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The Cosmic Origins Spectrograph (COS) was installed on the Hubble Space Telescope (HST) in 2009. The majority of science spectra are taken with the Far Ultraviolet (FUV) channel, which uses a cross delay line detector. This detector is subject to gain sag, which causes its sensitivity to decrease at locations where counts have fallen. Increasing the high voltage can be used to compensate for this effect, but when the voltage limit is reached, the spectra are moved to a different Lifetime Position (LP) by offsetting them in the cross-dispersion direction on the two-dimensional detector. However, since the optical design was optimized for the initial spectral position, other LPs show a change in the shape of the point spread function, with distortions that lead to a loss of resolution.
In anticipation of a move to a seventh lifetime position (LP7) in ~2025, we have conducted exploratory work to model the instrument performance over the full range of possible positions for the spectra on the detector. This effort includes (1) optical modeling of the spectrograph; (2) creating simulated science spectra and evaluating the resolving power and other spectral properties at a range of positions; (3) evaluating the detector properties at potential lifetime positions while considering mechanical limitations of the hardware; and (4) reserving sufficient space for later LPs. We have used this information to formulate preliminary plans for moving to the next position, which includes identifying which observing modes should be adjusted, and providing our initial thoughts on future Lifetime Positions.
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