On the optical alignment of the PLATO cameras

PLATO is an exoplanet hunting mission from the European Space Agency. It is a medium-class mission, with alaunch foreseen in 2026. Its prime objective is to uncover Earth-sized planets residing in the habitable zone of theirhost star. The payload consists of 26 cameras with a very wide field-of-view. While the operational temperatureof the cameras will be−80◦C, the focal plane of each camera will be integrated with its telescope assembly(bearing the optics) at room temperature. The degradation of the optical quality at ambient, combined with thedetector dark current and with the very high accuracy required from the alignment process bring a number ofinteresting challenges. In the present article, we review the alignment concept, present optical simulations of themeasurements at ambient along with their analysis, and present an error budget for the optical measurements.The derivation of this error budget is easily applicable to all optical measurements to be performed during thealignment, i.e. the definition of the best image plane at the operational temperature and the optical alignmentitself, at room temperature