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This article describes the implementation of a focal plane based wavefrontcontrol loop on the high-contrast imaging instrument SCExAO (SubaruCoronagraphic Extreme Adaptive Optics). The sensor relies on the Fourieranalysis of conventional focal-plane images acquired after an asymmetric maskis introduced in the pupil of the instrument. This absolute sensor is used herein a closed-loop to compensate the non-common path errors that normally affectsany imaging system relying on an upstream adaptive optics system.This specificimplementation was used to control low order modes corresponding to eightzernike modes (from focus to spherical). This loop was successfully run on-skyat the Subaru Telescope and is used to offset the SCExAO deformable mirrorshape used as a zero-point by the high-order wavefront sensor. The paperprecises the range of errors this wavefront sensing approach can operate withinand explores the impact of saturation of the data and how it can be bypassed,at a cost in performance. Beyond this application, because of its low hardwareimpact, APF-WFS can easily be ported in a wide variety of wavefront sensingcontexts, for ground- as well space-borne telescopes, and for telescope pupilsthat can be continuous, segmented or even sparse. The technique is powerfulbecause it measures the wavefront where it really matters, at the level of thescience detector.

Closed-loop focal plane wavefront control with the SCExAO instrument