  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.
