The COS FUV detectors convert each ultraviolet photon into a shower of electrons, for which the detector electronics calculate the X and Y coordinates and the total charge, or pulse height. Prolonged exposure to light causes the FUV detectors to become less efficient at this photon-to-electron conversion, a phenomenon called "gain sag." As a result, the peak of the pulse-height distribution slowly decreases. As it approaches the minimum threshold imposed by calcos, target photons may be rejected as background events. As it falls still farther, target photons may be permanently lost. Gain sag appears first in regions of the detector that are illuminated by bright airglow lines, but eventually affects the entire spectrum. Localized effects can be worked around by discarding the photons falling on severely sagged parts of the detector and filling in the missing wavelengths by using data taken at a different FP-POS position, but as more detector regions have become affected, this has proved increasingly difficult.
To overcome these gain sag effects, the position of spectra on the COS FUV detector will shortly be moved to a fresh part of the detector which has not yet experienced significant gain sag. This new position will be displaced in the cross-dispersion direction by 3.5", corresponding to ~41 pixels, and should, at least for the next two to three years, eliminate the deleterious effects of gain sag on detector throughput and flat fielding.
We currently plan to implement this change in late July, 2012. When implemented, the change in position will be applied to all pending COS FUV science and COS FUV target acquisition exposures, including those in unexecuted visits from current programs. The change will be transparent to users preparing their Phase II proposals with APT. No changes will be made to the location of the COS NUV spectra on that detector.
The spectral and spatial resolution at the new lifetime position are expected to be within ~10% of those at the original position. Small changes to the wings of the line spread function are predicted; new LSF models have been computed and will be made available to users once they are validated by calibration observations at the new lifetime position.
The flux and wavelength calibrations of the new lifetime position are expected to be very similar to those at the original position. Calibration observations will be executed shortly after moving to the new lifetime position and updates to calibration reference files, if needed, will be promptly made. These observations include measurements of the resolution, verification of the wavelength scales, verification of the FUV BOA operations, and flux and flat field calibration observations.
Changes have already been implemented in the calibration pipeline and associated calibration reference files so that data taken at the new lifetime position can be properly calibrated.