The payload is based around a single frame-transfer back-side illuminated CCD detector in the focal plane (FPA) of a 32 cm diameter on-axis telescope. The telescope feeds a re-imaging optic which supports the stray light suppression concept by providing a position for a field stop and reducing the impact of scattering from the baffling system reaching the detector directly.
The optical design is based on a Ritchey-Chretien style telescope with additional lenses to provide a de-focussed image of the target star with a point spread function (PSF) covering an area of ~765 px.
The optical design allows incorporation of a central baffle “tower” on which the field stop is mounted. Scattering of reflected earthshine from the baffle onto the primary mirror is the largest source of stray light in the current baffle design. By increasing the length of the baffle, both scattering from the spider and scattering from the baffle to the secondary mirror are minimised. Combined with an inner and outer baffling system the major stray light requirement (reaching < 1 ph/px/s) can be met. A comparison of the point source transmission functions of CoRoT, the Cassini orbiter narrow angle camera, and our preliminary design is made in Fig. 1.