Studying the precision of ray tracing techniques with Szekeres models
  The simplest standard ray tracing scheme employing the Born and Limber
approximations and neglecting lens-lens coupling is used for computing the
convergence along individual rays in mock N-body data based on Szekeres swiss
cheese and onion models. The results are compared with the exact convergence
computed using the exact Szekeres metric combined with the Sachs formalism. A
comparison is also made with an extension of the simple ray tracing scheme
which includes the Doppler convergence. The exact convergence is reproduced
very precisely as the sum of the gravitational and Doppler convergences along
rays in Lemaitre-Tolman-Bondi models. This is not the case when the models are
based on non-symmetric Szekeres models. For such models, there is a significant
deviation between the exact and ray traced paths and hence also the
corresponding convergences.
