Correcting for Quasi-static Wavefront Error Drifts in High-contrast and Wide-field Imaging Telescopes

Abstract

The study of both Exoplanets and Dark Matter provide valuable information on how we humans came to exist and whether we are alone in the universe. Directly imaging planets requires long integration times (hours) with a coronagraphic instrument due to the limited number of photons. The wavefront must be stable over a small field-of-view on the same time scale, which is often difficult in space due to time-varying wavefront errors from thermal gradients and other mechanical instabilities. In order to directly observe a habitable exoplanet, a series of masks are used to redistribute and block the starlight in the region where the planet might be; these masks are referred to as coronagraphs. Wavefront errors from misalignments and manufacturing defects of the optical surfaces must be corrected via deformable mirrors in order to achieve the required level of starlight suppression. Dark Matter mapping via cluster weak lensing, like exoplanet imaging, requires a very stable wavefront over hour-long integration times but also requires a large field-of-view. Dark Matter acts as a gravitational lens and the Dark Matter in a foreground galaxy cluster will distort the light travelling from the background galaxy cluster. The level of distortion can be used to infer the mass of Dark Matter present in the foreground galaxy cluster. When performing weak lensing measurements from a balloon-borne telescope, there are thermal and structural changes throughout the integration time which can introduce quasi-static wavefront error drifts that introduce errors in the Dark Matter measurement. The instruments required to directly image an exoplanet versus to measure Dark Matter via weak lensing are quite different but both suffer from the effects of quasi-static wavefront error drifts. I present solutions for exoplanet imaging that utilize active optics and focal plane wavefront sensing and control techniques. I also provide a study to inform the design of an active-optics system for a weak-lensing application. For directly imaging exoplanets, an overview of a Dark Zone Maintenance algorithm is provided which combines an Extended Kalman Filter as an estimator and Electric Field Conjugation as a controller. Also provided are laboratory demonstrations that show the algorithm maintaining the dark zone presence of various wavefront error drifts. Deformable Mirrors are used to inject wavefront error drifts both for monochromatic and broadband (10% bandwidth at 635nm) experiments. Low-photon cases are provided where the images obtained from the testbed are processed prior to being passed to the estimator to mimic the pixel sampling, photon rate, and detector noise expected on the Roman Space Telescope. I also demonstrate detection of fake planets injected into the testbed data using various post-processing techniques. Experiments are performed on the High-contrast imager for Complex Aperture Telescopes at the Space Telescope Science Institute, and both the In Air Coronagraph Testbed and Occulting Mask Coronagraph testbed at the Jet Propulsion Laboratory. For weak lensing, I perform a study of the stability of the image quality of the 2023 Super-pressure Balloon-borne Imaging Telescope flight results. These results are then used to inform the design for a new 1.3m optical-near-UV balloon-borne telescope (GigaBIT). GigaBIT has a 0.5 degree field of view over which the wavefront must be stable on timescales of an hour as the instrument moves in azimuth, elevation, and yaw to remain locked on a target. The Super-pressure Balloon-borne Imaging Telescope was designed as a pathfinder experiment and we use the thermal, mechanical, and pointing behaviour to determine a sufficient active optics design for the next generation instrument.