Metrology sensor characterization and pointing control for the formation interferometer testbed (FIT)

StarLight is a NASA/JPL sponsored mission that will demonstrate formation flying and interferometric technologies needed for future missions such as Terrestrial Planet Finder (TPF). The StarLight stellar interferometer is composed of two spacecraft, a combiner spacecraft and a collector spacecraft, forming separated apertures. In this paper, the metrology pointing sensor of the StarLight ground instrument prototype is characterized and a siderostat pointing control loop is closed between two optical benches with a 10 meter separation. The Formation Interferometer Testbed (FIT) metrology pointing sensor (MPS) uses four photodiodes to detect gradients in the metrology laser caused by relative motion of the two optical benches. By differencing across the photodiodes, a measurement of the laser beam centroid can be obtained. In the flight design, this signal is passed to the combiner spacecraft, via an interspacecraft communication link, and used as a feedback signal to point a siderostat on the combiner spacecraft.