Design and manufacturing of a multi-zone phase-shifting coronagraph mask for extremely large telescopes

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
e-mail: patrice.martinez@oca.eu
² Astrobiology Center, National Institutes of Natural Sciences, 2-21-1, Osawa, Mitaka, Tokyo, Japan
³ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁴ National Astronomical Observatory of Japan, Subaru Telescope, National Institutes of Natural Sciences, Hilo, HI 96720, USA
⁵ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
⁶ Silios Technologies, Rue Gaston Imbert prolonge, ZI de Peynier-Rousset, 13790 Peynier, France
⁷ Centre National d’Etudes Spatiales, 18 avenue Edouard Belin, 31401 Toulouse cedex 9, France
⁸ University of Arizona, Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85719, USA

Received: 11 October 2019
Accepted: 30 January 2020

Abstract

Context. High-contrast imaging of exoplanets around nearby stars with future large-segmented apertures requires starlight suppression systems optimized for complex aperture geometries. Future extremely large telescopes (ELTs) equipped with high-contrast instruments operating as close as possible to the diffraction limit will open a bulk of targets in the habitable zone around M-stars. In this context, the phase-induced amplitude apodization complex mask coronagraph (PIAACMC) is a promising concept for high-efficiency coronagraphic imaging at small angular separations with segmented telescopes.

Aims. The complex focal plane mask of the PIAACMC is a multi-zone, phase-shifting mask comprised of tiled hexagons that vary in depth. The mask requires micro-fabrication techniques because it is generally made of hundreds micron-scale hexagonal zones with depths ranging over a few microns. We aim to demonstrate that the complex focal plane mask of a PIAACMC with a small inner working angle can be designed and manufactured for segmented apertures.

Methods. We report on the numerical design, specifications, manufacturing, and characterization of a PIAACMC complex focal plane mask for the segmented pupil experiment for exoplanet detection facility.

Results. Our PIAACMC design offers an inner working angle of 1.3 λ/D and is optimized for a 30% telescope-central-obscuration ratio including six secondary support structures (ESO/ELT design). The fabricated reflective focal plane mask is made of 499 hexagons, and the characteristic size of the mask features is 25 μm, with depths ranging over ±0.4 μm. The mask sag local deviation is measured to an average error of 3 nm and standard deviation of 6 nm rms. The metrological analysis of the mask using interferential microscopy gives access to an in-depth understanding of the component’s optical quality, including a complete mapping of the zone depth distribution zone-depth distribution. The amplitude of the errors in the fabricated mask are within the wavefront control dynamic range.

Conclusions. We demonstrate the feasibility of fabricating and characterizing high-quality PIAA complex focal plane masks.