Wavefront sensing and closed-loop control for the Fizeau interferometry testbed

Stellar Imager (SI) is a proposed 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 sensing and control is required to maintain alignment during science observations and after array reconfigurations. The Fizeau Interferometry Testbed (FIT), developed at the NASA/Goddard Space Flight Center, is being used to study wavefront sensing and control methodologies for Stellar Imager and other large, sparse aperture telescope systems. FIT initially consists of 7 articulated spherical mirrors in a Golay pattern, and is currently undergoing expansion to 18 elements. FIT currently uses in-focus whitelight sparse aperture PSFs and a direct solve phase retrieval algorithm to sense and control its wavefront. Ultimately it will use extended scene wavelength, with a sequential diversity algorithm that modulates a subset of aperture pistons to jointly estimate the wavefront and the reconstructed image from extended scenes. The recovered wavefront is decomposed into the eigenmodes of the control matrix and actuators are moved to minimize the wavefront piston, tip and tilt in closed-loop. We discuss the testbed, wavefront control methodology and ongoing work to increase its bandwidth from 1 per 11 seconds to a few 10's of Hertz and show ongoing results.