26 September 2014 The vector apodizing phase plate coronagraph: prototyping, characterization and outlook
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The Apodizing Phase Plate (APP) is a phase-only pupil-plane coronagraph that suppresses starlight in a D-shaped region from 2 to 7 λ D around a target star. Its performance is insensitive to residual tip-tilt variations from the AO system and telescope structure. Using liquid crystal technology we develop a novel and improved version of the APP: the broadband vector Apodizing Phase Plate (vAPP). The vAPP prototype consists of an achromatic half-wave retarder pattern with a varying fast axis encoding phase structure down to 25 microns. The fast axis encodes the required phase pattern through the vector phase, while multiple twisting liquid crystal layers produce a nearly constant half-wave retardance over a broad bandwidth. Since pupil phase patterns are commonly designed to be antisymmetric, two complementary PSFs are produced with dark holes on opposite sides. We summarize results of the characterization of our latest vAPP prototype in terms of pupil phase reconstruction and PSF contrast performance. The liquid crystal patterning technique allows us to manufacture more extreme phase patterns than was possible before. We consider phase-only patterns that yield higher contrasts and better inner working angles than previous APPs, and patterns that produce dark regions 360 degrees around the PSF core. The possibility of including a phase ramp into the coronagraph is demonstrated, which simplifies the vAPP into a single optic. This additional phase ramp removes the need for a quarter-wave plate and a Wollaston prism, and enables the simplified implementation of a vAPP in a filter wheel at a pupil-plane location. Since the phase ramp is analogous to a polarization grating, it generates a (polarized) spectrum of a planet inside the dark hole, and thus allows for instantaneous characterization of the planet.
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Gilles P.P. L. Otten, Frans Snik, Matthew A. Kenworthy, Matthew N. Miskiewicz, Michael J. Escuti, Johanan L. Codona, "The vector apodizing phase plate coronagraph: prototyping, characterization and outlook", Proc. SPIE 9151, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 91511R (26 September 2014); doi: 10.1117/12.2056096; https://doi.org/10.1117/12.2056096

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