Active correction of low-order aberrations for exo-Earth imaging with future large observatories

Imaging exoplanets, especially exo-Earths, is a considerable technological challenge because of the high contrast, up to 10^-10, and the small angular separation, less than 50 milliarcseconds, between the star and the planet. Large telescopes represent an attractive option to achieve the resolution and sensitivity to access the light flux from the planet. These future observatories will use segmented primary mirrors, leading to a total diameter of more than 30 m on the ground and more than 6 m in space. In addition, combining these telescopes with a coronagraph that rejects starlight is an encouraging solution to make the contrast more favorable for exoplanet studies.Recent designs of coronagraphs coupled with active optics allow digging a dark hole in the image of an observed star, thus providing the required contrast levels. Nevertheless, many sources of instability, such as thermal variations, vibrations, or mechanical constraints, generate optical aberrations that prevent the maintenance of contrast during observations. The combined use of several wavefront sensors and deformable mirrors appears to be a critical means to control these aberrations in real time.By revisiting adaptive optics (AO) methods designed to correct for the effects of atmospheric turbulence on the image of a star, this thesis shows approaches for active stabilization of a coronagraph contrast. The proposed solutions cover various spatial or temporal frequency regimes of aberrations, in the case of large telescopes on the ground or in space. In the presented approach, a Zernike wavefront sensor (ZWFS) is implemented to take advantage of its excellent sensitivity and thus to optimally exploit the available photons.For future large, segmented ground-based telescopes, such as the Extremely Large Telescope or the Thirty Meter Telescope, the first solution presented in this thesis is to use a second stage AO inserted between the first stage and a coronagraph. This ZWFS-based strategy enables the measurement and the correction for several types of aberrations as close as possible to the coronagraph. These aberrations include non-common path aberrations that are invisible by the AO, uncorrected turbulence residuals, or cophasing errors of the primary mirror segments. Experimental tests on the MITHiC testbed have validated the control of these errors at the nanometric level.In the prospect of post-James Webb telescopes, a second approach is then proposed to stabilize a coronagraph designed for space applications. In this case, the ZWFS is installed in the light rejected by the coronagraph and containing information on the low-order aberrations. The sensor thus leaves the signal intact for observations. A first demonstration of a control loop is presented on the HiCAT testbed. The experiment validates the correction at the sub-nanometric level of aberrations introduced by testbed perturbations, as well as the blind stabilization of the contrast in the focal plane. In the perspective of a complete high-contrast imaging architecture, the correction loop is then combined with the dark hole algorithm with common deformable mirrors, paving the way for parallel use of multiple wavefront control systems.These coronagraph stabilization results represent an essential step towards the realization of observatories capable of imaging and spectrally analyzing Earth-like planets.