ARC: an Achromatic Rotation-shearing Coronagraph

Abstract

Context. Because of the larger size of future telescopes, the star leakage effect due to the finite diameter of the stars will become a major problem for coronagraphs with very small inner working angles (IWA). Aims. To reduce this star leakage, we propose a new instrumental concept, the Achromatic Rotation-shearing Coronagraph (ARC), that provides a variable IWA that is easily tunable to ensure the observation of Solar-like systems with an extremely large telescope. Methods. The ARC belongs to the nuller class of coronagraphs. It takes advantage of the achromatic dark output of a Mach-Zehnder interferometer (MZI). Field rotators set in the arms of the MZI rotate the π-phase shifted images of the MZI by an angle ψ. An on-axis point source (the star) is nulled as twin-images of an off-axis point source (the planet) are formed. For ψ = π, the ARC is equivalent to the achromatic interfero-coronagraph. For small ψ, the twin images of the off-axis source are formed close to each another, and eventually nulled if too close. In this way, the ARC can get rid of star leakage while optimizing the transmission of the planet. We describe the compensation of polarization effects induced by field rotators. An application to diluted apertures is shortly presented. Results. We perform the mathematical analysis of the technique using Cartesian and polar coordinates, and assuming that the system is optically perfect. Analytic expressions are given for the leakage in both the focal and aperture planes. Simplified expressions for the contrast and signal-to-noise ratio (SNR) in the focal plane are given for a circular aperture, explicitly showing that the level of star leakage decreases as sin 2 (ψ/2), while the SNR increases as the inverse of sin(ψ/2). Conclusions. The ARC is appropriate for the detection of exoplanets and is not intended to provide images of the environment of stars.