COS Instrument Handbook for Cycle 24

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2.1	COS FUV Detector Lifetime Positions

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 (LP2). This changed the COS aperture position location from its original lifetime 1 (LP1) 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 temporarily eliminated the “Lyman-α” gain sag holes and other gain sag artifacts for data obtained during Cycle 20, 21, and the early parts of Cycle 22. By February 9, 2015, new gain-sagged regions at LP2 degraded enough to require a move of most FUV modes to the third lifetime position (LP3).

For LP3 a smaller shift of ~2.5” from LP1 has been performed in the opposite direction from that done for LP2. This closer placement to LP1 is needed to keep the spectral resolution as high as possible, though it places the FUV spectra significantly closer to the severely gain-sagged detector regions of LP1. Observations with the G130M 1055 and 1096 central wavelength settings, which have wide cross-dispersion profiles that would be severely impacted by this overlap, continue to be executed at LP2. The G130M 1222 central wavelength will execute at LP3, but has been operated at a higher voltage setting to minimize the impact of gain-sagged regions.

For point source observations at LP3, the close proximity to the sagged detector regions require the implementation of a new extraction algorithm that uses the shape of a point source profile to define the region over which counts are included in the extracted spectrum and to decide when bad pixels in the profile wings compromise the accuracy of the spectral extraction. Sources that have substantial spatial extent may have significant overlap with the gain-sagged regions and may require specialized extractions that are currently not performed with calcos. For these reasons, observations of extended sources will not be optimally calibrated. Users should set the optional APT parameter EXTENDED=YES to flag such sources (see Section 5.9), even if the calcos pipeline calibration will not treat extended sources differently from point sources.

Throughput, and most other calibrations at LP3, are very similar to those at the original position, but the spectral resolution may be up to 15-20% lower than it was at the original lifetime position and about 10% lower than at LP2. See the COS website and the COS Instrument Science Reports (ISRs) for additional information about the calibration of the second and third lifetime positions.

Gain sag is an inevitable result of using the detector. Gain sag holes will eventually appear at LP3 as well, 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.