Precision Formation Flying at Megameter Separations for Exoplanet Characterization

Starshade missions offer a near-term capability to measure the spectra of Earth-sized exoplanets, searching for possible bio-indicators. To function, a starshade and telescope separated by approximately 50Mm must align to the meter-level on the line to the target star. From the telescope׳s perspective, this alignment in turn requires sensing the bearing between target star and starshade to approximately 1 milli-arcsecond (5nrad). Previously, several fine bearing sensors have been proposed based on pupil images of the starshade׳s shadow. In this paper, a fine bearing sensor is presented based on measuring in the focal plane the bearing between a laser beacon on the starshade and the diffracted centroid of the target star that “leaks” around the starshade outside the science wavelengths. Coarse and medium bearing sensors are also introduced that allow for autonomous operation. The performance of extended Kalman filters using the bearing sensors is presented, as well as deadbanding performance in science mode.