COS Sensitivity Trends in Cycle 17

Details COS Sensitivity Trends in Cycle 17 COS Sensitivity Trends in Cycle 17

Jul 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010hstc.worke...5o&link_type=abstract

"2010 Space Telescope Science Institute Calibration Workshop - Hubble after SM4. Preparing JWST, held 21-23 July 2010 at Space T

Physics

Scientific paper

After the initial on-orbit determination of the absolute flux calibration of the Cosmic Origins Spectrograph was performed, we have been monitoring the instrument's spectroscopic sensitivity regularly. The bare Aluminum gratings G225M and G285M exhibited sensitivity degradation on the ground relative to the Al+MgF2- coated gratings, measured during semi-annual grating efficiency tests. The rates of degradation were about 1.8% per year for the G225M and 5.4% per year for the G285M grating relative to the G230L grating. Observations of spectroscopic white dwarf standard stars have shown a decline in sensitivity in the G225M and G285M gratings on the NUV channel, while the gratings with Al+MgF2 coatings (G230L and G185M) appear stable. The trends seen in external targets confirm those seen using internal exposures, and appear to be wavelength-independent but grating dependent. All gratings on the FUV channel are experiencing wavelength-dependent sensitivity degradation, which is worse at longer wavelengths. At the shortest FUV wavelengths the sensitivity decline is around 5% per year, increasing to about 12%/year at 1800
Å. The sensitivity changes on the NUV and FUV channels appear to be uncoupled. The sensitivity decline of the NUV bare Al gratings appears to be caused by continued growth of an oxide layer, either from additional deposition of atomic oxygen in space or migration of existing atomic oxygen in the system to the outside. Detector QE loss due to localized exposure of the FUV cross-delay line detector to UV irradiation can be ruled out by several tests which examine the sensitivity decline versus total counts in specific regions of the detector. The wavelength dependence of the FUV sensitivity degradation is the opposite sense for contamination to be an issue. The source of the FUV sensitivity decline does not appear to be due to shifts in the pulse-height distribution from gain sag. The loss of quantum efficiency shows the classic signature of photocathode degradation.

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