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Hubble Space Telescope
FOC Instrument Science Report # 87

FOC Instrument Science Report # 87			12 Sept 1995

"The New f/96 Geometric Correction Models"
by P. Greenfield

Abstract excerpted from the report:

Described are the new geometric distortion models determined
for the post-COSTAR f/96 relay based on multiple, overlapping
observations of crowded star fields. New distortion models were
determined for the following f/96 formats: 512zx1024, 512x512, 
256x1024, 256x256, and 128x128 using the method outlined in the
previous Instrument Science Report FOC-086. Unlike the previous
observations, the offsets between exposures appear to be as 
expected. The quality of the fits are good despite an apparent
change of ~0.16% in plate scale over the course of all the 
observations. The consistency of fit star positions with the
previous model is good. The consistency of the distortion fit 
with the previous 512x512 distortion fit using the new method is
very good whereas the consistency with the previous 512zx1024 fit
is less so. The evidence is strong that the photometric variations
at the scan line beginning are not a direct result of distortion.