Simulations for In-Flight Stellar Calibration Aimed at Monitoring Space Instruments’ Optical Performance

Stellar in-flight calibrations have a relevant impact on the capability of space optical instruments, such as telescopes or cameras, to provide reliable scientific products, i.e., accurately calibrated data. Indeed, by using the in-flight star images, instrument optical performance can be checked and compared with the on-ground measurements. The analysis of star images carried out throughout the entire lifetime of the instrument in space will enable tracking changes in instrument performance and sensitivity due to degradation or misalignment of the optical components. In this paper, we present the concept, the necessary input and the available outputs of the simulations performed to predict the stars visible in the field of view (FoV) of a specific space instrument. As an example of the method, its application to two specific cases, the Metis coronagraph onboard Solar Orbiter and the stereo camera STereo Channel (STC) onboard BepiColombo, are given. Due to their proximity to the Sun, and to Mercury for STC, both instruments operate under harsh environmental conditions in terms of radiation exposure ((e.g., cosmic rays and SEP), high temperatures and significant temperature variations. Therefore, it is crucial to monitor their optical performances.