When
Where
Title: Advancing Wavefront Control Algorithms for Dark Hole Creation and Maintenance
Abstract
To directly image exoplanets, advanced active and adaptive optics methods will be required to generate and maintain high contrast regions in an image plane. This is due to the fact that host-stars and exoplanets will have relatively small angular separations and very high flux ratios. As an example, an Earth-Sun analog system at a distance of 10 parsecs would have an angular separation of 100 mas and a flux ratio approaching 1E-10. This combination of challenges means an exoplanet will be obscured by diffracted starlight in the image plane. Many coronagraph concepts have been developed to reject the on-axis signal of a star, but manufacturing errors on the telescope and instrument optics will still result in diffracted residuals known as ``speckles'' that often limit coronagraphs to contrasts near the 1E-6 regime. To suppress these speckles, a family of active optics known as high-order wavefront sensing and control (HOWFSC) techniques have been developed using deformable mirrors (DMs). Most common among these algorithms is Electric Field Conjugation (EFC), but any of these techniques also require the wavefront to be stabilized for long observation periods using a low-order wavefront sensing and control (LOWFSC) scheme.
Here, various HOWFSC algorithms have been implemented and tested both with simulations and testbed experiments to understand and advance their capabilities. The first work studied the implicit EFC (iEFC) method in the context of the Roman Coronagraph by using an end-to-end physical optics model to simulate performance of iEFC and compare it with the standard EFC approach. We found that using iEFC, a contrast of 1E-8 can be achieved for a specific mode of the Roman Coronagraph and mitigate potential risks associated with EFC. The second study compared EFC with the newer adjoint-EFC (aEFC) technique for a vortex coronagraph. While EFC requires a Jacobian, which can take extended compute times to generate along with large memory allocation space, aEFC uses reverse mode algorithmic differentiation (RMAD) to solve for the DM commands with nonlinear optimization. For this work, an adjoint model for a vortex coronagraph is hand-derived and implemented using open-source programming and demonstrated aEFC achieving sub-1E-8 contrasts on the Space Coronagraph Optical Bench (SCoOB). Lastly, SCoOB was used to perform experiments that demonstrate how to combine a HOWFSC method with a Lyot-based LOWFSC scheme using a vortex coronagraph. Here, LOWFSC is used to correct for low-order aberrations injected with SCoOB's DM while iEFC is simultaneously used to create a dark hole. This work demonstrates that the iEFC algorithm can be calibrated and achieve 1E-8 contrasts while LOWFSC maintains a stable wavefront.