Non-common path aberration compensation and a dark hole loop with a pyramid adaptive optics system: Application to SAXO+

In ground-based high-contrast instruments, residual aberrations known as non-common path aberrations (NCPAs) limit detection performance, as they are unseen by the adaptive optics (AO) wavefront sensor but impact the astrophysical image, creating quasi-static speckles. SAXO+, the upgrade of the SAXO (SPHERE AO system) includes a second loop of AO downstream of the SAXO loop. This second loop is equipped with a near-infrared pyramid wavefront sensor whose nonlinearities, usually described with modal optical gains, might be challenging for removing quasi-static speckles. We investigated two methods of quasi-static speckle removal : NCPA compensation and a dark hole loop, behind a pyramid AO system, measuring the interest of compensating for the pyramid optical gains. We performed end-to-end numerical simulations of SAXO+ under various astrophysical conditions (seeing, star magnitude). We offset the pyramid wavefront sensor operating point to apply both the speckle suppression methods, calibrating or not calibrating the optical gains. We evaluated their performance by measuring the residual starlight in the coronagraph image. A by-product of our study is an on-sky calibration method of measuring the pyramid optical gains. Non-common path aberration compensation reduces the residual starlight in the coronagraph image by a factor of up to 20 for seeing between 0.7" and 1" for a bright star and a factor of 2 at 0.7" for a faint star. Optical gains compensation enhances the performance at poor seeing and small pyramid modulation radius with a bright star, but shows a useless or even negative impact due to estimation inaccuracies at faint targets. On the other hand, the dark hole loop reduces the residual starlight by a factor of up to 200. The optical gain calibration enhances the dark hole performance behind a single pyramid AO system but is useless behind the SAXO+ system. Our parametric study gives baseline values for the efficient control of the dark hole loop for the SAXO+ system.