Improved Medium Resolution Line Spread Functions for COS FUV Spectra

We have developed an improved line spread function (LSF) for the medium resolution farultraviolet gratings (G130M and G160M) of the Cosmic Origins Spectrograph (COS) by taking into account scattering in the far wings due to microroughness on the HST primary mirror. We start with the preliminary model LSFs that include the broad, non-Gaussian wings caused by mid-frequency wave-front errors (MFWFEs) that are produced by the zonal polishing errors on the HST primary and secondary mirrors. Since these LSFs only extend to ±50 pixels from line center, we extend these profiles to the full width of ±100 pixels observed for geocoronal Lyα using a power law profile. We optimize the normalization and index of the power-law extension by convolving a grid of model PSFs with high-resolution STIS echelle spectra of the star Sk-155 and comparing these to onorbit COS spectra. The best fits give a power-law index of -2.25 and show that additional light of ~3% in the extended wings is required. This low level of scattering is less than prelaunch expectations for HST, as also found by Hasan & Burroughs (1993) in longerwavelength WFPC observations. It also implies that COS internal scattering is nearly negligible. Overall, the system as a whole has impressively low scattering. We demonstrate the improvement in our characterization of the LSFs by deconvolving the Sk 155 COS spectrum in regions of saturated interstellar absorption lines. These lines are black and saturated in the deconvolved spectra, as seen in high-resolution STIS echelle spectra of the same target. We have placed the new model LSFs online for use by COS observers in analyzing their COS spectra.