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With a 100mx110m off-axis paraboloid dish, the Green Bank Telescope (GBT) isthe largest fully steerable radio telescope on Earth. A major challenge facinglarge ground-based radio telescopes is achieving sufficient pointing accuracyfor observing at high frequencies, up to 116 GHz in the case of the GBT.Accurate pointing requires the ability to blindly acquire source locations andperform ad hoc corrections determined by observing nearby calibrator sources inorder to obtain a starting position accurate to within a small margin of errorof the target's location. The required pointing accuracy is dependent upon thehalf-power beamwidth, and for the higher-frequency end of GBT observing, thismeans that pointing must be accurate to within a few arcseconds RMS. The GBT'soff-axis design is advantageous in that it eliminates blockage of the dish andreduces sidelobe interference, and there is no evidence that the resultingasymmetric structure adversely affects pointing accuracy. However, factors suchas gravitational flexure, thermal deformation, azimuth track tilt andirregularity, and small misalignments and offset errors within the telescope'sstructure cause pointing inaccuracies. A pointing model was developed for theGBT to correct for these effects. The model utilizes standard geometricalcorrections along with metrology data from the GBT's structural temperaturesensors and data from measurements of the track levels. In this paper weprovide a summary of the GBT's pointing model and associated corrections, aswell as a discussion of relevant metrology systems and an analysis of itscurrent nighttime pointing accuracy.

Green Bank Telescope: Overview and analysis of metrology systems and pointing performance