Scalar Vortex Coronagraphs for Imaging Habitable Exoplanets

Citation

Desai, Niyati K. (2024) Scalar Vortex Coronagraphs for Imaging Habitable Exoplanets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wnma-x832. https://resolver.caltech.edu/CaltechTHESIS:05282024-210649680

Abstract

Of the over 5,600 exoplanets detected to date, less than 2% have ever been directly imaged. Direct imaging is crucial for the study of habitable exoplanets around Sun-like stars because it offers the potential to characterize their atmospheres and detect biosignatures. However, the extreme contrast between star and planet light poses immense challenges which coronagraphs aim to address. Future telescopes, like NASA's upcoming Habitable Worlds Observatory, necessitate coronagraphs capable of suppressing starlight to contrast levels of 10^-10 and operating in broadband light to directly image and characterize habitable planets. To meet these ambitious goals, innovations in focal plane mask technologies and wavefront sensing and control strategies are imperative.

This thesis investigates the viability of scalar vortex coronagraphs for direct imaging of habitable exoplanets. The first part of this thesis focuses on simulation efforts for modeling various coronagraph mask topographies and laboratory testing. Analysis of current scalar vortex topographies found phase wrapping is favorable over classic vortex designs. The chromatic performance of one such design — the wrapped staircase scalar vortex coronagraph — is investigated and a laboratory demonstration is presented.

Next, this thesis explores the behavior of different wavefront sensing and control methods combined with the wrapped staircase scalar vortex coronagraph. Three techniques were implemented on a high contrast imaging testbed and competitive performance between model-free and model-based techniques was found, particularly with increasingly complex mask designs.

Lastly, new scalar vortex mask designs which combine radially and azimuthally varying features are investigated. Specifically, the benefits of adding central phase dimples to scalar masks to improve broadband performance are explored. Hybrid designs incorporating phase dimples are found to suppress chromatic leakage and show substantial improvement in broadband contrast over current scalar vortex designs.

Overall, this thesis advances the understanding and development of scalar vortex coronagraphs for exoplanet direct imaging, explores their potential for future space telescopes and highlights avenues for further research and experimentation.

Thesis Files