TYPES OF TELESCOPES

Refracting Telescopes

Light enters through a large convex (outward curving) glass lens called the objective. This lens refracts, or bends, the light toward the eyepiece, where it produces an image. One of the main problems with refracting telescopes is a phenomenon referred to as chromatic aberration, which causes objects to appear to be surrounded by rainbow-colored halos. To compensate for this problem, a special second lens is mounted behind the main objective to focus more of the colors of light at a single point. Because they can produce "true" images (rather than the inverted images seen with reflectors), refracting telescopes are useful both for stargazing and land observation, such as bird-watching.

Reflecting Telescopes

Reflecting telescopes use a series of mirrors to gather and focus light. In a Newtonian reflector, light passes through the open tube of the telescope until it reaches the primary parabolic mirror. This curved mirror reflects the light rays to a secondary flat mirror, which reflects the light to the eyepiece of the telescope. There the image (inverted) is brought into focus. Because it is technically simpler to build and support large, high-quality mirrors rather than lenses, most large optical telescopes are of the reflecting type. Reflectors in general yield the largest aperture per dollar and account for about 90 percent of telescopes used by amateurs. Advanced amateurs commonly use an aperture of at least 152 millimeters (6 inches).

Catadiatropic Telescope

Invented by Bernhard Schmidt in 1930, the catadiatropic telescope — the name means "reflecting and refracting" — employs a combination of lenses and mirrors. Light passes through a thin lens on its way to a spherical primary mirror. The reflected light then hits a convex secondary mirror and is transmitted to the focal point through a small opening in the primary mirror. This configuration allows the optics to be housed in a short tube. Compact and portable, catadiatropic telescopes are popular among experienced amateur astronomers.