COS External Spectroscopic Performance: Spectral and Spatial Resolution

During the SM4 Servicing Mission Observatory Verification (SMOV) we discovered that the on-orbit shape of the COS LSF with the HST optical tele- scope assembly (OTA) exhibits broad wings. The wings are caused by mid-frequency wave-front errors (MFWFEs) that are produced by the zonal (polishing) errors on the HST primary and secondary mirrors; these errors could no tb e simulated during ground testing. The MFWFEs are particularly noticeable in the FUV. The on-orbit LSF has up to 40% of its total power distributed in non-Gaussian wings (as com- pared to 26% for a Gaussian). The power in these wings is largest at the shortest wavelengths covered by the COS FUV medium-resolution gratings (! 1150 ˚ The e! ect decreases with increasing wavelength but has a non-negligible e! ect on encir- cled energies even at the longest wavelengths. We have calculated optical models incorporating the MFWFE e! ects into the LSF for the whole spectral range covered by the FUV and NUV medium-resolution gratings. We show that for the FUV, the convolution of these model LSFs with high-resolution STIS echelle spectra yields an excellent match to the on-orbit COS spectra of the same targets. The model LSFs are available online and can be used by COS observers to asses st he impact of the MFWFE broadening on their COS spectra. In ground tests it was shown that COS can spatially resolve two equally bright objects separated by 1 !! in the cross-dispersion direction in the FUV. Using the FUV spectra of white dwarfs acquired during the Cycle 17 COS Spectroscopic Sensitivity Monitoring program ,w e show the on-orbit spatial resolution (as defined by the full-width half maximu mo f the spectrum along cross-dispersion) meets this specification, though in a wavelength-dependent man- ner. The wavelength dependence is primarily due the astigmatism introduced by the FUV gratings in cross-dispersion. The spatial resolution also depends on the central wavelength (CENWAVE) setting used, with spatial resolution monotonically improving with increasing CENWAVE settings.