Aug 2, 2010 - 2 CASA, University of Colorado, Boulder, CO. 3 Space Telescope Science Institute/Computer Sciences Corporation, Baltimore, MD. August 2 ...
SPACE TELESCOPE SCIENCE INSTITUTE
Operated for NASA by AURA
Instrument Science Report COS 2010-15 (v1)
Early Results from the COS Spectroscopic Sensitivity Monitoring Programs Rachel A. Osten1 , Parviz Ghavamian1, Sami-Matias Niemi1 , Derck Massa1 , Steve Penton2 , Alessandra Aloisi1, Tony Keyes1, Steve Osterman2 , Charles Proffitt3 1
3
Space Telescope Science Institute, Baltimore, MD 2 CASA, University of Colorado, Boulder, CO Space Telescope Science Institute/Computer Sciences Corporation, Baltimore, MD August 2, 2010
ABSTRACT We report initial results from the Cosmic Origins Spectrograph spectroscopic sensitivity monitoring calibration programs. For the NUV channel we use both internal and external measurements to determine the time dependence of any sensitivity changes, while for the FUV channel we use only external targets. Time-dependent sensitivity (TDS) trends for the NUV channel appear to be wavelength-independent but gratingdependent. Internal Grating Efficiency Tests (GETs) performed in the NUV show that the bare-Al gratings G225M and G285M are exhibiting declines in efficiency, while the Al+MgF2 -coated G185M and G230L gratings appear to be quite stable. The changes in efficiencies for the bare-Al gratings were first noted on the ground and continue in orbit. GET data obtained on the ground indicate efficiency declines relative to the G230L grating of −1.8 ± 0.9%/year for G225M and −5.4 ± 0.6%/year for G285M. The onorbit NUV TDS data obtained using external targets confirms the trends seen with the internal wavecal exposures. In the first 9 months of Cycle 17, the weighted means of the TDS slopes using external targets for G230L and G185M are −1.1 ± 0.4%/year and −0.8 ± 0.4%/year, respectively, while the G225M and G285M gratings on the NUV channel have exhibited on-orbit declines of −3.3 ± 0.3%/year and −10.8 ± 0.2%/year, respectively. We have considered the effect which potentially inaccurate target acquisition had on early FUV monitoring observations. The absolute values of the corrections are small (generally
