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.
