaces even if you're careful to cover the tube in storage, and this will mean occasional cleaning. Also, the aluminized surfaces of a reflector's mirrors may need to be sent off for recoating every 10 or 20 years — more frequently if you live in a badly air-polluted urban area or by the sea.

The Best of Both Worlds

Then there's the third category of instruments, the catadioptric or compound telescope. These were invented in the 1930s out of a desire to marry the best characteristics of refractors and reflectors: they employ both lenses and mirrors to form an image. The greatest appeal of these instruments is that, in their commonly encountered forms (the Schmidt-Cassegrain and Maksutov-Cassegrain), they are very compact. Their tubes are just two to three times as long as wide, an arrangement allowed by "optical folding" of the light. The smaller tube can use a lighter and thus more manageable mounting. The upshot is that you can obtain a large-aperture, long-focus telescope that's very transportable.

But here too there are caveats. Like the Newtonian, the Schmidt-Cassegrain needs occasional optical collimation that lessens its appeal to those disinclined to tinker. Their fields of view can be rather narrow, too. In terms of cost, aperture for aperture, the catadioptric lies midway between the reflector and the refractor. Like a Newtonian, the popular forms of compound telescopes have a secondary mirror in the light path of the instrument, and this slightly degrades performance for critical lunar and planetary observations. Even so, when well made, a Schmidt-Cassegrain or Maksutov will deliver very fine images of a wide variety of celestial objects.

In common with refractors, the tubes of catadioptrics are sealed so that dirt and dust are largely excluded — a big plus for an instrument that you're going to take out into the country. But if you live in an area where dew occurs (which is almost everywhere), some sort of collar or extension to prevent misting of the exposed corrector plate at the front of the tube is a must.

In practice, many people seeking a highly versatile, very portable (for the aperture) scope that can be used for all sky subjects and for astrophotography will tend to opt for some form of compound instrument. Scopes of this type also tend to be the most highly "technologized," with many options such as computerized pointing and photographic adaptations. In short, they're excellent general-purpose scopes that can use a wide variety of accessories.

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