Extending HST/COS/G130M Coverage Down to 900Å with Two New Central Wavelengths.

Details Extending HST/COS/G130M Coverage Down to 900Å with Two New Central Wavelengths. Extending HST/COS/G130M Coverage Down to 900Å with Two New Central Wavelengths.

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21725428p&link_type=abstract

American Astronomical Society, AAS Meeting #217, #254.28; Bulletin of the American Astronomical Society, Vol. 43, 2011

Physics

Optics

Scientific paper

The far-ultraviolet (FUV) channel of the Cosmic Origins Spectrograph (COS) is designed to operate between 1130Å and 1850Å, limited at shorter wavelengths by the reflectivity of the MgF2 protected aluminum reflective surfaces on the Hubble Space Telescopes’ (HST) Optical Telescope Assembly (OTA) and on the COS FUV diffraction gratings. However, because the detector for the FUV channel is windowless, it retains some sensitivity at shorter wavelengths due to the first surface reflection from the MgF2 coated optics. On-orbit observations verified that the COS G140L/1230 mode was sensitive down to at least the Lyman limit with 10-20 cm2 effective area between 912-1070Å, and rising rapidly to over 1000 cm2 beyond 1150Å. Following this initial work we explored the possibility of using the G130M grating out of band to provide coverage down to 900Å.
We present on-orbit calibration results (wavelength solutions, spectral resolution, cross-dispersion profiles, sensitivity results, etc.) and supporting ray trace simulations for these two new observing HST/COS/G130M modes. These modes will be available in HST Cycle 19 as the new G130M central wavelength settings of 1055Å and 1096Å. These new modes provide effective areas twice that previously reported for the G140L ranging from 20 cm2 at 912 Å to 30 cm2 at 1070 Å then rapidly rising to 2000 cm2 at > 1140 Å. Unlike the G140L modes, the new COS G130M modes are designed to place the FUV segment-B below the rapid increase in effective area so that targets bright at > 1140 Å can be observed by disabling the FUV segment-A.

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