High-performance AO systems: impact of the chromatic pupil shift in the case of RISTRETTO

RISTRETTO is an instrument developed by the University of Geneva and the Laboratoire d'Astrophysique de Marseille, designed to characterise temperate rocky planets around M-dwarfs, such as Proxima b, using reflected light. Achieving this requires a contrast of approximately 10−7, necessitating a highly efficient extreme adaptive optics (AO) system. The high performance targeted by the RISTRETTO instrument translates into a very restrictive error budget for the AO system. New error terms that used to be neglected for previous AO systems must be carefully evaluated such as the Chromatic Pupil Shift (CPS). In this proceeding, we introduce the concept of CPS and we propose a method to compute it. Then, we inject this errors into-end to-end simulation to obtain its value. We found that the chromatic shift creates, for a science wavelength of 750nm and a zenith angle of 38°, a wavefront error of 5.24nm RMS at a 1400nm wavefront sensing wavelength and 2.53nm at 925nm. We conclude that the chromatic pupil shift is twice lower than the temporal error for a close-loop at 5kHz with a gain of 0.4 and an one frame delay but is in the order of other lower error terms such as aliasing and we must consider it in RISTRETTO's error budget.