Adaptive High-order Wavefront Control Algorithms for High-contrast Imaging on the Decadal Survey Testbed

Introduction: This work will increase the TRL of continuous high-order wavefront sensing and control (HOWFSC) systems for space-based coronagraphs. NASA exoplanet missions need to reach a contrast of 1e-10 and maintain it for many-hours-long observations to detect and characterize exoplanets. “Coronagraph contrast stability” is a tier 1 gap for Exoplanet Exploration (ExEP) in NASA’s 2019 Astrophysics Biennial Technology Report. The Astro2020 decadal survey identified “adaptive wavefront control algorithms that are more efficient and improve tolerance to instabilities” as one of the “high-priority technologies to mature” for the recommended IR/O/UV Large Strategic Mission. The Coronagraph Instrument (CGI) on the 2.4-meter Roman Space Telescope (RST) plans to mitigate contrast instability by slewing to a bright reference star several times a day for re-calibrating. However, this introduces its own instability as the solar angle of the telescope changes, affecting thermal distribution. Slewing to a reference star is still feasible with RST’s target contrast, but may be infeasible for the larger Astro2020-recommended 6-meter IR/O/UV telescope with 2-3 orders of magnitude higher contrast. High-order wavefront perturbations would need to be continuously controlled (dark hole maintenance) on time scales of minutes. In experiments on the High-contrast Imager for Complex Aperture Telescopes (HiCAT) at the Space Telescope Science Institute, dark hole maintenance rejected artificially-introduced time-dependent wavefront aberrations at the 1e-8 contrast level, at TRL 3. We propose increasing the TRL to 4 by demonstrating advanced HOWFSC algorithms at the 1e-9 to 1e-10 contrast on the Decadal Survey Testbeds (DST) at JPL. Approach: We will explore the trade space of dark hole maintenance (DHM) on HiCAT at 1e-8 contrast, so that we optimize our time on DST for a true TRL 4 demonstration. On HiCAT, we will first run our basic DHM scheme for a large space of parameter such as wavefront drift magnitude and photon flux. We will then compare our experimental results to theory and simulation of HiCAT. In order to improve agreement between the three, we will implement advanced algorithms such as modal DHM (to improve performance relative to theory) and system-identification (to improve match with simulations). We will then bring basic DHM to TRL 4 by demonstrating it on DST at 1e-10 contrast and comparing its performance to theory. We will also validate the advanced algorithms on DST. Expected Results: 1) TRL 4 for continuous contrast maintenance via demonstration in a vacuum laboratory at a contrast level necessary to detect Exo-earths and analysis of (closed-loop) contrast degradation in the presence of various wavefront instabilities. 2) Assessment of the benefits of using advanced/adaptive algorithms, including modal wavefront control and system identification techniques. 3) Observing scenario data package (similar to OS 9 for Roman Coronagraph) scaled to represent the IR/O/UV mission’s baseline. Significance: This work will lead to an improved understanding of the risks and rewards associated with wavefront control on a space coronagraph. We will assess how well we can maintain contrast in the presence of wavefront instabilities in an environment relevant to the IR/O/UV telescope. New HOWFSC approaches will lead to better contrast stability and higher science yield, and reduce missions cost and risk as larger structures in the observatory would not need to be as stable.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

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