The Phase Induced Amplitude Apodizer and Nuller

High transmission, high dispersion coronagraphy at 2λ/D

¹ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Division of Space and Planetary Sciences, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

^★ Corresponding author: nicolas.blind@unige.ch

Received: 14 March 2025
Accepted: 26 August 2025

Abstract

Context. Proxima Cen b is the prime target in a search for life around a nearby exoplanet based on characterization of its atmosphere in reflected light. Due to its very high star-companion contrast (≤10⁻⁶), High Dispersion Coronagraphy is the most promising technique to perform such a characterization.

Aims. With a maximum separation of 37 mas, Proxima b can be observed with one of the VLT’s UT in the visible. This requires a coronagraph that provides high contrast (≤10⁻⁴) very close to the star (≤2 λ/D ) over a broad spectral range (∼30%)and with high transmission of the companion (≥50%). We searched for an optimal solution that benefits from the properties of single-mode fibers.

Methods. We introduce the Phase Induced Amplitude Apodizer and Nuller (PIAAN), a coronagraphic integral field unit, designed to feed a diffraction-limited spectrograph. It uses pupil remapping optics with moderate apodization combined with a single-mode fiber integral field unit. It exploits the properties of single-mode fibers to null starlight without reducing the companion coupling. The study focuses on a proper tolerance analysis and proposes a wavefront optimization strategy. A prototype has been built to demonstrate its performance.

Results. We show that the PIAAN can theoretically provide contrasts of 7 · 10⁻⁷ and a transmission of 72% at 2 λ/D over a bandwidth of 30%. We built and characterized a prototype and demonstrate the proposed wavefront control strategy. The prototype reached contrast levels of 3 · 10⁻⁵ over the full bandwidth, as expected from the tolerance analysis.

Conclusions. The new PIAAN coronagraph achieves performances that enable the study of exoplanets in reflected light. It is the main coronagraph candidate for the RISTRETTO instrument to observe Proxima Cen b on the VLT and its first technological milestone. It is also a solution of high interest for exploiting the spatial resolution and the large collecting area of future ELTs.