Other
Scientific paper
Oct 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009cos..rept....1g&link_type=abstract
COS Instrument Science Report 2009-01(v1), 23 pages
Other
Hubble Space Telescope, Hst, Space Telescope Science Institute, Cos, Cosmic Origins Spectrograph
Scientific paper
We present a preliminary analysis of the line spread function (LSF) of the Cosmic Origins Spectrograph (COS) using FUV and NUV stellar spectra acquired during the SM4 Servicing Mission Observatory Verification (SMOV). Our results indicate that the on-orbit shape of the COS LSF with the HST optical telescope assembly (OTA) deviates from the profile measured in ground testing due to the appearance of broad non-Gaussian 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 not be simulated during ground testing. The MFWFE effects are particularly noticeable in the FUV. While the pre-launch FUV LSF is well described by a Gaussian, the on-orbit FUV LSF has up to 40% of its total power distributed in non-Gaussian wings. The power in these wings is largest at the shortest wavelengths covered by the COS medium-resolution gratings (~ 1150 Å). The effect diminishes with increasing wavelength but has a non- negligible effect on encircled energies even at the longest wavelengths. The effects of the MFWFEs are also present in the COS NUV LSF, particularly for the shorter wavelength gratings (G185M and G225M), although at a lower level than in the FUV. Optical models incorporating the MFWFE effects into the LSF have been calculated 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 assess the impact of the MFWFE broadening on their COS spectra. While these LSF models are not perfect and may not always provide an exact match to the actual on-orbit LSFs, they should prove to be an extremely useful tool for the analysis and interpretation of COS spectra. We anticipate that the effects of the MFWFE wings on the on-orbit LSF will be most important for observations that require the full expected 15 km/s resolution of the G130M, G160M, and G185M gratings, with more modest impacts for science programs targeting broad lines, continuum sources, or which use other COS gratings. However, the impact on the detectability of faint, narrow features is important for all gratings, leading to an increase between 20% and 40% of the minimum detectable equivalent width (these values assume a 3σ detection of an unresolved line, superimposed on a continuum with S/N = 10 per pixel).
Aloisi Alessandra
Delker Thomas
Ghavamian Parviz
Hartig George
Keyes Tony
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