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Island effect (IE) aberrations are induced by differential pistons, tips, andtilts between neighboring pupil segments on ground-based telescopes, whichseverely limit the observations of circumstellar environments on the recentlydeployed exoplanet imagers (e.g., VLT/SPHERE, Gemini/GPI, Subaru/SCExAO) duringthe best observing conditions. Caused by air temperature gradients at the levelof the telescope spiders, these aberrations were recently diagnosed withsuccess on VLT/SPHERE, but so far no complete calibration has been performed toovercome this issue. We propose closed-loop focal plane wavefront control basedon the asymmetric Fourier pupil wavefront sensor (APF-WFS) to calibrate theseaberrations and improve the image quality of exoplanet high-contrastinstruments in the presence of the IE. Assuming the archetypal four-quadrantaperture geometry in 8m class telescopes, we describe these aberrations as asum of the independent modes of piston, tip, and tilt that are distributed ineach quadrant of the telescope pupil. We calibrate these modes with the APF-WFSbefore introducing our wavefront control for closed-loop operation. We performnumerical simulations and then experimental tests on a real system usingSubaru/SCExAO to validate our control loop in the laboratory and on-sky.Closed-loop operation with the APF-WFS enables the compensation for the IE insimulations and in the laboratory for the small aberration regime. Based on acalibration in the near infrared, we observe an improvement of the imagequality in the visible range on the SCExAO/VAMPIRES module with a relativeincrease in the image Strehl ratio of 37%. Our first IE calibration paves theway for maximizing the science operations of the current exoplanet imagers.Such an approach and its results prove also very promising in light of theExtremely Large Telescopes (ELTs) and the presence of similar artifacts.

Calibration of the island effect: Experimental validation of closed-loop focal plane wavefront control on Subaru/SCExAO