Optimization of coronagraph design for segmented aperture telescopes

Society of Photo-Optical Instrumentation Engineers 2017-09-13

The goal of directly imaging Earth-like planets in the habitable zone of other stars has motivated the design of coronagraphs for use with large segmented aperture space telescopes. In order to achieve an optimal trade-off between planet light throughput and diffracted starlight suppression, we consider coronagraphs comprised of a stage of phase control implemented with deformable mirrors (or other optical elements), pupil plane apodization masks (gray scale or complex valued), and focal plane masks (either amplitude only or complex-valued, including phase only such as the vector vortex coronagraph). The optimization of these optical elements, with the goal of achieving 10 or more orders of magnitude in the suppression of on-axis (starlight) diffracted light, represents a challenging non-convex optimization problem with a nonlinear dependence on control degrees of freedom. We develop a new algorithmic approach to the design optimization problem, which we call the ”Auxiliary Field Optimization” (AFO) algorithm. The central idea of the algorithm is to embed the original optimization problem, for either phase or amplitude (apodization) in various planes of the coronagraph, into a problem containing additional degrees of freedom, specifically fictitious ”auxiliary” electric fields which serve as targets to inform the variation of our phase or amplitude parameters leading to good feasible designs. We present the algorithm, discuss details of its numerical implementation, and prove convergence to local minima of the objective function (here taken to be the intensity of the on-axis source in a ”dark hole” region in the science focal plane). Finally, we present results showing application of the algorithm to both unobscured off-axis and obscured on-axis segmented telescope aperture designs. The application of the AFO algorithm to the coronagraph design problem has produced solutions which are capable of directly imaging planets in the habitable zone, provided end-to-end tel

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Optimization of coronagraph design for segmented aperture telescopes
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https://authors.library.caltech.edu/84865/1/104000H.pdf
Jewell, Jeffrey and Ruane, Garreth and Shaklan, Stuart and Mawet, Dimitri and Redding, Dave (2017) Optimization of coronagraph design for segmented aperture telescopes. In: Techniques and Instrumentation for Detection of Exoplanets VIII. Proceedings of SPIE. No.10400. Society of Photo-Optical Instrumentation Engineers , Bellingham, WA, Art. No. 104000H. https://resolver.caltech.edu/CaltechAUTHORS:20180216-092345021
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