A device for detecting and/or imaging objects (generally in space) which emit electromagnetic radiation in the radio bands of the spectrum. Most are composed of one or more dish-shaped antennas. Smaller antennas, as used in multi-dish arrays, may be mounted in such a way as to be steerable; this kind can be pointed at various parts of the sky. Larger, fixed dishes cannot be aimed at will; the Earth's turning gradually brings targets in and out of the field of view.

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