RISTRETTO: a VLT XAO design to reach Proxima Cen b in the visible

RISTRETTO is the evolution of the original idea of coupling the VLT instruments SPHERE and ESPRESSO, aiming at High Dispersion Coronagraphy. RISTRETTO is a visitor instrument that should enable the characterization of the atmospheres of nearby exoplanets in reflected light, by using the technique of high-contrast, high-resolution spectroscopy. Its goal is to observe Prox Cen b and other planets placed at about 35mas from their star, i.e. 2λ/D at λ=750nm. The instrument is composed of an extreme adaptive optics, a coronagraphic Integral Field Unit, and a diffraction-limited spectrograph (R=140.000, λ=620-840nm). We present the RISTRETTO XAO architecture that reach the specification, providing contrasts down to 5× 10−5 at 2λ/D from the star in the visible, in the presence of atmosphere and low wind effect. This performance is allowed by a new two-sensors-one-dm architecture, some variations to the already known concepts of unmodulated pyWFS and zWFS, and exploiting to the maximum of their capabilities the state-of-the-art high speed, low noise cameras and fast DM. We present the result of end-to-end simulations, that demonstrate stable closed loop operation of an unmodulated pyramid and a zernike WFS (together), and in presence of low wind effect.