Demonstration of extreme wavefront sensing performance on the TPF high contrast imaging testbed

The Terrestrial Planet Finder (TPF) high contrast imaging testbed (HCIT) facilitates the investigation into the diversity of engineering challenges presented by the goal of direct exo-planet detection. For instance, HCIT offers a high-density deformable mirror to control the optical wavefront errors, a configurable coronagraph to control the diffracted light, and translatable cameras for measuring the focal and pupil planes before and after the coronagraph. One of the principle challenges for a coronagraphic space telescope is the extreme level of wavefront control required to make the very faint planet signal reasonably detectable. A key component, the extremely accurate sensing of the wavefront aberrations, was recently shown to be achievable using a sufficiently constrained image-based approach. In this paper, we summarize the experimental performance a focus-diverse phase-retrieval method that uses symmetrically defocus point-spread function measurements that are obtained about the coronagraph occulter focal plane. Using the HCIT, we demonstrate the high level of wavefront sensing repeatability achieved with our particular choices of focus diversity, data fidelity and processing methodologies. We compare these results to traceable simulations to suggest a partitioning of the error sources that may be limiting the experimental results.