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. The attached figure shows the effect of gain sag on the COS FUV detectors. The data were obtained in 2010 September using the grating setting G160M/1623/FP=4. The blue curve was constructed using only photon events with pulse heights in the range 4-30 (the limits used by calcos in late 2010). The red curve includes all photon events. Arrows mark two regions that suffer gain sag: the region near pixel 7200 is illuminated by Lyman alpha when grating setting G130M/1309/FP=3 is used, and that near pixel 9100 is illuminated by Lyman alpha when the setting is G130M/1291/FP=3. Note that the red curve lies slightly above the blue curve, even in regions where the gain sag is small; changing the pulse-height thresholds alters the effective sensitivity of the instrument.
To combat these effects, we have developed short, medium, and long-term solutions. In the short term, we will flag low-gain pixels and exclude them when combining spectra taken at multiple FP-POS positions, and we will employ time- and position-dependent pulse-height thresholds within calcos. These pipeline improvements should be implemented in early 2011. In the medium term, we are investigating the possibility of raising the high-voltage level of the FUV detectors. This technique was successfully employed by FUSE, which used detectors similar to those of COS. In the long term, it is possible to move the spectrum in the cross-dispersion direction onto a previously-unused portion of the detector by offsetting the aperture mechanism.
Spectra showing the effects of gain sag on segment B. Raw counts are plotted against raw X pixel number in this G160M/1623/FP=4 observation, binned by 20 pixels. The blue curve includes only photon events with pulse heights in the range 4-30. The red curve includes all photon events. The spectral features marked with arrows are not astrophysical, but represent the effect of gain sag on regions of the detector illuminated by Lyman alpha when other gratings are employed.