In the Cycle 19 Call for Proposals, users were strongly encouraged to use multiple FP-POS positions for a given CENWAVE of the COS FUV detector as this strategy improves the limiting S/N ratio and allows the effects of flat-field artifacts to be minimized. Using multiple FP-POS positions proved especially important for observations that overlap the locations where the Lyman alpha line illuminates the detector during G130M and G140L 1105 exposures, as, over time, exposure to this bright geo-coronal emission causes local degradation of the COS FUV detector
To minimize such local degradation at the new COS lifetime position and maximize COS FUV data quality, proposers are strongly encouraged to use all four FP-POS positions (FP-POS=ALL) at any given CENWAVE when planning their observations in Cycle 20 as well. Even for very short exposures, observers should try to use at least two FP-POS positions per CENWAVE setting, as described in the Cycle 20 HST Primer and COS Instrument Handbook.
STScI will keep monitoring the ongoing and planned usage of the different COS CENWAVE and FP-POS positions at the new COS lifetime position. Observers may be contacted after their Phase II submission to evaluate whether changes in their planned usage - in order to improve their data quality and minimize localized impact on the FUV detector- are compatible with their scientific goals.