We describe a method of implementing a common-path phase-shifting point diffraction interferometric wavefront sensor suitable for extreme adaptive optics. The sensor simultaneously gives two phase-shifted outputs which can be used to drive a phase-only wavefront corrector. The device can also give a null output which can be used to calibrate any scintillation. Simulations are performed showing the utility of the device and experimental results of a high speed single channel closed loop system are presented.
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