Gain reduction in an offset Gregorian antenna due to gravity-induced deformations and its compensation

A fully-steerable 100-meter radio telescope is under construction at Green Bank, WV and is scheduled for completion in 1995. This telescope, known as the Green Bank Telescope (GET), GET uses a double-offset design where the aperture of the antenna is completely free from blockage. The asymmetric primary reflector is a paraboloid of focal length 60 meters. The telescope uses a Gregorian configuration for frequencies above 1.15 GHz. The asymmetric subreflector is 7.5 meters in diameter. At prime focus the telescope provides continuous coverage in the 290-1230 MHz frequency band. A three-dimensional structure which includes the feed/subreflector support arm has bean carried out. This analysis gives the deformations of the structure induced by gravity as a function of elevation angle. Using this information, a scheme has been worked out to minimize the resultant reduction in beam peak gain by translation of the subreflector. At prime focus, compensation is achieved by translating the feed to track the focus of the best fit parabola. The compensation schemes will work for the slowly varying thermal and gravitational deformations but not for the rapid wind-induced disturbances. The secondary focus compensation scheme is presented.<>