Time-Dependent Spectroscopic Sensitivity Monitoring of the Cosmic Origins Spectrograph

Details Time-Dependent Spectroscopic Sensitivity Monitoring of the Cosmic Origins Spectrograph Time-Dependent Spectroscopic Sensitivity Monitoring of the Cosmic Origins Spectrograph

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21546420o&link_type=abstract

American Astronomical Society, AAS Meeting #215, #464.20; Bulletin of the American Astronomical Society, Vol. 42, p.502

Astronomy and Astrophysics

Astronomy

Scientific paper

After the initial on-orbit determination of spectroscopic sensitivities was made for the near ultraviolet (NUV) and far ultraviolet (FUV) channels on the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST), additional monitoring has continued in Cycle 17 as part of the Cycle 17 COS Calibration Program. Low- and medium-resolution gratings are observed roughly monthly and quarterly, respectively, using external observations of spectrophotometric standard stars.
Of particular interest are two of the NUV medium resolution gratings. The G225M and G285M gratings are coated with bare aluminum. Because of concerns about the long term stability of the coatings, grating efficiency testing (GET) was conducted semi-annually from 2003 until payload integration in April, 2009 using the COS internal PtNe lamps to compare the strengths of spectral features common to multiple NUV gratings. The GET procedure was executed early in the Servicing Mission Observatory Verification (SMOV) and will be repeated approximately semi-annually during Cycle 17. The first on-orbit efficiency measures were consistent with the ground-based trends, which indicated an efficiency loss of 4% per year (relative) for the G285M grating and less than 2% per year for the G225M grating. Monitoring of the time-dependent sensitivity (TDS) using external targets for these gratings occurs at a monthly frequency to capture any on-orbit changes.
We present intial determination of TDS changes in the FUV and NUV channels through December 2009, using both external targets and GET data.

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