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 FoV of a specific space instrument. As an example of the method, its application to two specific cases, i.e. the Metis coronagraph on board Solar Orbiter and the stereo camera STC on board BepiColombo, will be given.
Due to their proximity to the Sun, and also to Mercury for STC, both instruments operate under hard conditions subjected to high temperatures and significant temperature variations. Therefore, performance optical monitoring is crucial.