Design of the Focal Plane Assembly for the Scientific Payload of the Small Satellite Mission “AsteroidFinder/SSB”

Kurzfassung

This work presents and discusses an athermal design concept of the focal plate for the proposed full-C/SiC telescope of the AsteroidFinder/SSB mission. Key design drivers are the very low detector temperature of < -80 °C, a very small misalignment tolerance due to the fast Cook TMA optic design, the required high stiffness of the mechanical telescope interface and the exposure to significant mechanical loads. As the detectors can only be cooled by passive means, conductive and radiative heat flows from other subunits in proximity of the focal plate need to be minimized. A detailed discussion of the telescope interface was necessary at the beginning of the design process which resulted in a trade-off between a kinematic coupling and a flexure mount. A so-called kinematic “Maxwell mount” has been chosen due to its significantly higher stiffness, higher thermal resistance, reproducible and rather easy alignment and analytically predictable behavior. After a detailed investigation of candidate materials and their properties at quasi-cryogenic temperatures, a machinable AlN-BN ceramic has been selected for the focal plate's main structure due to its excellent match to the coefficient of thermal expansion of the telescope and CCD subcarrier material, its high thermal conductivity avoiding temperature gradients and greatly simplified manufacturing. Consequently, a design in accordance to the special characteristics of ceramics and the discussed requirements has been developed and documented, including the definition of the focal plate's mechanical interface to the telescope, thermal interface to a spacecraft radiator panel and electrical interface to the front-end electronic boxes. Finally, a finite element model has been used both for a first steady-state thermal analysis and a related investigation of thermally-induced stress and strain. Five representative load cases have been defined and studied, making clear that in a typical operational scenario, just one third of the total heat flow is dissipated by electronic components on the focal plate, while two thirds emerge from conductive and radiative heat flows from the telescope and front-end electronic. This emphasizes the need of a careful accommodation of all subunits and a well-studied structural and thermal design of the whole instrument. The results of the strain and displacement analysis validate the feasibility of the design concept and illustrate the excellent dimensional stability over the very large temperature range the assembly is going to experience. Still, the remaining deformation of the focal plate will cause a displacement of the active CCD areas on the optical axis, which has been estimated to be on the order of 13 - 15 µm in a worst case scenario compared to the position at room temperature. This estimate equals already about half of the initially defined misalignment budget, illustrating the great challenges the opto-mechanical design of the instrument is going to face.