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Combining large segmented space telescopes, coronagraphy and wavefrontcontrol methods is a promising solution to produce a dark hole (DH) region inthe coronagraphic image of an observed star and study planetary companions. Thethermal and mechanical evolution of such a high-contrast facility leads towavefront drifts that degrade the DH contrast during the observing time, thuslimiting the ability to retrieve planetary signals. Lyot-style coronagraphs arestarlight suppression systems that remove the central part of the image for anunresolved observed star, the point spread function, with an opaque focal planemask (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 toretrieve information about low-order aberrations. We propose an active controlscheme using a Zernike wavefront sensor (ZWFS) to analyze the light rejected bythe FPM, control low-order aberrations, and stabilize the DH contrast. Theconcept formalism is first presented before characterizing the sensor behaviorin simulations and in laboratory. We then perform experimental tests tovalidate a wavefront control loop using a ZWFS on the HiCAT testbed. Bycontrolling the first 11 Zernike modes, we show a decrease in wavefront errorstandard deviation by a factor of up to 9 between open- and closed-loopoperations using the ZWFS. In the presence of wavefront perturbations, we showthe ability of this control loop to stabilize a DH contrast around 7x10^-8 witha standard deviation of 7x10^-9. Active control with a ZWFS proves a promisingsolution in Lyot coronagraphs with an FPM-filtered beam to control andstabilize low-order wavefront aberrations and DH contrast for exoplanet imagingwith future space missions.

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