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100-meter Green Bank Telescope
Construction has been completed on the world's largest fully steerable radio telescope at the National Radio Astronomy Observatory's site in Green Bank, Pocahontas County, West Virginia (79° 50' 23.40" W, 38° 25' 59.23" N : NAD83). The GBT achieved "first light" at 403 MHz on August 22, 2000 with observations of the radio galaxy 1140+223 and the pulsar B1133+16. The GBT was dedicated as the Robert C. Byrd Green Bank Telescope in a ceremony on August 25, 2000. Outfitting and commissioning of the GBT commence this autumn (2000). The GBT is described as a 100-meter telescope, but the actual dimensions of the surface are 100 by 110 meters. The overall structure of the GBT is a wheel-and-track design that allows the telescope to view the entire sky above 5 degrees elevation. The track, 64 m (210 ft) in diameter, is level to within a few thousandths of an inch in order to provide precise pointing of the structure while bearing 7300 metric tons (16,000,000 pounds) of moving weight. The GBT is of an unusual design. Unlike conventional telescopes, which have a series of supports in the middle of the surface, the GBT's aperture is unblocked so that incoming radiation meets the surface directly. This increases the useful area of the telescope and eliminates reflection and diffraction that ordinarily complicate a telescope's pattern of response. To accommodate this, an off-axis feed arm cradles the dish, projecting upward at one edge, and the telescope surface is asymmetrical. It is actually a 100-by-110 meter section of a conventional, rotationally symmetric 208-meter figure, beginning four meters outward from the vertex of the hypothetical parent structure. The GBT's lack of circular symmetry greatly increases the complexity of its design and construction. The GBT is also unusual in that the 2,004 panels that make up its surface are mounted at their corners on actuators, little motor-driven pistons, which will make it easier to adjust the surface shape. Such adjustment will be crucial to the high-frequency performance of the GBT in which an accurate surface figure must be maintained. The GBT is being equipped with a novel laser-ranging system. Beams of light will be reflected within the structure and between the telescope and a series of ground stations surrounding the telescope in a broad ring. Monitoring of these beams will show the deformation of the figure under such stresses as gravity, wind and temperature differences, and will allow the telescope's motors, subreflector and surface panel actuators to compensate for any ill effects. |
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