Stray light overview for the Origins Space telescope

Abstract

The Origins Space Telescope (OST) is a mission concept being studied in preparation for the 2020 Decadal Survey. OST will be a large space based astronomical telescope operating at mid and far Infrared wavelengths. The desire is to have the radiometric sensitivity of observations be limited by the natural celestial sky background. This will require that OST be operated at cryogenic temperatures to limit the self-generated thermal emission. The architecture has the telescope exposed to space, with a protective shield blocking exposure to direct illumination from the sun, earth, and moon. The telescope design limits the self-emission to stray light to only come from the telescope structure, baffles, and optics that are thermally controlled to ≈ 4 K. A reverse try trace technique is used to determine the susceptibility of light from any part of the sky getting to the instrument focal plane and producing a stray light background. A Radiance Transfer Function (RTF) is derived that relates the background stray light produced at the focal plane by a patch of sky to the radiance of that patch of sky. The RTF is defined relative to the observatory reference frame. For a given pointing direction of the Observatory on the sky, the sky radiance mapped in ecliptic coordinates is transformed into the telescope reference frame and multiplied by the RTF to calculate the stray light. The sky radiance maps are from data obtained from the Cosmic Background Explorer (COBE) mission. In addition to the sky source of stray light, a separate calculation is used to determine the self-generated IR thermal background.