Radio telescopes with off-axis feeds, such as the (E)VLA, suffer from "beam
squint" in which the two orthogonal circular polarizations sampled have
different pointing centers on the sky. Its effects are weak near the beam
center but become increasingly important towards the edge of the antenna power
pattern where gains in the two polarizations at a given sky position are
significantly different. This effect has limited VLA measurements of circular
polarization (Stokes V) and introduced dynamic range limiting, wide-field
artifacts in images made in Stokes I. We present an adaptation of the
visibility-based deconvolution CLEAN method that can correct this defect "on
the fly" while imaging, correcting as well the associated self-calibration. We
present two examples of this technique using the procedure "Squint" within the
Obit package which allows wide-field imaging in Stokes V and reduced artifacts
in Stokes I. We discuss the residual errors in these examples as well as a
scheme for future correction of some of these errors. This technique can be
generalized to implement temporally- and spatially-variable corrections, such
as pointing and cross-polarization leakage errors.Comment: 9 pages, 6 figures (five of them double), to appear in Astronomy &
Astrophysics (accepted: May 9, 2008). High-resolution versions of the figures
(gzipped, tar,gzipped) can be downloaded from
http://www.cv.nrao.edu/~juson/technical/squint/squint_figures.g
