| aperture synthesis |
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A technique developed by radio astronomers, more recently applied also in the
infrared and optical wavebands, that makes it possible to obtain maps or images with the
resolving power of a very large aperture by combining the observations made with a number of
smaller antennas or mirrors. In the simplest radio astronomy example, two antennas are used as a radio interferometer, and the phase and amplitude of the combined radio signal are measured continuously. As the Earth rotates in the course of a day, one antenna is automatically carried around the other, effectively sweeping out a ring. On successive days the separation between the two antennas is changed, so that a large elliptical area is gradually covered. When the records are combined in a computer, it is possible to produce a radio map of the section of sky under observation with the detail resolved as if the telescope aperture were the size of the total area swept out. In practice, more than two antennas are normally used to speed up the process and give greater flexibility. It is also possible to combine observations made at different sites, separated by thousands of kilometres, to obtain even better resolution. Technological developments in the 1990s have made it possible to apply the same physical principle to obtain high-resolution images in the optical and infrared. Pioneering instruments include the Cambridge Optical Aperture Synthesis Telescope (COAST) in the UK and the Navy Prototype Optical Interferometer (NPOI) at the US Naval Observatory's site near Flagstaff, Arizona. See also: Earth rotation synthesis, interferometer. |