The National Radio Astronomy
Observatory (NRAO), funded by the National Science Foundation and operated by Associated Universities,
Inc., is the largest radio astronomy
observatory in the United States. The NRAO has its headquarters and computing center in Charlottesville, Va., and maintains
instruments in Green Bank, W.Va.; at Kitt Peak National Observatory, near Tucson, Ariz.; and in Socorro, N.Mex. One of the main instruments was a radio telescope, 91.5 m (300 ft) in diameter, that operated from 1962 to 1988, when it collapsed. It is being replaced by a 100-m (330-ft) antenna shaped like an oyster shell to avoid radiation scattering.Other instruments include a 43-m (140-ft) telescope (1965), the world's largest equatorially mounted radio telescope; a 37-m-long (120-ft) calibration horn antenna; and an interferometer array of three 26-m (85-ft) dishes mounted on a 1,525-m (5,000-ft) baselineÑtwo of the dishes are movable, and the third, the fixed-position Howard E. Tate telescope, is the NRAO's oldest antenna (1959). At an elevation of 1,800 m (6,000 ft) on Kitt Peak, NRAO has an 11-m (36-ft) millimeter-wave radio telescope. At a site 83 km (52 mi) west of SocorroÑon the vast Plains of San AugustinÑNRAO built (1981) a huge, movable, Y-shaped array of twenty-seven 25-m (82-ft) radio telescopes called the Very Large Array (VLA). The VLA has a resolution equivalent to that of a single dish about 30 km (20 mi) in diameter (see aperture synthesis).