Automatized alignment of the focal plane assemblies on the PLATO cameras

Abstract

PLATO (PLAnetary Transits and Oscillation of stars) is a medium-class space mission part of the ESA Cosmic vision program. Its goal is to find and study extrasolar planetary systems, emphasizing on planets located in habitable zone around solar-like stars. PLATO is equipped with 26 cameras, operating between 500 and 1000nm. The alignment of the focal plane assembly (FPA) with the optical assembly is a time consuming process, to be performed for each of the 26 cameras. An automatized method has been developed to fasten this process. The principle of the alignment is to illuminate the camera with a collimated beam and to vary the position of the FPA to search for the position which minimizes the RMS spot diameter. To reduce the total number of measurements which is performed, the alignment method is done by iteratively searching for the best focus, decreasing at each step the error on the estimated best focus by a factor 2. Because the spot size at focus is similar to the pixel, it would not be possible with this process alone to reach an alignment accuracy of less than several tens of microns. Dithering, achieved by in-plane translation of the focal plane and image recombination, is thus used to increase the sampling of the spot and decrease the error on the merit function.

The PLATO Mission

“…The other way around, the heat dissipated by the CCDs is conductively transferred directly to the baffle that can then dissipate it into cold space. This TCS is also used to fine tune the cameras focus in space and make sure that each individual camera is being used in its best possible focus position (Pertenais et al, 2021a;Clermont et al, 2018). By changing the temperature of the camera, the characteristics of the optical system are changing leading to a slight shift of the focus.…”