SAXO+, the second-stage adaptive optics for SPHERE: high contrast performance analysis

Abstract

The SAXO+ system is a planned second-stage Adaptive Optics (AO) upgrade to the VLT/SPHERE eXtreme Adaptive Optics (SAXO) system. This consists of adding a second adaptive optics stage using a near-infrared pyramid wavefront sensor to record images of fainter exoplanets around redder stars. In this work, we compare the performance of SAXO and SAXO+. We look for the optimal values of the key system parameters of SAXO+ for various science cases and turbulence conditions. We performed numerical simulations using the COMputing Plateform for Adaptive opticS Systems (COMPASS), an end-to-end (E2E) adaptive optics simulation tool. We simulated perfect coronagraph images of an on-axis point source, and we minimized the residual starlight intensity between 3 and 5 λ/D as a performance criterion. The explored parameter space includes science cases, turbulence conditions, and key system parameters. This study shows that the future SAXO+ system will outperform the current SAXO system in all studied cases. The main drivers of the SAXO+ upgrade are: 1) to increase the contrast closer to the optical axis at short separations ranging from 0.1 to 0.5 arcseconds; 2) to observe fainter and redder stars compared to those observed with SPHERE; 3) to decrease the impact of fast-evolving turbulence thanks to a faster AO loop and the use of predictive control algorithms. SAXO+ will also serve as a technological demonstrator for the future Planetary Camera and Spectrograph (PCS) of the European ELT.