On-board target acquisition for CHEOPS

The CHaracterising ExOPlanet Satellite (CHEOPS) is the first ESA S-class and exoplanetary follow-up mission headed for launch in 2018. It will perform ultra-high-precision photometry of stars hosting confirmed exoplanets on a 3-axis stabilised sun-synchronous orbit that is optimised for uninterrupted observations at minimum stray light and thermal variations. Nevertheless, due to the satellites structural design, the alignment of the star trackers and the payload instrument telescope is affected by thermo-elastic deformations. This causes a high pointing uncertainty, which requires the payload instrument to provide an additional acquisition system for distinct target identification. Therefor a star extraction software and two star identification algorithms, originally designed for star trackers, were adapted and optimised for the special case of CHEOPS. In order to evaluate these algorithms reliability, thousands of random star configurations were analysed in Monte-Carlo simulations. We present the implemented identification methods and their performance as well as recommended parameters that guarantee a successful identification under all conditions.