Wavefront Control Techniques for the Direct Imaging of Exoplanets

Abstract

Over two decades ago, the first planet around a star other than the Sun was discovered. With each passing year, more and more such exoplanets are discovered as new technologies and methods of discovery are developed and enhanced. As these techniques continue to mature, humanity gets closer to finally being able to answer the question: are we alone in the universe? Improvements to Adaptive Optics (AO) have enabled ground-based observation to expand to including high-contrast imaging instruments called coronagraphs that are meant to make the direct imaging of exoplanet light possible.Direct imaging is a method of observation that gives astronomers the ability to determine if a planet exhibits signatures of life via spectroscopic analysis for biomarkers. This is a difficult task for three major reasons: the planet orbits very close to its host star if it is located in the so-called habitable zone, the planet light is up to 10^-10 times fainter than the host star light, and static and quasi-static aberration being present during the observation degrades both coronagraph performance and post-processing technique efficacy. In this dissertation, I explore two methods for estimating non-common path aberration (NCPA) in the science instrument of AO enabled, ground-based telescopes. The first is a method called the Differential Optical Transfer Function (dOTF), which is a simple, non-iterative, non-interferometric technique to estimate the complex amplitude field in the exit pupil of an optical system exploiting the properties of the functional derivative of the Optical Transfer Function. dOTF is demonstrated in both simulation and lab based experiments, showing several possible applications, including AO system self calibration, segment cophasing, and estimating systematic NCPA using an off-axis light source. The second method is known as Frazin's algorithm, which is a statistical regression framework that uses wavefront sensor (WFS) and science camera (SC) telemetry with advanced computational models of optical systems to estimate any NCPA and any present exoplanet signals. I develop the history of the method starting from its inception in 2013 and its extension to potential real-time use in 2018, followed by the conception of an improved version that is fully realizable. Three separate estimators are presented within the framework, and then are demonstrated via comprehensive end-to-end simulation of an AO system running at 1kHz frame rate with a Lyot Coronagraph in the science arm. Finally, preliminary future extensions of the work done on Frazin's algorithm are presented to guide future steps to evolve the method to improve the current limits of ground-based direct imaging of exoplanets.