Major Improvements to the Green Bank Telescope Surface Accuracy from Conventional Holography

Details Major Improvements to the Green Bank Telescope Surface Accuracy from Conventional Holography Major Improvements to the Green Bank Telescope Surface Accuracy from Conventional Holography

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21544210h&link_type=abstract

American Astronomical Society, AAS Meeting #215, #442.10; Bulletin of the American Astronomical Society, Vol. 42, p.408

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The NRAO 100m diameter Green Bank Telescope (GBT) was constructed with an active surface of 2209 actuators in order to achieve and maintain an accurate paraboloidal shape. The actuator home positions were set originally via photogrammetry performed nearly 10 years ago, resulting in a surface accuracy of about 400 microns rms. In order to improve this performance, we have undertaken a campaign of conventional, high-resolution radio holography at Ku band (11.7 GHz). With a 28Hz sampling rate, on-the-fly phase and amplitude beam maps are acquired by raster scanning over a 2 degree region with 1400 points in the scan direction and 201 points in the perpendicular direction, requiring approximately 3-4 hours. Surface features as small as 0.5m are visible, compared to the typical panel size of 2m by 2.5m. By measuring the deviations from a perfect paraboloid, proportional adjustments are made to the actuator home positions stored in a configuration file. We have also performed manual adjustments at two sets of panel corners which showed particularly noticeable errors not correctable by motion of the associated actuator, most of which are shared by 4 panels. Following these improvements, the 90 GHz aperture efficiency estimated using the MUSTANG bolometer camera had improved from 10% to 20% as of May 2009. Since then, further iterations of holography have been completed. Based on the observed increase in gain at 43GHz, the radiometric surface accuracy is now 230-260 microns rms under stable thermal conditions. Because the small scale contribution to the surface error is now only 200 microns rms, the removal of temperature-dependent large-scale errors via the near-realtime out-of-focus holography technique (AutoOOF) remains an important observing tool. New measurements of the 90 GHz efficiency using MUSTANG are pending and will be presented.
NRAO is a facility of the NSF operated under cooperative agreement by AUI.

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