  Island effect (IE) aberrations are induced by differential pistons, tips, and
tilts between neighboring pupil segments on ground-based telescopes, which
severely limit the observations of circumstellar environments on the recently
deployed exoplanet imagers (e.g., VLT/SPHERE, Gemini/GPI, Subaru/SCExAO) during
the best observing conditions. Caused by air temperature gradients at the level
of the telescope spiders, these aberrations were recently diagnosed with
success on VLT/SPHERE, but so far no complete calibration has been performed to
overcome this issue. We propose closed-loop focal plane wavefront control based
on the asymmetric Fourier pupil wavefront sensor (APF-WFS) to calibrate these
aberrations and improve the image quality of exoplanet high-contrast
instruments in the presence of the IE. Assuming the archetypal four-quadrant
aperture geometry in 8m class telescopes, we describe these aberrations as a
sum of the independent modes of piston, tip, and tilt that are distributed in
each quadrant of the telescope pupil. We calibrate these modes with the APF-WFS
before introducing our wavefront control for closed-loop operation. We perform
numerical simulations and then experimental tests on a real system using
Subaru/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 in
simulations and in the laboratory for the small aberration regime. Based on a
calibration in the near infrared, we observe an improvement of the image
quality in the visible range on the SCExAO/VAMPIRES module with a relative
increase in the image Strehl ratio of 37%. Our first IE calibration paves the
way for maximizing the science operations of the current exoplanet imagers.
Such an approach and its results prove also very promising in light of the
Extremely Large Telescopes (ELTs) and the presence of similar artifacts.
