  The ingot wavefront sensor (I-WFS) has been proposed, for ELT-like apertures,
as a possible pupil plane WFS, to cope with the geometrical characteristics of
a laser guide star (LGS). Within the study and development of such a WFS,
on-going in the framework of the MAORY project, the final purpose of the I-WFS
simulation is to estimate its performance in terms of wavefront aberration
measurement capability. The first step of this analysis is to translate
incoming wavefronts into the three pupil images, produced by the optical
system. The intrinsic geometrical characteristics of the ingot optical element,
designed to be coupled with the LGS elongated image, make the system
conceptually different with respect to other pupil WFSs (like the Pyramid WFS,
P-WFS) also in terms of the simulation technique to be selected, within the
ones which can be found in literature. In this paper, we aim to report the
considerations and derivations which led to the selection of a ray-tracing
method for ingot pupil images simulation, and the geometrical assumptions and
approach made to optimize the computing time.
