Baffles design of the PROBA-V wide FOV TMA

Proba-V payload is a successor of the Vegetation instrument, a multispectral imager flown on Spot-4 and subsequently on Spot-5, French satellites for Earth Observation and defence. The instrument, with its wide field of view, is capable of covering a swath of 2200 km, which, in combination with a polar low Earth orbit, guarantees a daily revisit. The lifetime of Spot-5 expires in early 2013, and to ensure the continuity of vegetation data, BELSPO, the Belgian Federal Science Policy Office, supported the development of an instrument that could be flown on a Proba type satellite, a small satellite developed by the Belgian QinetiQ Space (previously known as Verhaert Space). The challenge of this development is to produce an instrument responding to the same user requirements as Vegetation, but with an overall mass of about 30 kg, while the Vegetation instrument mass is 130 kg. This development had become feasible thanks to a number of new technologies that have been developed since the nineties, when Vegetation was first conceived, namely Single Point Diamond Turning fabrication of aspherical mirrors and efficient VNIR and SWIR detectors. The Proba-V payload is based on three identical reflective telescopes using highly aspherical mirrors in a TMA (Three Mirrors Anastigmat) configuration. Each telescope covers a field of view of 34° to reach the required swath. One of the challenges in the development of the PROBA-V instrument is the efficient reduction of stray light. Due to the mass and volume constraints it was not possible to implement a design with an intermediate focus to reduce the stray light. The analysis and minimization of the in-field stray light is an important element of the design because of the large FOV and the surface roughness currently achievable with the Single Point Diamond Turning. This document presents the preliminary baffle layout designed for the Three Mirrors Anastigmatic (TMA) telescope developed for the Proba-V mission. This baffling is used to avoid 1st order stray light i.e. direct stray light or through reflections on the mirrors. The stray light from the SWIR folding mirror is also studied. After these preliminary analyses the mechanical structure of the TMA is designed then verified in term of vignetting and stray light.