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Hubble Space Telescope
Effect of COS FUV Gain Sag on FUV Dispersed Light Target Acquisitions

Another consequence of the gain sag in the FUV detector is the mis- registration of events along the cross-dispersion or y-direction, commonly referred to as y-walk. While this y-walk has no effect in extracted spectra, it could potentially affect FUV dispersed light target acquisitions which use information about where the target's spectrum is found in the cross- dispersion direction vs where it is supposed to be found (determined from the location of the wavecal) to center the target in the aperture. Targets that are initially well centered in the aperture will be more affected by y-walk than targets that are not well centered initially, as the latter will fall on a more pristine area of the detector where the gain has not sagged as much. We estimate that currently FUV dispersed light target acquisitions could lead to target mis-centerings of the order of ~0.4"-0.5" with G160M, ~0.35" with G140L, and 0.3" with G130M.

Analysis of data obtained during SMOV showed that the target's flux starts to be compromised when the mis-centering is ~0.5" or larger. There are currently no Cycle 18 dispersed light target acquisitions using the G160M grating and only a few using the G140L grating (Calibration program). We are in the process of executing tests on orbit to raise the gain, in addition to considering possible modifications to the dispersed target acquisition algorithms to avoid this issue.