Focal plane modal estimation of high-order wavefront drifts on coronagraphic testbeds

Abstract

The upcoming Habitable Worlds Observatory requires precise wavefront stability over long observation periods to directly image Exo-Earths of interest, which are 10 orders of magnitude dimmer than their host star. Dark zone maintenance (DZM) has been proposed as a focal plane high-order wavefront sensing and control (HOWFSC) strategy to mitigate instabilities caused by thermal and mechanical vibrations that contribute to slow-evolving high-order wavefront drifts, which ultimately degrade contrast. Here we present a modal dark zone maintenance (MDZM) algorithm that uses an Extended Kalman Filter (EKF) to estimate the drifting wavefront modes in the basis of the deformable mirror (DM) actuators used for control. The estimator derives information across the entire focal plane from probe-pair images, addressing deficiencies of DZM, and can directly obtain a DM command, eliminating the need for Electric Field Conjugation (EFC) in this algorithm and the regularization parameter usually used for control. MDZM has been demonstrated in simulation, and in hardware on the Decadal Survey Testbed (DST) at NASA JPL. These experiments have maintained a normalized intensity (NI) of ∼ 3 × 10−10 for over 36 hours, and have reduced the gap between the photon-derived theoretical bounds as compared to DZM. Furthermore, it provides an estimate of naturally occurring on the testbed, providing potential system identification capabilities. To evaluate the robustness of MDZM, a parameter scan is conducted of MDZM on DST, spanning exposure times and drift rates. Additionally, realistic observing scenarios for HWO will provide information in the focal plane as broadband images, with a 20% band, rather than narrowband images previously used in MDZM of 2.5%. Hence, we tested the MDZM formulation using broadband sensing images with a 10% band, in simulation using the FALCO software package, modeling on DST and compared the results to performance using narrowband images.