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Hubble Space Telescope
GHRS Instrument Science Report 044

GHRS Instrument Science Report 044

SSA Spiral Search Tests Results for Centering and Throughput  10/92

Ronald L. Gilliland

Abstract excerpted from the report:

The primary GHRS science aperture is the 0.25 arcsec SSA with a size
matched to the spectroscopic resolution element. Because a planned flux
peakup capability in the SSA did not work well in SV testing, precise
acquisitions to the SSA center have remained a problem. Without a robust
means of centering a star in the SSA, throughput measurements have also
remained inaccurate.

Without an SSA centering algorithm placement in the SSA requires: 1) a
precise centering within the LSA, and 2) a precise blind offset to the SSA.
Two calibration proposals developed for Cycles 1 and 2 have been used to test
positioning to the SSA following A2 acquisitions. We document how a successful
PDB update to improve the initial centering in the LSA was derived. Also 
discussed are determinations of the SSA throughput which with perfect centering
is considerably higher than that found during SV. These SSA spiral search 
tests provide information necessary for simulations of a planned SSA flux
peakup capability.