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.