Radio Interferometers A very important development of the early 1960s and subsequent years was the application of groups of radio antennas as radio interferometers. In this approach, several antennas are connected together simultaneously, usually by an ordinary electrical cable connection but sometimes via radio links over distances of more than 80 km (50 mi). Alternatively, the signals received at various antennas can be tape-recorded and subsequently played into a common radio receiver simultaneously. The radio interferometer permits extremely high resolution and a very large equivalent antenna collecting area at much less cost than would be called for if these were achieved with a single large antenna. This procedure is the basis of most of the major instruments recently constructed or planned. One of the most important developments to grow out of the successful application of interferometers was the process of aperture synthesis, pioneered by Sir Martin Ryle. When two parabolic antennas are connected together as an interferometer, the pair gives the same information to the radio receiver as two points on a much larger paraboloid. Research showed mathematically that if a pair of antennas was moved so that the many lengths and orientations of the line connecting them duplicated all the lines that occur in a paraboloid, then the information from the antennas could be combined to give exactly the same performance as the larger paraboloid would have achieved. This method made it feasible to duplicate the performance of a large paraboloidal radio telescope, for example, 1.6 km (1.0 mi) in diameter, by moving two antennas to a large number of positions within a circle on the Earth 1.6 km (1.0 mi) in diameter. In aperture synthesis the signals received at the antennas must be recorded precisely. When all the moves of the antennas have been achieved, the recorded signals can be combined to construct a picture of a small portion of the sky that would have the same clarity as though a 1.6-km (1.0-mi) telescope, in this example, had been built. This approach has proven effective. In existing aperture synthesis systems, large numbers of antennas are used, with each antenna connected to every other antenna; this procedure allows observance of a large number of antenna spacings and orientations at a given instant. The antennas themselves are physically moved to change the length of the lines connecting the antennas, and the rotation of the Earth is utilized to achieve an effective rotation with respect to the sky. One large operating aperture synthesis is located at Westerbork, the Netherlands, where antennas are spaced along a line one kilometer long.
Another powerful system is operated by Cambridge University. The largest system now in existence is the Very Large Array, which the National Radio Astronomy Observatory has built on the Plains of San Augustin near Socorro, N.Mex. This system possesses 27 steerable paraboloids, each 25 m (82 ft) in diameter, arranged along three railroad tracks. The three rail lines form an equiangular "Y" shape, and each is 20 km (12 mi) long. All the antennas are connected to the other antennas, giving 351 interferometer pairs at any given time. The resolution that is achieved by this instrument is a few tenths of an arc-second, about the same as that of the largest optical telescopes under the best atmospheric conditions. The entire array provides astronomers with the equivalent performance of a fully steerable radio dish 27 km (17 mi) in diameter. Another powerful application of interferometry is called very long baseline interferometry (VLBI). In this technique two or more radio telescopes at different locations observe the same region simultaneously. Atomic clocks are used at each telescope to control the radio telescope electronics and to synchronize the observations to accuracies that are better than one-thousandth of a second. At each station videotape recorders are used, with atomic clock synchronization, to record the signals received. The tapes are then brought to a processing location where they are played simultaneously into a device that mimics the electronic operation of the interferometer electronics that would have been used if the telescopes had been connected together in the conventional way. The output is a normal output from an interferometer but with the telescopes widely separated. This procedure is used with telescopes spread across the United States and also with telescopes in Australia, England, and Russia. Resolutions can be obtained that are comparable to those which would be provided by a fully steerable radio dish nearly the size of the Earth.