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

II. Concurrent operation with stroke minimization

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France
e-mail: raphael.pourcelot@oca.eu
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Thales Alenia Space, 5 Allée des Gabians, B.P. 99, 06156 Cannes la Bocca Cedex, France
⁴ Aix-Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁵ LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, 5 place Jules Janssen, 92195 Meudon, France
⁶ Hexagon Federal, 14291 Park Meadow Dr Suite 350, Chantilly, VA 20151, USA
⁷ Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08540, USA
⁸ NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA

Received: 1 September 2022
Accepted: 26 December 2022

Abstract

Context. Wavefront sensing and control (WFSC) will play a key role in improving the stability of future large segmented space telescopes while relaxing the thermo-mechanical constraints on the observatory structure. Coupled with a coronagraph to reject the light of an observed bright star, WFSC enables the generation and stabilisation of a dark hole (DH) in the star image to perform planet observations.

Aims. While WFSC traditionally relies on a single wavefront sensor (WFS) input to measure wavefront errors, the next generation of instruments will require several WFSs to address aberrations with different sets of spatial and temporal frequency contents. The multiple measurements produced in such a way will then have to be combined and converted to commands for deformable mirrors to modify the wavefront subsequently.

Methods. We asynchronously operate a loop controlling the high-order modes digging a DH and a control loop that uses the rejected light by a Lyot coronagraph with a Zernike wavefront sensor to stabilize the low-order aberrations. Using the HiCAT testbed with a segmented telescope aperture, we implement concurrent operations and quantify the expected cross-talk between the two controllers. We then present experiments that alternate high-order and low-order control loops to identify and estimate their respective contributions.

Results. We show an efficient combination of the high-order and low-order control loops, keeping a DH contrast better than 5 × 10⁻⁸ over a 30 min experiment and stability improvement by a factor of 1.5. In particular, we show a contrast gain of 1.5 at separations close to the DH inner working angle, thanks to the low-order controller contribution.

Conclusions. Concurrently digging a DH and using the light rejected by a Lyot coronagraph to stabilize the wavefront is a promising path towards exoplanet imaging and spectroscopy with future large space observatories.