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Instrument composed of one or more lenses or mirrors to gather and focus electromagnetic radiation. They increase the observed angular size of objects, as well as their apparent brightness. Used mainly in astronomy since one was first turned on the sky by Galileo Galilei, an Italian scientist.

Those telescopes in which the primary light-gathering surface is a lens are called refractive telescopes, those in which it is a mirror are reflective telescopes. The sensitivity and angular resolution of a telescope is determined to a great extent by the area of this light-gathering surface, termed it's "aperture". Because of the difficulty involved in manufacturing and manipulating large-aperture lenses, nearly all large research-grade astronomical telescopes are reflective telescopes.

The current generation of telescopes being constructed have a primary mirror of between 6 and 8 meters of diameter (for ground based telescopes).

Refractive telescopes are similar in basic design and function to microscopes, and have with them a shared history.

The word telescope alone usually refers to optical/infrared/ultraviolet light telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation For example, there are radio telescopes, which are essentially highly focused radio antennas, x-ray telescopes, gamma-ray telescopes, etc.