  Focal plane wavefront sensing and control is a critical approach to reducing
non-common path errors between the a conventional astronomical adaptive optics
(AO) wavefront sensor (WFS) detector and science camera. However, in addition
to mitigating non-common path errors, recent focal plane wavefront sensing
techniques have been developed to operate at speeds fast enough to enable
"multi-WFS" AO, where residual atmospheric errors are further corrected by a
focal plane WFS. Although a number of such techniques have been recently
developed for coronagraphic imaging, here we present one designed for
non-coronagraphic imaging. Utilizing conventional AO system components, this
concept additionally requires (1) a detector imaging the focal plane of the WFS
light source and (2) a pupil plane optical chopper device that is non-common
path to the first WFS and is synchronized to the focal plane imager readout.
These minimal hardware requirements enable the temporal amplitude modulation to
resolve the sine ambiguity of even wavefront modes for both low, mid, and high
wavefront spatial frequencies. Similar capabilities have been demonstrated with
classical phase diversity by defocusing the detector, but such techniques are
incompatible with simultaneous science observations. This optical chopping
technique, however, enables science imaging at up to a 50% duty cycle. We
present both simulations and laboratory validation of this concept on SEAL, the
Santa Cruz Extreme AO Laboratory testbed.
