Fizeau Interferometry Testbed: Wavefront Control

Stellar Imager (SI) is a potential NASA space-based UV imaging interferometer to resolve the stellar disks of nearby stars. SI would consist of 20 30 separate spacecraft flying in formation at the Earth-Sun L2 libration point. Onboard wavefront control would be required to initially align the formation and maintain alignment during science observations and after array reconfiguration. The Fizeau Interferometry Testbed (FIT) is a testbed currently under development at the NASA/Goddard Space Flight Center to develop and study the wavefront control methodologies for Stellar Imager and other large, sparse aperture telescope systems. FIT consists of 7 articulated spherical mirrors in a Golay pattern, expandable upto 30 elements, and reconfigurable into multiple array patterns. FIT’s purpose is to demonstrate image quality versus array configuration and to develop and advance the wavefront control for SI. FIT uses extended scene wavelength, focus and field diversity to estimate the wavefront across the set of apertures. The recovered wavefront is decomposed into the eigenmodes of the control matrix and actuators are moved to minimize the wavefront piston, tip and tilt. Each mirror’s actuators are 3 degrees of freedom, however, they do not move each of the mirrors about a point on each mirrors surface, thus the mapping from wavefront piston, tip/tilt to mirror piston, tip/tilt is not diagonal. We initially estimate this mapping but update it as part of wavefront sensing and control process using system identification techniques. We discuss the FIT testbed, wavefront control methodology, and show initial results from FIT.