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Hubble Space Telescope
GHRS Instrument Science Report 062

GHRS Instrument Science Report 062

"First Measures of Sensitivity for the Post-COSTAR GHRS, With Interim
Values for Data Analysis"

Stephen J. Hulbert, Lisa E. Sherbert, and David R. Soderblom

Abstract excerpted from the report:

Observations were made with the GHRS through both apertures after
the COSTAR mirrors were deployed to evaluate changes in instrument
sensitivity. The ratio of post- to pre-COSTAR count rates are 
examined and illustrated for a series of first-order grating 
observations and an OSCAN with Echelle-B. These ratios behave about
as expected for wavelength > 2500A (about 1 for the LSA; about 2 for 
the SSA) but show a broad deficit in response from about 1500 to 2500A,
as well as a peak in response near 1300A. These features are very 
similar to what has been seen with the blue side of the Faint Object 
Spectrograph, although the underlying origins are not yet understood.

A full ab initio determination of GHRS Sensitivity will be made
early in Cycle 4. As an interim measure for science data calibration
we provide here a relationship between pre-COSTAR sensitivities and
those for the post-COSTAR instrument. The ratios of post- to pre-
COSTAR count rates for both the LSA and SSA have been fitted with
functions. These functions have been applied to the pre-COSTAR
sensitivities that are used to flux calibrate science observations,
and copies have been posted on STEIS and delivered to the Calibration
Data Base System (CDBS).