Optical modeling of new lifetime positions for the Cosmic Origins Spectrograph (COS)

In order to prolong the life of the detector in the FUV channel of the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), the positions of the spectra are adjusted by offsetting targets in the cross-dispersion direction. Since the installation of COS in 2009, five Lifetime Positions (LPs) have been used, and a sixth is scheduled to be commissioned in late 2022. The location of each LP is chosen based on both the evolving detector gain sag and the optical properties of the spectrograph. The COS optical design was optimized for use at its initial Lifetime Position (LP1), and offsetting the spectra introduces aberrations which affect the spectral resolution. In order to understand the effects of these aberrations on the data quality, optical models have been constructed in order to predict the performance of the spectrograph over the permissible range of target offsets and grating focus values. This modeling work - and how it is used in combination with on-orbit data when determining where to locate new Lifetime Positions - is discussed.