|Space Telescope Science Institute|
|COS Instrument Handbook|
Prolonged exposure to light causes the COS FUV detectors to become less efficient at photon-to-electron conversion, a phenomenon called “gain sag”. The more a particular region of the detector has been used, the smaller the “pulse height” of the charge cloud generated by an individual photon becomes. As long as all pulse heights are above the minimum threshold needed to distinguish real photons from background events there is no loss in sensitivity. However, as the average pulse height at a location on the detector approaches, and then drops below, this threshold real photon pulses are increasingly misidentified as background and the effective throughput decreases. Since the amount of gain sag increases with the total amount of previous illumination, these effects appear first on regions of the detector that are illuminated by the bright Lyman-α airglow line, but eventually the entire spectrum becomes affected. STScI is undertaking a number of actions to mitigate the effects of gain sag and extend the lifetime of the COS FUV XDL detector. These are discussed in detail in Chapter 4.On July 23, 2012 the COS FUV spectral location was moved to its second lifetime position. This changed the COS aperture position by 3.5" in the HST field of view, and shifted the spectral location by about 1 mm to a part of the detector where significant gain sag had not yet occurred. This change eliminated the “Lyman-α” holes and other gain sag artifacts for data obtained during Cycle 20 and much of Cycle 21. However, because of heavy use of the COS detector during Cycle 21, it is possible that a move to a third lifetime position will occur early in Cycle 22, or ealier if necessary.Throughput, and most other calibrations at the second lifetime position, are very similar to those at the original position, but the spectral resolution may be up to 10% lower than it was at the original lifetime position. See the COS website and the COS Instrument Science Reports (ISRs) for additional information about the calibration of the second lifetime position. The resolution that will be available at the third lifetime position is not yet known, but it is expected to be about 80–85% of that at the original lifetime position.Gain sag is an inevitable result of using the detector. Gain sag holes will eventually appear at the new position, with the timing of their appearance depending on the locally-accumulated signal. The use of all four FP-POS positions, which is now required for most COS FUV observations (see Section 4.1.7), will distribute the high geocoronal Lyman-α flux more uniformly over the detector, and thus will significantly delay the re-appearance of these holes.