  Astronomical imaging using aperture synthesis telescopes requires
deconvolution of the point spread function as well as calibration of
instrumental and atmospheric effects. In general, such effects are
time-variable and vary across the field of view as well, resulting in
direction-dependent (DD), time-varying gains. Most existing imaging and
calibration algorithms assume that the corruptions are direction independent,
preventing even moderate dynamic range full-beam, full-Stokes imaging. We
present a general framework for imaging algorithms which incorporate DD errors.
We describe as well an iterative deconvolution algorithm that corrects known DD
errors due to the antenna power patterns and pointing errors for high dynamic
range full-beam polarimetric imaging. Using simulations we demonstrate that
errors due to realistic primary beams as well as antenna pointing errors will
limit the dynamic range of upcoming higher sensitivity instruments and that our
new algorithm can be used to correct for such errors. We have applied this
algorithm to VLA 1.4 GHz observations of a field that contains two ``4C''
sources and have obtained Stokes-I and -V images with systematic errors that
are one order of magnitude lower than those obtained with conventional imaging
tools. Our simulations show that on data with no other calibration errors, the
algorithm corrects pointing errors as well as errors due to known asymmetries
in the antenna pattern.

Astrophysics