The Orbiting Astronomical Observatory (OAO) program was a series of four U.S. scientific
satellites launched between April
1966 and August 1972. They were designed to study astronomical phenomena at
ultraviolet and X-ray wavelengths inaccessible to terrestrial observatories. The basic structure of each satellite was an octagonal aluminum cylinder into which were fitted the control, data-processing, and telemetry systems, along with a cluster of
telescopes. Attached to the cylinder were two paddlelike solar arrays.The first and third satellites in the series failed to function. The successful second satellite, OAO-2, launched Dec. 7, 1968, carried a cluster of four 32-cm (12.5-in) ultraviolet telescopes, a 41-cm (16-in) telescope, four 20-cm (8-in) photoelectric telescopes, and two ultraviolet spectrometers. OAO-2 scanned the sky in four ultraviolet wavelengths and studied the ultraviolet radiation of stars and gas clouds. The satellite operated until February 1973.The fourth satellite, named Copernicus, was launched on Aug. 21, 1972. It carried an 82-cm (32-in) reflecting ultraviolet telescope and three smaller X-ray telescopes. Copernicus continued to function until early 1981. An essential and at the time unique feature of Copernicus and OAO-2 was their ability to point their optics sufficiently accurately to focus on a single chosen star. The two satellites revolutionized astronomy in the same way that terrestrial radio telescopes had, by opening a new window in the spectrum of radiation through which to view the universe.