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An instrument for the collection, detection and analysis of radio waves from any cosmic source. All such telescopes consist of a radio antenna feeding an amplifier and a detector. The large range of frequencies covered by radio astronomy means that radio telescopes vary greatly because different techniques are used for different parts of the spectrum. A fundamental problem in radio astronomy is obtaining adequate angular resolution. A telescope with a diameter of 100 wavelengths has a resolving power of only 1°. To reach a resolution of half an arc second, comparable to that of a good optical telescope, a diameter of 50,000 wavelengths constructed to an accuracy of a tenth of a wavelength is required. At a wavelength of 21 centimetres, the diameter of dish needed is 100 kilometres! Single steerable dishes are used mainly for studies of interstellar matter, through the twenty-one centimetre line, and variable sources, such as pulsars. Fully steerable dishes are limited to apertures of about 100 metres by the weight of the structure. The higher angular resolution needed to map structure in objects such as radio galaxies and quasars is obtained by linking arrays or networks of telescopes to form a radio interferometer. See also: aperture synthesis, radio astronomy, very-long-baseline interferometry. |