Mitigate the impact of ELT architecture on AO performance: Learn from todaýs telescopes to characterize and prevent the island effect

Island effects (IE) are critical piston difierential aberrations between neighbouring pupil segments on ground-based telescopes that degrade the quality of Adaptive optics (AO) images on 8m-class telescopes in good observing conditions. Due to telescope architecture, these residual errors lead to a loss of observing time, up to 20% as observed on the VLT during the direct imaging observations of planetary companions with SPHERE. These effects are expected to be even more critical with ELTs as these observatories will present more complex architectures. Measuring these effects however prove very challenging for most of the state-of-the-art AO wavefront sensors. A first successful diagnosis of these aberrations has recently been obtained on SPHERE with ZELDA, the Zernike wavefront sensor for the measurement of residual aberrations in coronagraphic systems. In this communication, we focus on the Asymmetric Pupil Fourier-wavefront sensor (APF-WFS) that relies on point source images to retrieve wavefront errors with an interferometric analysis. Assuming the archetypal four-quadrant aperture geometry in 8m class telescopes, we define aberration modes based on piston, tip, and tilt over the pupil quadrants and derive a calibration matrix for the LWE measurement in closed-loop operation with our sensor. Using the sensor mask present in Subaru/SCExAO, we perform first tests on a real system with an internal source and on sky, showing the ability of our sensor path to control these wavefront errors in the small aberration regime. Our approach looks very promising for the IE issue in the perspective of ELTs.