Because of its nicely chromatic behavior, Calcium Fluoride (CaF 2 ) is a nice choice for an optical designer as it can easily solve a number of issues, giving the right extra degree of freedom in the optical design tuning. However, switching from tablet screens to real life, the scarcity of information -and sometimes the bad reputation in term of fragility-about this material makes an overall test much more than a "display determination" experiment. We describe the extensive tests performed in ambient temperature and in thermo-vacuum of a prototype, consistent with flight CTEs, of a 200mm class camera envisaged for the PLATO (PLAnetary Transit and Oscillations of Stars) mission. We show how the CaF 2 lens uneventfully succeeded to all the tests and handling procedures, and discuss the main results of the very intensive test campaign of the PLATO Telescope Optical Unit prototype.

Abstract-PLATO (PLAnetary Transits and Oscillation of stars) is a candidate for the M3 Medium-size mission of the ESA Cosmic Vision programme (2015-2025 period). It is aimed atEarth-size and Earth-mass planet detection in the habitable zone of bright stars and their characterisation using the transit method and the asterosismology of their host star. That means observing more than 100 000 stars brighter than magnitude 11, and more than 1 000 000 brighter than magnitude 13, with a long continuous observing time for 20 % of them (2 to 3 years). This yields a need for an unusually long term signal stability. For the brighter stars, the noise requirement is less than 34 ppm.hr -1/2 , from a frequency of 40 mHz down to 20 μHz, including all sources of noise like for instance the motion of the star images on the detectors and frequency beatings.Those extremely tight requirements result in a payload consisting of 32 synchronised, high aperture, wide field of view cameras thermally regulated down to -80 °C, whose data are combined to increase the signal to noise performances. They are split into 4 different subsets pointing at 4 directions to widen the total field of view; stars in the centre of that field of view are observed by all 32 cameras. 2 extra cameras are used with color filters and provide pointing measurement to the spacecraft Attitude and Orbit Control System (AOCS) loop. The satellite is orbiting the Sun at the L2 Lagrange point. This paper presents the optical, electronic and electrical, thermal and mechanical designs devised to achieve those requirements, and the results from breadboards developed for the optics, the focal plane, the power supply and video electronics.

Spacecraft simulators have been used intensively at CNES (Centre National d'Etudes Spatialesthe French space agency) for many years during the development of new missions, and especially during their qualification phases. Up until now, the spacecraft simulators developed at CNES have shown themselves to be fairly representative when compared with actual spacecraft behaviour in flight and provide operators with acceptable computational performance (usually up to 5 times faster than real time). However, the thermal predictions computed by these types of simulator have always suffered from a lack of accuracy. At the current time, simulating the thermal characteristics of a spacecraft at real time frequency or even faster is very challenging. This paper describes a novel thermal simulation concept being developed at CNES as a joint effort between the "Thermal" and "Simulation" departments to improve the current situation. After describing the strategy, the paper reports on the performance obtained using this solution in terms of computing time and the extent to which it is representative, finally giving various future prospects for the application of this approach.

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