  We analyse a method to compensate for anisotropy effects in the spatial
distribution of parametric down-conversion (PDC) radiation in bulk crystals. In
this method, a single nonlinear crystal is replaced by two consecutive crystals
with opposite transverse walk-off directions. We implement a simple numerical
model to calculate the spatial distribution of intensity and correlations, as
well as the Schmidt mode structure, with an account for the anisotropy.
Experimental results are presented which prove the validity of both the model
and the method.
