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dishA type of antenna used in radio telescopes and consisting of a large spherical or parabolic metal reflector, usually circular in outline, by means of which radio waves are brought to a focus above the dish center. The waves are collected at the focus by a secondary antenna, called a feed, or are reflected from a curved secondary to be brought to a focus elsewhere (see Cassegrain configuration). At low radio frequencies the feed may be a dipole mounted in front of a small reflector; at high frequencies it is usually a horn antenna (see waveguide). The signals picked up by the feed are transferred by means of the feeder to the receiver for amplification and analysis. Both the angular resolution and the sensitivity of the telescope to a point source increase with the area of the dish.
The dish is usually mounted so that it can be steered to point in different directions and can be made to track a moving object. Some dishes, however, can move in only one coordinate and rely on the Earth's rotation to provide coverage in the other coordinate. The 305-meter dish mounted in a natural hollow in the ground at the Arecibo radio telescope, Puerto Rico, achieves partial beam steering by moving the feed.