COS FUV External Spectroscopic Performance

Details COS FUV External Spectroscopic Performance COS FUV External Spectroscopic Performance

Sep 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010cos..rept....9g&link_type=abstract

Instrument Science Report COS 2010-09(v1), 37 pages

Other

Hubble Space Telescope, Hst, Space Telescope Science Institute, Cos, Cosmic Origins Spectrograph

Scientific paper

We have performed an analysis of the spectral and spatial resolution of the Cosmic Origins Spectrograph (COS) in its far-ultraviolet channel using spectra acquired during the SM4 Servicing Mission Observatory Verification (SMOV) and during the Cycle 17 FUV sensitvity monitoring program. As described in ISR 2009-01(v1), our results indicate that the on-orbit FUV spectral resolution is reduced compared to values obtained in ground testing due to the redistribution of light from the core of the line spread function (LSF) into the wings. The wings are caused by mid-frequency wavefront errors (MFWFEs) that are produced by zonal (polishing) errors on the HST OTA. Here we compare the effective resolving power obtained from our models of the COS LSF, for cases where the MFWFEs are included and excluded. The spectral resolving power obtained when MFWFEs are included is 16,000-20,000 for G130M and G160M and 2300-3500 for G140L (λ > 1250 Å). On the other hand, our analysis of the on-orbit spatial resolution perpendicular to dispersion shows that it depends strongly on both wavelength and central wavelength (CENWAVE) setting. This dependence is primarily caused by the known astigmatism introduced by the COS FUV gratings. Over most of its spectral range, the spatial resolution of the FUV channel (as defined by the full width half maximum of a point source spectrum perpendicular to dispersion) meets or exceeds the values measured in ground testing, for both segments of the detector. Across the full range of CENWAVE settings, we obtain a spatial resolution of 0.5" to 1.8" for the G130M grating, 0.3" - 0.6" for G160M and 0.4"- 2.0" for the G140L grating. The corresponding spatial resolution for the BOA aperture is 0.5" - 1.5" for G130M, 0.4" - 0.9" for G160M and 0.7" - 1.5" for G140L. Note that the real values for these gratings will be ~ 15% larger (i.e., slightly lower spatial resolution than quoted) due to our use of a single component fit to the strongly asymmetric BOA spatial profiles. For both the PSA and BOA spectra we present third- and fourth-order polynomial fits to the widths of the crossdispersion profile as a function of wavelength, for a range of CENWAVE settings. This allows users to estimate the wavelength dependence of the spatial resolution in the FUV. We also present calculations of the plate scale and aperture throughput curves for the FUV channel with the PSA aperture. Both quantities also meet pre-launch specifications.

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