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Hubble Space Telescope
New COS Far Ultraviolet Time-Dependent Spectroscopic Sensitivity Reference File

During the first several months of on-orbit operations of the COS FUV detector, it became apparent that the spectroscopic sensitivity was changing with time at a rate larger than previously anticipated. The sensitivity decline was affecting all FUV gratings, with an increase in degradation at the longest FUV wavelengths. The initial results of this monitoring were described in Osten et al. (2010), and resulted in the delivery of a time-dependent sensitivity reference file for use in the flux calibration of spectroscopic data. Regular monitoring of spectrophotometric white dwarf standard stars continues, and has revealed a change in the rate at which the sensitivity decline is occurring, as well as its behavior with wavelength. The new trends show that around mid-March 2010, the rate of sensitivity decline slowed markedly and became nearly wavelength independent. Prior to this, the rates at which the sensitivity was declining ranged from 5% per year near 1200 A to as high as 15% per year at the longest FUV wavelengths. There was variation in behavior at a given wavelength with different gratings and detector segments. Since mid-March 2010, the rate of sensitivity decline has become much more consistent across the FUV gratings and detector segments, with a nearly grey wavelength dependence. The sensitivity decline is now in the range 3-4 % per year between 1100 and 1800 A.

As the changes in the rate of sensitivity decline became apparent, other detector effects such as gain sag were also being noticed. Prior to December 21, 2010, FUV data were filtered with a minimum pulse-height amplitude of 4. There were regions of the detector where the peak of the pulse-height distribution had shifted towards low enough values that valid events were being removed. This caused an apparent increase in the rate of sensitivity decline. The application of a new pulse-height filtering, set to a minimum of 2, has removed this effect, and reveals no additional changes in sensitivity beyond that described above.

These data were used to establish time-dependent sensitivity (TDS) reference files which were delivered on March 18, 2011 to the calibration database system for use with COS/FUV. These files are now routinely used in on-the-fly-reprocessing of COS data obtained through the archive. These files should correct fluxes to an accuracy of +/-2%. The updated results of the spectroscopic sensitivity monitoring will be described in an ISR currently in preparation (Osten et al. 2011).