Low-order wavefront control using a Zernike sensor through Lyot coronagraphs for exoplanet imaging

TL;DR

This paper introduces a Zernike wavefront sensor-based active control scheme for Lyot coronagraphs, effectively stabilizing low-order aberrations and dark hole contrast in exoplanet imaging, validated through simulations and laboratory experiments.

Contribution

It presents a novel active wavefront control method using a Zernike sensor with Lyot coronagraphs, demonstrating significant error reduction and contrast stabilization.

Findings

Wavefront error reduced by up to a factor of 9

Dark hole contrast stabilized around 7x10^-8

Validated control loop performance in laboratory tests

Abstract

Combining large segmented space telescopes, coronagraphy and wavefront control methods is a promising solution to produce a dark hole (DH) region in the coronagraphic image of an observed star and study planetary companions. The thermal and mechanical evolution of such a high-contrast facility leads to wavefront drifts that degrade the DH contrast during the observing time, thus limiting the ability to retrieve planetary signals. Lyot-style coronagraphs are starlight suppression systems that remove the central part of the image for an unresolved observed star, the point spread function, with an opaque focal plane mask (FPM). When implemented with a flat mirror containing an etched pinhole, the mask rejects part of the starlight through the pinhole which can be used to retrieve information about low-order aberrations. We propose an active control scheme using a Zernike wavefront sensor (ZWFS) to analyze the light rejected by the FPM, control low-order aberrations, and stabilize the DH contrast. The concept formalism is first presented before characterizing the sensor behavior in simulations and in laboratory. We then perform experimental tests to validate a wavefront control loop using a ZWFS on the HiCAT testbed. By controlling the first 11 Zernike modes, we show a decrease in wavefront error standard deviation by a factor of up to 9 between open- and closed-loop operations using the ZWFS. In the presence of wavefront perturbations, we show the ability of this control loop to stabilize a DH contrast around 7x10^-8 with a standard deviation of 7x10^-9. Active control with a ZWFS proves a promising solution in Lyot coronagraphs with an FPM-filtered beam to control and stabilize low-order wavefront aberrations and DH contrast for exoplanet imaging with future space missions.