Observing with HST below 1150Ã : Extending the Cosmic Origins Spectrograph Coverage to 900Ã

Abstract

The far-ultraviolet (FUV) channel of the Cosmic Origins Spectrograph (COS) is designed to operate between 1130Ã and 1850Ã , limited at shorter wavelengths by the reflectivity of the MgF2 protected aluminum reflective surfaces on the Optical Telescope Assembly and on the COS FUV diffraction gratings. However, because the detector for the FUV channel is windowless, it was recognized early in the design phase that there was the possibility that COS would retain some sensitivity at shorter wavelengths due to the first surface reflection from the MgF2 coated optics. Preflight testing of the flight spare G140L grating revealed ~5% efficiency at 1066Ã , and early on-orbit observations verified that the COS G140L/1230 mode was sensitive down to at least the Lyman limit with 10-20 cm^2 effective area between 912Ã and 1070Ã , and rising rapidly to over 1000 cm2 beyond 1150Ã . Following this initial work we explored the possibility of using the G130M grating out of band to provide coverage down to 900Ã . We present calibration results and ray trace simulations for these observing modes and explore additional configurations that have the potential to increase spectroscopic resolution, signal to noise, and observational efficiency below 1130Ã .

6 pages, 5 figures. To appear in 2010 HST Calibration Workshop Proceedings