Paper detail

The Phase Induced Amplitude Apodizer and Nuller -- High transmission, high dispersion coronagraphy at 2λ/D

Context. Proxima Cen b is the prime target for the search of life around a nearby exoplanet by characterizing its atmosphere in reflected light. Due to the very high star/companion contrast (<1E-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 a VLT in the visible. It requires a coronagraph providing high contrast (< 1E-4 ) very close from the star (< 2 λ/D ) over a broad spectral range (~30%), with a high transmission of the companion (> 50%). We look for an optimal solution that takes benefit of 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 a pupil remapping optics with moderate apodization, combined to a single mode fiber integral field unit. It exploits the properties of single mode fibers to null the star light without reducing the companion coupling. The study focuses on a proper tolerance analysis and proposes a wavefront optimization strategy. A prototype is built to demonstrate its performance. Results. We show that the PIAAN can theoretically provide contrasts of 7E-7 and a transmission of 72% at 2 λ/D over a bandwidth of 30%. A prototype is built and characterized and the proposed wavefront control strategy is also demonstrated in the lab. We reach contrast levels of 3E-5 over the full bandwidth, as expected from the tolerance analysis. Conclusions. We demonstrated the potential of the new PIAAN coronagraph, from simulations to prototype. Its performance will eventually be limited by the XAO capabilities. It is the main coronagraph candidate for the RISTRETTO instrument to observe Proxima Cen b on the VLT and its first technology milestone.