  Wavefront sensing in solar adaptive-optics is currently done with correlating
Shack-Hartmann sensors, although the spatial- and temporal-resolutions of the
phase measurements are then limited by the extremely fast computing required to
correlate the sensor signals at the frequencies of daytime
atmospheric-fluctuations. To avoid this limitation, a new wavefront-sensing
technique is presented, that makes use of the solar brightness and is
applicable to extended sources. The wavefront is sent through a modified
Mach-Zehnder interferometer. A small, central part of the wavefront is used as
reference and is made to interfere with the rest of the wavefront. The contrast
of two simultaneously measured interference-patterns provides a direct estimate
of the wavefront phase, no additional computation being required. The proposed
optical layout shows precise initial alignment to be the critical point in
implementing the new wavefront-sensing scheme.
