Abstract: To achieve the highest accuracy boresight pointing performance the Hubble Space Telescope uses attitude feedback from the Fine Guidance Sensors (FGS). There are three FGS's on board HST. During normal operations, one sensor monitors spacecraft pitch and yaw, another monitors roll and the third is the redundant unit. Each FGS senses wavefront tilt interferometrically and converts that tilt into spacecraft pointing error. The presence of spherical aberration affects the signal from the instrument causing a reduction in acquisition and tracking performance on targets whose magnitudes are fainter than 14. This paper documents the efforts to optimize uplinkable, FGS parameters in order to increase the probability of target acquisition that is better than 98 percent over the entire field of view. To this end, the paper describes the Monte Carlo simulator used in deriving the optimized values for the FGS acquisition and discusses methods for testing the new parameters prior to on-orbit verification. It reports on improvements predicted by the acquisition simulator and evaluates on orbit performance with the optimized values. In addition, the paper discusses the commissioning of FGS 1R, installed during the February 1997 servicing mission, with regard to operational options predicted by the simulator. It also reports on how well the new FGS, with its on board alignment capability, is working with the new acquisition parameters determined by the simulator. !5
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