Ground-based Coronagraphy with High-order Adaptive Optics

We summarize the theory of coronagraphic optics and identify a dimensionless fine-tuning parameter, ℱ, which we use to describe the Lyot stop size in the natural units of the coronagraphic optical train and the observing wavelength. We then present simulations of coronagraphs matched to adaptive optics (AO) systems on the Calypso 1.2 m, Palomar Hale 5 m, and Gemini 8 m telescopes under various atmospheric conditions and identify useful parameter ranges for AO coronagraphy on these telescopes. Our simulations employ a tapered, high-pass filter in spatial frequency space to mimic the action of adaptive wave front correction. We test the validity of this representation of AO correction by comparing our simulations with recent K-band data from the 241 channel Palomar Hale AO system and its dedicated Palomar High Angular Resolution Observer (PHARO) science camera in coronagraphic mode. Our choice of monochromatic modeling enables us to distinguish between underlying halo suppression and bright Airy ring suppression in the final coronagraphic images. For a given telescope-AO system combination, we find that AO systems delivering images with Strehl ratios below a threshold value are not well suited to diffraction-limited coronagraphs. When Strehl ratios are above this threshold, an optimized coronagraph with occulting image plane stops as small as 4λ/D creates a region around the AO target where dynamic range is significantly enhanced.