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Hubble Space Telescope
GHRS Instrument Science Report 027

GHRS Instrument Science Report 027

"Calibration of the Photometric Sensitivity of some Low and Medium
Resolution Gratings" February 8, 1991

L. Ferrarese and J. R. Walsh

Excerpted from the summary:

The photometric sensitivity of low and medium resolution gratings
has been derived using standard analysis tools in STSDAS.HRS. G140L and
G160M spectra of UV grating sensitivity functions against the pre-launch
curves and between calibrations derived from different standard stars 
are shown. The throughput for the G140L grating is down by a factor
~50% in the 1500-2000 A region in comparison with the pre-launch values,
and it is closer to the nominal throughput at shorter wavelengths (note
that the sharp peak near 1275A in the pre-launch sensitivity curve is not
real). Spherical aberration is responsible for a 20% loss of throughput
for the LSA, and for an 80% loss for the SSA.

Similar behavior to the G140L is exhibited for the G160M grating,
which however employs a different detector to the G140L grating.  
Comparison of the SSA to LSA throughput shows a mean value of 0.28 in the 
1100 - 2200A region but with a trend to increasing value at longer 
wavelengths.