  Cosmic shear probes the distribution of dark matter via gravitational lensing
of distant, background galaxies. We describe our cosmic shear survey consisting
of deep blank fields observed with the Keck II telescope. We have found biases
in the standard weak lensing analysis, which are enhanced by the elongated
geometry of the Keck fields. We show how these biases can be diagnosed and
corrected by masking edges and chip defects.
