The COS FUV dark rate is monitored regularly by taking 5 1330s dark exposures every 2 weeks. These exposures, after excluding those that graze the SAA, are used to determine the dark rate trend vs. time for both FUVA and FUVB detectors.
Background counts in FUV observations can come from a variety of sources including radioactive decay of atoms in the detector glass, external charged particles, and scattered light. Because COS observations can be taken in TIME-TAG mode, individual events that are identified as background events by their PHA can be screened out during calibration. Analysis of the dark rate on this page has been done using PHA limits of 2-30, the limits currently imposed by CalCOS.
For more information on the COS FUV dark rate see ISR 2010_11: COS FUV Detector Dark Rates During SMOV and Cycle 17
Most Recent Dark Monitoring Exposures: