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Thermal effects in PLATO are analyzed in terms of uniform temperature variations, longitudinal and lateral temperature gradients. We characterize these effects by evaluating the PSF centroid shifts and the Enclosed Energy variations across the whole FoV. These patterns can then be used to gauge the thermal behavior of each individual telescope in order to improve the local photometric calibration across the PLATO field of view.

PLATO stands for PLAnetary Transits and Oscillation of stars and is a Medium sized mission selected as M3 by the European Space Agency as part of the Cosmic Vision program. The strategy behind is to scrutinize a large fraction of the sky collecting lightcurves of a large number of stars and detecting transits of exo-planets whose apparent orbit allow for the transit to be visible from the Earth. Furthermore, as the transit is basically able to provide the ratio of the size of the transiting planet to the host star, the latter is being characterized by asteroseismology, allowing to provide accurate masses, radii and hence density of a large sample of extra solar bodies. In order to be able to then follow up from the ground via spectroscopy radial velocity measurements these candidates the search must be confined to rather bright stars. To comply with the statistical rate of the occurrence of such transits around these kind of stars one needs a telescope with a moderate aperture of the order of one meter but with a Field of View that is of the order of 50 degrees in diameter. This is achieved by splitting the optical aperture into a few dozens identical telescopes with partially overlapping Field of View to build up a mixed ensemble of differently covered area of the sky to comply with various classes of magnitude stars. The single telescopes are refractive optical systems with an internally located pupil defined by a CaF2 lens, and comprising an aspheric front lens and a strong field flattener optical element close to the detectors mosaic. In order to continuously monitor for a few years with the aim to detect planetary transits similar to an hypothetical twin of the Earth, with the same revolution period, the spacecraft is going to be operated while orbiting around the L2 Lagrangian point of the Earth-Sun system so that the Earth disk is no longer a constraints potentially interfering with such a wide field continuous uninterrupted survey.

The project PLAnetary Transits and Oscillations of stars (PLATO) is one of the selected medium class (M class) missions in the framework of the ESA Cosmic Vision 2015-2025 program. The mean scientific goal of PLATO is the discovery and study of extrasolar planetary systems by means of planetary transits detection. The opto mechanical subsystem of the payload is made of 32 normal telescope optical units (N-TOUs) and 2 fast telescope optical units (FTOUs). The optical configuration of each TOU is an all refractive design based on six properly optimized lenses. In the current baseline, in front of each TOU a Suprasil window is foreseen. The main purposes of the entrance window are to shield the following lenses from possible damaging high energy radiation and to mitigate the thermal gradient that the first optical element will experience during the launch from ground to space environment. In contrast, the presence of the window increases the overall mass by a non-negligible quantity. We describe here the radiation and thermal analysis and their impact on the quality and risks assessment, summarizing the trade-off process with pro and cons on having or dropping the entrance window in the optical train.

The project PLAnetary Transits and Oscillations of stars (PLATO) is one of the selected medium class (M class) missions in the framework of the ESA Cosmic Vision 2015-2025 program. The main scientific goal of PLATO is the discovery and study of extrasolar planetary systems by means of planetary transits detection. According to the current baseline, the scientific payload consists of 34 all refractive telescopes having small aperture (120mm) and wide field of view (diameter greater than 37 degrees) observing over 0.5-1 micron wavelength band. The telescopes are mounted on a common optical bench and are divided in four families of eight telescopes with an overlapping line-of-sight in order to maximize the science return. Remaining two telescopes will be dedicated to support on-board star-tracking system and will be specialized on two different photometric bands for science purposes. The performance requirement, adopted as merit function during the analysis, is specified as 90% enclosed energy contained in a square having size 2 pixels over the whole field of view with a depth of focus of +/-20 micron. Given the complexity of the system, we have followed a Montecarlo analysis approach for manufacturing and alignment tolerances. We will describe here the tolerance method and the preliminary results, speculating on the assumed risks and expected performances.