Updated Results from the COS Spectroscopic Sensitivity Monitoring Program

Details Updated Results from the COS Spectroscopic Sensitivity Monitoring Program Updated Results from the COS Spectroscopic Sensitivity Monitoring Program

Jun 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011cos..rept....2o&link_type=abstract

Instrument Science Report COS 2011-02, 29 pages

Physics

Hubble Space Telescope, Hst, Space Telescope Science Institute, Cos

Scientific paper

We report updated results from the Cosmic Origins Spectrograph spectroscopic sensitivity monitoring programs utilizing data taken through the end of Cycle 17 and beginning of Cycle 18. Earlier results (reported in Osten et al. 2010) had indicated a wavelengthdependent decline of the FUV sensitivity which was worse at longer wavelengths. Since mid-March 2010, the rate of this sensitivity decline has become much smaller and mostly wavelength independent, and the rate of decline is now between 2 and 5%/year for all Far-Ultraviolet (FUV) gratings with sufficient signal to characterize. The characteristics of the FUV sensitivity decline are consistent with degradation of the quantum efficiency of the CsI photocathode of the FUV detector. The initial steep decline may have been caused by water vapor outgassing after COS's installation, while the subsequent decline may be due to exposure to ambient atomic oxygen present at HST's orbital altitude. New FUV Time-Dependent Sensitivity (TDS) reference files have been delivered to correct the pipeline flux calibration, however, even after the application of these TDS corrections there remain discrepancies in the absolute flux calibration which appear to depend on central wavelength and FP-POS, and can be up to 5-10%. Further investigation reveals that some of this discrepancy may be due to additional sensitivity degradation during initial on-orbit operations. As reported in Osten et al. (2010), the two NUV bare-aluminum gratings (G225M and G285M) are also showing sensitivity declines, which appear to continue trends seen during ground testing, and which may be due to ongoing evolution of an oxide layer. In contrast, the throughputs of the NUV gratings coated with MgF2 (G185M and G230L) remain stable, showing little to no sensitivity decline. The NUV bare-aluminum sensitivity decline appears to be a continuing trend from that seen on the ground.

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