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Telescope, instrument used to detect or examine distant objects. It consists of a series of lenses and mirrors capable of producing a magnified image and of collecting more light than the unaided eye.

The refracting telescope (refractor) essentially consists of a tube with a lens system at each end. Light from a distant object first strikes the objective lens, which produces an inverted image at its focal point. In the terrestrial telescope the second lens system, the eyepiece, produces a magnified, erect image of the focal image, but in instruments for astronomical use, where the image is usually recorded photographically, the image is not reinverted, thus reducing light losses.

The reflecting telescope (reflector) uses a concave mirror to gather and focus the incoming light. The various types of instruments use different combinations of mirrors and lenses to view the focal image with fewer optical aberrations.

The size of a telescope is measured in terms of the diameter of its objective. Up to about 12 in (30 cm) diameter, the resolving power (the ability to distinguish finely separated points) increases with size, but for larger objectives the only gain is in light gathering. A 200-in (508-cm) telescope can thus detect much fainter sources but resolve no better than a 12-in (30-cm) instrument. Because mirrors can be supported more easily than large lenses, the largest astronomical telescopes are all reflectors.

See also: Galileo Galilei.

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