Holographic Measurement and Improvement of the Green Bank Telescope Surface

Abstract

We describe the successful design, implementation, and operation of a 12 GHz\nholography system installed on the Robert C. Byrd Green Bank Telescope (GBT).\nWe have used a geostationary satellite beacon to construct high-resolution\nholographic images of the telescope mirror surface irregularities. These images\nhave allowed us to infer and apply improved position offsets for the 2209\nactuators which control the active surface of the primary mirror, thereby\nachieving a dramatic reduction in the total surface error (from 390 microns to\n~240 microns, rms). We have also performed manual adjustments of the corner\noffsets for a few panels. The expected improvement in the radiometric aperture\nefficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The\nimprovement in the telescope beam pattern has also been measured at 11.7 GHz\nwith greater than 60 dB of dynamic range. Symmetric features in the beam\npattern have emerged which are consistent with a repetitive pattern in the\naperture due to systematic panel distortions. By computing average images for\neach tier of panels from the holography images, we confirm that the magnitude\nand direction of the panel distortions, in response to the combination of\ngravity and thermal gradients, are in general agreement with finite-element\nmodel predictions. The holography system is now fully integrated into the GBT\ncontrol system, and by enabling the telescope staff to monitor the health of\nthe individual actuators, it continues to be an essential tool to support\nhigh-frequency observations.\n