Monitoring of the COS FUV channel has shown that during the last few months its sensitivity has begun to decline significantly more rapidly than previously. It is now declining at rates comparable to those seen in late 2009 and early 2010 as documented in COS ISR 2011-02.
This may be related to the increased atmospheric density at HST's orbital altitude as the solar cycle approaches maximum. This increases the amount of highly reactive atomic oxygen that can reach the CsI photocathode of the windowless COS detector.
While throughputs at the present time are still close to ETC estimates, if the currently observed rate of decline continues over the next year, at some wavelengths it could have a significant impact on the exposure times needed for Cycle 20 proposals. An update of the ETC with an improved estimate of COS throughputs for mid-Cycle 20 will be released by the end of January. An announcement will be sent out once the ETC has been updated.
Scientists writing HST proposals should note that even with this projected decline in sensitivity, the COS FUV channel will still be significantly more sensitive than are comparable STIS modes. This, combined with the very low detector background rate of the COS FUV channel, means that during Cycle 20 COS will remain the spectrograph of choice for most faint FUV targets.