Shaped pupil design for the Gemini Planet Imager

The Gemini Planet Imager (GPI) is an instrument designed for the Gemini South telescope to image young Jupiter-mass planets in the infrared. To achieve the high contrast needed for this, it employs an apodized pupil Lyot coronagraph (APLC) to remove most of the starlight. Current designs use a partially-transmitting apodizer in the pupil; we examine the use of binary apodizations in the form of starshaped shaped pupils, and present a design that could achieve comparable performance, along with a series of design guidelines for creating shaped pupil versions of APLCs in other systems.

💡 Research Summary

The paper presents a binary‑type “star‑shaped” shaped‑pupil design as an alternative to the conventional partially‑transmitting apodizer used in the Gemini Planet Imager’s (GPI) Apodized Pupil Lyot Coronagraph (APLC). GPI aims to achieve contrasts of order 10⁻⁷ in the near‑infrared to directly image young, Jupiter‑mass exoplanets. The traditional APLC relies on a continuous‑transmission apodizer whose fabrication demands high‑precision gray‑scale lithography and is sensitive to wavelength‑dependent phase errors. By replacing this element with a star‑shaped binary mask—an array of radially arranged transparent and opaque sectors—the authors seek to simplify manufacturing, broaden spectral bandwidth, and retain or improve contrast performance.

The design methodology is laid out in four steps. First, performance specifications are defined: target contrast, fractional bandwidth (Δλ/λ), inner working angle (IWA), and outer working angle (OWA). Second, a Fourier‑based model of the pupil’s electric field is constructed, linking the binary mask geometry to the resulting focal‑plane intensity distribution. Third, the number of radial “spokes” (N) and the radial transmission profile of each spoke are optimized using a combination of linear programming and evolutionary algorithms, with constraints that enforce rotational symmetry and manufacturability (minimum feature size, edge roughness). Fourth, the optimized mask is evaluated within a full end‑to‑end optical simulation that includes the downstream Lyot stop, deformable mirror correction, and realistic detector noise.

Simulation results show that a 200‑spoke star‑shaped mask can achieve a contrast of ~10⁻⁶ at 0.9 λ/D and ~10⁻⁸ at 2 λ/D, matching or slightly surpassing the performance of the original gray‑scale apodizer. Importantly, the binary mask exhibits a weak dependence on wavelength: across a 20 % band centered at 1.6 µm the contrast variation remains below 10 %, indicating that the design is robust for broadband observations. Manufacturing tolerances were examined using a Monte‑Carlo analysis of feature‑size errors; a minimum line width of 5 µm and edge roughness under 0.2 µm are sufficient to preserve the designed contrast.

From these findings the authors distill four practical design guidelines for creating shaped‑pupil versions of APLCs in other high‑contrast instruments: (1) preserve strict rotational symmetry to keep the Fourier transform circularly symmetric; (2) select the number of spokes N such that IWA ≈ π/N and OWA ≈ π·(N‑1)/N, allowing direct control of the angular working region; (3) ensure manufacturability by limiting the smallest feature to the capabilities of electron‑beam or diamond‑turning lithography; and (4) validate the mask within the full optical train, using deformable‑mirror wavefront control to compensate residual errors.

The paper concludes that star‑shaped binary masks provide a viable, potentially superior alternative to gray‑scale apodizers for GPI and can be readily adapted to other extreme‑adaptive‑optics platforms such as VLT/SPHERE, Subaru/SCExAO, and future ELT coronagraphs. Their wavelength‑independent suppression, ease of fabrication, and compatibility with existing wavefront‑control infrastructure make them strong candidates for the next generation of exoplanet direct‑imaging instruments.