Low-Order Wavefront Sensing for Future Space-Based Coronagraphic Missions with a Sparse Aperture Mask

Abstract

In the quest for extra-terrestrial life, we have landed on the moon and sent rovers to other planets in the Solar System. The discovery of the first Jupiter-sized exoplanet, two decades ago, has led to an exponential increase in exoplanet research and public interest in existence of life outside our Solar System. Since then, indirect techniques have led to the discovery of hundreds of exoplanets but are limited in their scope of exoplanet characterization. Direct imaging, however, will provide the capability to thoroughly characterize an exoplanet's atmosphere, composition, temperature, size, and orbit. This would help scientists look for biomarkers of life. Diffraction due to the finite size of telescopes, $10^{10}$ difference in brightness between the host star and earth-like planets, and a small angular separation between them pose a huge challenge to direct imaging. The state of the art coronagraphs can create dark holes at smaller inner working angles, making it possible to image dim planets with smaller orbits. The contrast degradation caused by optical aberrations, however, limits the coronagraphs from achieving their theoretical limits. Robust estimation and control algorithms that can estimate and control these aberrations are being tested in various high-contrast imaging labs. However, to achieve the required contrast, it is also essential to estimate and control the dynamic aberrations that degrade the coronagraphic performance.

This thesis focuses on a novel technique that estimates these dynamic aberrations. This technique utilizes the starlight rejected by the coronagraph and a sparse aperture mask (SAM) to infer the dynamic aberrations. In this work, we will present the working principle of the SAM wavefront sensor, detail the SAM optimization, and compare its performance with other existing wavefront sensors. We will also present an adaptive estimation and control technique that can be used with a Kalman filter for line-of-sight (LoS) jitter estimation and control in the absence of the jitter frequency data. The work presented in this thesis would help designers pick low-order wavefront sensing and control hardware and software for future coronagraphic missions.