Recent progress in vortex coronagraphy

The Optical Vortex Coronagraph (OVC) is a phase-based coronagraph that can enable high-contrast imaging observations very near bright stars and can make use of smaller telescope diameters than most alternative techniques. This paper first briefly describes the basic principles of operation of the vortex coronagraph, which applies an azimuthal phase spiral to the focal plane point spread function, and then turns to recent advances, both in understanding and in the needed technology development. In particular, vortex phase masks based on circularly-symmetric half-wave plates made of both liquid-crystal polymers and photonic crystals have now achieved very good contrast. Moreover, a dual-stage vortex coronagraph configuration can be used to achieve high contrast in the case of an on-axis telescope, i.e., in the presence of obscuration due to a secondary mirror and a secondary support structure. Further development of the relevant vortex techniques could potentially enable a range of high-contrast coronagraphic space missions, from an initial explorer class mission to a large flagship class exoplanet imaging mission. Of particular interest in this regard is the use of one of the two former 2.4 m National Reconnaissance Office telescopes for coronagraphic observations.