From the Greek word teleskopos, meaning far-seeing, telescopes gather, magnify, and focus light, making it possible to view objects too far away to be seen with the unaided eye. Optical telescopes are identified by which system they use to magnify light - refraction, reflection, or catadioptric. All three types have two common principles and share two basic elements - light gathering and focusing component (aperture), and light magnification element (eyepiece lens).
These were the first telescopes to be developed - they originally used a combination of convex and concave glass lenses as aperture and eyepiece. Holland's Hans Lippershey invented the refracting telescope in 1608, and Italian astronomer Galileo was the first to employ it to observe planets and stars. In 1611, Johannes Kepler improved the design by using two convex lenses instead of the concave-convex arrangement. His improved version is still widely used today. Because the refraction lenses needed for large-diameter apertures are both difficult and expensive to make, refractors are impractical for observing the faint light from deep-sky galaxies and nebulae.
Mirrors rather than lenses are used to catch and focus light inside reflection telescopes. First developed by Sir Isaac Newton in about 1680, the Newtonian telescope uses a concave primary mirror in the back of the scope to collect light and reflect it to a focal point. A secondary mirror then sends the focused beam into the eyepiece lens for magnification and viewing. Because of their large apertures, reflectors are well suited for deep-sky observation.
Also called compound telescopes, these use a combination of both lenses and mirrors. Although there are several catadioptric designs, they are all variations of German astronomer Bernhard Schmidt's original 1930 model. The Schmidt Compound used a corrector lens to modify light entering the front of the telescope and refract it onto the primary mirror in the back. Due to the fact that the Schmidt model had no secondary mirror or eyepiece, light from the primary mirror could be focused directly onto photographic rolls or plates. The Schmidt-Cassegrain telescope invented in 1960 remains the most popular catadioptric style.
These are simple, easy-to-build Newtonian reflectors made from plastic, fiberglass, or plywood. Because large-aperture primary mirrors can now be easily made, Dobsonian telescopes are inexpensive and very popular among amateur astronomers for both deep-sky and planetary observation.