Types of telescopes

1. Optical These consist of an "objective" (the major component) and an "eyepiece" lens. "Reflectors" have mirrors as objectives, "refractors" have lenses. Newtonian reflectors direct the light out of the side of the telescope; Cassegrain types reflect the image back through a hole in the mirror.
   

   Difficulties with optical telescopes:

   • Refractors exhibit "chromatic aberration." Different colors focus at different places. It is corrected with a double lens.

   • Reflectors, if spherical in shape, don't focus light all (even of a single color) at one point. This difficulty is called ``spherical abberation.'' It is corrected by shaping the surface as a paraboloid.

   • Paraboloidal reflectors have a small field of sharp focus. This is enlarged in the "Schmidt" telescope, which uses a spherical mirror and a refracting plate, together, to give a large field of view without spherical aberration.
2. Radio telescope These are almost all of the reflector type. They are typically much larger than optical telescopes because
   • signals are weaker, and
   • long wavelength requires large aperture for good resolution ("resolution" is discussed below).
3. Radar telescopes These differ from optical and radio in that they send radiation as well as receive it. They are "active", rather than "passive." All other types of telescopes are restricted to merely observing what naturally comes from the object under study. Radar astronomy is only usable within the solar system. We don't learn about the stars from radar.