  The role of the Coulomb potential of the atomic core for creation of caustics
in the photoelectron momentum distribution for tunneling ionization in a
linearly polarized strong laser field, usually termed as Coulomb focusing, is
investigated within classical theory beyond the dipole approximation. Coulomb
focusing is addressed by analytical calculation of Coulomb momentum transfer to
the tunneled electron due to rescatterings, while applying perturbation theory
and classifying the recollisions either as fast or as slow. The accuracy of the
obtained analytical formulas for the Coulomb momentum transfer is investigated
by analytical derivation of asymptotic photoelectron momentum distribution and
its comparison to exact numerical calculations. With the help of the applied
analytical treatment, we analyze the origin of the counterintuitive
energy-dependent bend of the Coulomb focusing cusp in the photoelectron
momentum distribution in a linearly polarized laser field in the non-dipole
regime, and its scaling with the field parameters. The importance of high-order
recollisions is also investigated and they are shown to be responsible for a
decrease of the bend of the cusp at very low energies in this regime. In
general, the momentum transfer caused by high-order rescattering events, while
being a perturbation with respect to the instantaneous electron momentum at the
rescattering, is shown to induce a significant non-perturbative contribution to
the total momentum transfer.
