COS2025 Policies

The COS FUV microchannel plate detector’s efficiency at converting incoming photons into detectable events decreases with usage. This depletion of the detector’s gain (i.e. gain sag) results in unusable regions of the COS/FUV detector. The main culprit of gain sag in the COS/FUV detector is Lyα geocoronal emission at 1216 Å that fills the COS 2.5” aperture whenever the G130M grating is used with the 1291/1300/1309/1318/1327 cenwaves on Segment B.

In order to mitigate this gain sag, a number of strategies have been employed since the installation of COS on HST. These range from moving to different lifetime positions, to managing the high voltage to extract a smaller amount of charge, to re-distributing the cenwave usage so that Lyα does not produce a gain-sag hole in a given location.

We are now at a point where none of the strategies above will, without any other changes, allow us to continue operating the COS/FUV detector to 2025. To address this, a COS2025 policy has been developed. This policy, with the goal of retaining full science capability of COS/FUV past 2025, places restrictions on the G130M cenwaves allowed at Lifetime Positions 5 and 7.

In addition to cenwave restrictions, new COS FUV lifetime-usage policies place caps on how much of any given detector location may be used by a single program. These policies are described in detail in the March 2025 COS STScI Analysis Newsletter (STAN), which provides guidance on estimating lifetime usage and the justification requirements for higher-impact observations.
 

Tables 1a and 1b summarize the default lifetime positions for each COS/FUV science mode in Cycles 33 and 32, respectively, following the COS2025 policy. Tables 1c and 1d summarize alternative lifetime positions that can be used for certain G130M and G160M observations in Cycles 33 and 32, respectively, with appropriate justification. Table 2a and 2b summarize the lifetime positions for COS/FUV dispersed-light acquisitions in Cycles 33 and 32, respectively.

To assist users in preparing their Phase II proposals we have developed a COS/FUV wavelength range tool that allows users to visualize the COS/FUV wavelength ranges in light of the restrictions on G130M setting usage put in place to maximize the lifetime of the COS/FUV detector.  For wavelength range information in tabular form, please see Section 5.10 of the COS Instrument Handbook and the tables below.

The COS2025 strategy allows users to obtain Segment = B data with G130M/1291/FP-POS=3,4. Over time, Lyα airglow will sag these detector regions, which will become un-usable. This means that other settings, projecting light into these detector regions will also be affected. Tables 3a) and 3b) below summarize the wavelength ranges that will be affected for all the gratings. Users should consider carefully whether a wavelength region of interest falls in any of the affected wavelength ranges given in Tables 3a) and 3b).