Telescope, Astronomical

an optical instrument used for observing celestial bodies. With respect to their optical systems, astronomical telescopes are divided into reflector, refractor, and catadioptric telescopes. Astronomical telescopes are employed to carry out visual, photographic, spectrographic, and photoelectric observations and may be equipped with photographic, television, optoelectronic, or other types of radiation detectors.

A visual telescope has an objective and an eyepiece. The magnifying power G of a telescope is defined as F/f, where F is the focal length of the objective and f is the focal length of the eyepiece. The stellar magnitude of the faintest stars visible through a telescope during observations at the zenith is known as the limiting magnitude m of the telescope. The limiting magnitude of a visual telescope can be estimated from Bowen’s formula

m = 3.0 + 2.5 log D + 2.5 log G

where D is the diameter of the objective in mm.

A photographic telescope, such as an astrograph or an astronomical camera, has an objective and a photographic plate in a holder and is not fundamentally different from a large camera. The limiting magnitude of a photographic telescope can be determined from the simplified formula

m₀ = 22^m + 1.25 log t + 2.5 log D

where D is the diameter of the objective in meters and t is the maximum possible exposure time in hours.

The limiting magnitude of an astronomical telescope depends to a large extent on the quality of the optics, the brightness of the sky, the atmospheric transparency, and the steadiness of the atmosphere.

With respect to its general structure, an astronomical telescope consists of a solid or open (framework) tube that is supported by a mounting provided with axes for aiming the telescope at the object of observation and tracking the object (seeTELESCOPE MOUNTING).

Astronomical telescopes can be classified into several types according to the use to which the telescope is put. Astrophysical telescopes, for example, are employed to investigate stars, planets, and nebulas. Artificial earth satellites are observed with satellite-tracking cameras. Other types include solar telescopes, astrometric telescopes, meteor cameras (seeMETEOR PATROL), and special telescopes for the observation of comets.