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Hubble Space Telescope
GHRS Instrument Science Report 077

GHRS Instrument Science Report 077

"Anomalies in the Calibration of GHRS Grating G270M"

Melissa A. McGrath

Oct 17, 1996


Two anomalies in the calibration of the GHRS G270M grating are
reviewed. The pre-COSTAR, in-flight sensitivity calibration used in
the pipeline processing between Nov. 11, 1991 and April 14, 1994 is
erroneous shortward of 2300. The pre-COSTAR vignetting correction for
grating G270M is inadequate shortward of 2300, and the post-COSTAR
vignetting correction shortward of about 2150 is not handled properly
by calhrs. The vignetting correction calibration problem is symptomatic
of the underlying problem, which is that the y-deflection has not
actually been measured below 2300.