Observing with HST below 1150{\AA}: Extending the Cosmic Origins Spectrograph Coverage to 900{\AA}

TL;DR

This paper demonstrates that the Cosmic Origins Spectrograph on HST can observe wavelengths below 1150{ A} down to 900{ A} by leveraging its windowless detector and specific grating configurations, expanding ultraviolet observational capabilities.

Contribution

The study presents calibration results and simulations showing how COS can be used for effective spectroscopy below 1150{ A}, extending its operational wavelength range.

Findings

COS retains sensitivity down to 900{ A} with specific configurations.

Calibration and ray trace simulations support extended wavelength coverage.

Potential for increased resolution and efficiency below 1130{ A}.

Abstract

The far-ultraviolet (FUV) channel of the Cosmic Origins Spectrograph (COS) is designed to operate between 1130{\AA} and 1850{\AA}, 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 1066{\AA}, 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 912{\AA} and 1070{\AA}, and rising rapidly to over 1000 cm2 beyond 1150{\AA}. Following this initial work we explored the possibility of using the G130M grating out of band to provide coverage down to 900{\AA}. 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 1130{\AA}.