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.
Highlights
Exploring the planetary paradigm to heretofore unseen regions including terrestrial planets represents an outstanding technological breakthrough for the forthcoming decades
The pupil exhibits 163 segments over a circular aperture shape to produce a complexity in the pupil similar to that of the ESO/extremely large telescopes (ELTs) primary mirror, with five times fewer segments, but 1.8 to 4.5 times more segments than any currently operating segmented telescopes, such as the SALT (Buckley 2001) or Keck (Nelson 1990) observatories, respectively, and a 30% central obscuration ratio with six spider struts separated by 60◦ for the ESO/ELT secondary mirror
The selected phase-induced amplitude apodization complex mask coronagraph (PIAACMC) design for segmented pupil experiment for exoplanet detection (SPEED) achieves starlight suppression by combining three elements: (i) a lossless apodization with aspheric mirrors to provide a point-spread function (PSF) with attenuated bright diffraction rings: the first mirror, namely PIAA-M1, compresses the beam into the desired pupil apodization profile and the second mirror, PIAA-M2, corrects optical path length errors that are introduced by the remapping; (ii) a complex focal plane mask that induces destructive interference inside the downstream pupil; (iii) a single Lyot stop that blocks diffracted light
Summary
Exploring the planetary paradigm to heretofore unseen regions including terrestrial planets represents an outstanding technological breakthrough for the forthcoming decades. A throughput improvement is critical because it leads to: (i) faster and better correction of dynamical speckles with wavefront control systems; (ii) better correction of quasi-static speckles with wavefront shaping systems; (iii) better sensitivity to planets in the post-processed images All these aspects improve contrast levels and detection sensitivity, and highlight the influence and importance of reaching small inner working angles (IWAs) and high throughput. While Knight et al (2017) demonstrated reliable means for evaluating the cosmetic quality of the FPM, as well as 1D-cut mask depth profile measurements and evaluation, we provide extensive analysis of our prototype with 2D mask depth measurements, hexagon by hexagon This allows us to model a numerical map of the purpose-built prototype that can be used in simulation for: (i) reliable performance evaluation, (ii) assessing the impact of manufacturing errors on the contrast. The paper is structured as follows: in Sect. 2, the FPM design optimization process is detailed, and we discuss the PIAACMC design simplification offered by the small IWA objective; in Sect. 3, the manufacturing process and the characterization of the prototype are presented; Sect. 4 investigates the expected performance of the purpose-built FPM with numerical simulations; in Sect. 5, we draw our conclusions
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