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The word "telescope" (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types.

An optical telescope is an optical tool that gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied.

Optical telescopes are used for astronomy and in many non-astronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses and spyglasses.

Single-dish Radio telescopes are focusing radio antennae often having a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. As of 2005, the current record array size is many times the width of the Earth, utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and Aperture Masking Interferometry at single telescopes.

X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. They use ring-shaped "glancing" mirrors, made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola. High energy particle telescopes detect a flux of particles, usually originating at an astronomical source.

History
The first telescopes may have been Assyrian crystal lenses[1], but the Visby lenses tentatively suggest that the technology was known to the Arabs and Persians, then to the Vikings, in the 10th century. Leonard Digges is sometimes credited with the invention in England in the 1570s, but usually credit for assembling the first telescope is given to an unknown Dutch spectacle maker in about 1608. Some name that person as Hans Lippershey (c. 1570 – c. 1619), but Jacob Metius and Zacharias Jansen also claimed to have invented a telescope during the same period. Even if Lippershey did not make the first one, he publicized it. Galileo Galilei made his own telescope in 1609, calling it at first a "perspicillum," and then using the terms "telescopium" in Latin and "telescopio" in Italian (from which the English word derives). Galileo is generally credited with being the first to use a telescope for astronomical purposes. Galileo's telescope consisted of a convex object lens and a concave eye lens, which is universally called a Galilean telescope (used as a viewfinder in many simple cameras). Later, Johannes Kepler described the optics of lenses (see his books Astronomiae Pars Optica and Dioptrice), including a new kind of astronomical telescope with two convex lenses (a principle often called the Kepler telescope). Optical interferometer arrays and arrays of radio telescopes were developed much more recently. Telescopes have been around for a while.