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Hubble Space Telescope
Change to COS FUV Pulse Height Filtering to Lower the Observed Dark Rate

A new Pulse Height Parameters Table reference file (PHATAB) was delivered for use in On the Fly Reprocessing (OTFR) on December 3, 2012. This table lowers the upper limit for pulse-height amplitude (PHA) screening in CalCOS from 30 to 23. This change is the result of analysis showing that COS Pulse Height Distributions (PHD) have always been below a modal PHA gain of 21, and that an upper limit of 23 will keep >99% of source counts while removing approximately 20% of the observed dark counts. The actual amount of the dark rate reduction depends on the PHD of the dark counts in each observation, which varies between observations and with solar cycle activity.

Because of the incredibly low dark rate on COS, the effect on most observations is very small. However, observations or spectral features that are background limited should see a moderate improvement.

Users wishing to have the new limits applied to their data can re-retrieve their data through OTFR.