  Commonly used wavefront sensors, the Shack Hartmann wavefront sensor and the
pyramid wavefront sensor, for example, have large dynamic range or high
sensitivity, trading one regime for the other. A new type of wavefront sensor
is being developed and is currently undergoing testing at the University of
Arizona's Center for Astronomical Adaptive Optics. This sensor builds on linear
optical differentiation theory by using linear, spatially varying halfwave
plates in an intermediate focal plane. These filters, along with the polarizing
beam splitters, divide the beam into four pupil images, similar to those
produced by the pyramid wavefront sensor. The wavefront is then reconstructed
from the local wavefront slope information contained in these images. The ODWFS
is ideally suited for wavefront sensing on extended objects because of its
large dynamic range and because it operates in a pupil plane which allows for
on chip resampling even for arbitrarily shaped sources. We have assembled the
ODWFS on a testbed using 32 by 32 square 1000 actuator deformable mirror to
introduce aberration into a simulated telescope beam. We are currently testing
the system's spatial frequency response and are comparing the resulting data to
numerical simulations. This paper presents the results of these initial
experiments.
