Extinction-controlled adaptive phase-mask coronagraph

European Southern Observatory, Alonso de Cordóva 3107, Vitacura, Santiago, Chile
e-mail: pbourget@eso.org

Received: 5 June 2012
Accepted: 26 October 2012

Abstract

Context. Phase-mask coronagraphy is advantageous in terms of inner working angle and discovery space. It is, however, still affected by drawbacks such as sensitivity to tip-tilt errors and chromatism. A nulling stellar coronagraph based on the adaptive phase-mask concept using polarization interferometry is presented in this paper.

Aims. Our concept aims at dynamically and achromatically optimizing the nulling efficiency of the coronagraph, making it more immune to fast low-order aberrations (tip-tilt errors, focus, etc.).

Methods. We performed numerical simulations to demonstrate the value of the proposed method. The active control system will correct for the detrimental effects of image instabilities on the destructive interference. The mask adaptability in size, phase, and amplitude also compensates for manufacturing errors of the mask itself and, potentially, for chromatic effects. Liquid-crystal properties are used to provide variable transmission of an annulus around the phase mask, but also to achieve the achromatic π phase shift in the core of the point spread function by rotating the polarization by 180°.

Results. We developed a new concept and showed its practical advantages using numerical simulations. This new adaptive implementation of the phase-mask coronagraph could advantageously be used on current and next-generation adaptive optics systems, enabling small inner working angles without compromising contrast.