astronomical instrument consisting of a radio receiver and an antenna system that is used to detect radio-frequency radiation emitted by extraterrestrial sources. Because radio wavelengths are much longer than those of visible light, radio telescopes must be very large in order to attain the resolution of optical telescopes.

The first radio telescope, built in 1937 by Grote Reber (Reber, Grote) of Wheaton, Ill., U.S., was a steerable paraboloid—i.e., a device with a parabolically shaped reflector, dubbed the “dish,” that focuses the incoming radio waves onto a small pickup antenna, or “feed.” The radio telescope at Jodrell Bank (Jodrell Bank Observatory), Cheshire, Eng., has a steerable paraboloid antenna 76 m (250 feet) in diameter. The reflecting surface of the telescope at Arecibo (Arecibo Observatory), P.R., fills a naturally occurring bowl-shaped depression 305 m (1,000 feet) in diameter. The Arecibo installation is equipped with a radar transmitter for the study of radar signals reflected from such celestial objects as planets and their satellites.

Some important radio telescopesRadio interferometers (radio interferometer) consist of two or more widely separated antennas connected by transmission lines. With their greatly increased resolving power, they can be used to determine the position or diameter of a radio source or to separate two closely spaced sources. Phase-array telescopes consist of large numbers of relatively small antenna elements arranged in any of various configurations over a relatively large area, yielding the effective sensitivity and resolution of an antenna much larger than could practicably be built. An example of such a system is the 27-antenna Very Large Array near Socorro, N.M., which is one of the world's largest and most sensitive radio telescopes. See also telescope. Some important radio telescopes