How mission requirements affect observations: case of the PICARD mission

Abstract : The scientific objectives of a space mission translate into instrumental developments and satellites operations to observe the astronomical objects of interest. The payload in its space environment is however subject to important thermal variations that affect the observations. This is well observed when images of the Sun are recorded with the constraint of having the solar rotational axis in a constant direction relatively to the camera reference system. Consequences are then clearly observed on the image positions that follow the thermal variations on the satellite orbit. This is particularly the case for the PICARD space mission. This phenomenon is similar to the defocus and motions of the images recorded with ground-based telescopes. We will present in this work some simulations showing this effect in space. We will then compare our results with real data obtained from the PICARD solar mission.