  A slicer mirror is a complex surface composed by many tilted and decentered
mirrors sub-surfaces. The major difficulty to model such a complex surface is
the large number of parameters used to define it. The Zemax's
multi-configuration mode is usually used to specify each parameters (tilts,
curvatures, decenters) for each mirror sub-surface which are then considered
independently. Otherwise making use of the User-Defined Surface (UDS-DLL) Zemax
capability, we are able to consider the set of sub-surfaces as a whole surface.
In this paper, we present such a UDS-DLL tool comparing its performance with
those of the classical multi-configuration mode. In particular, we explore the
use of UDS-DLL to investigate the cross-talk due to the diffraction on the
slicer array mirrors which has been a burden task when using
multi-configuration mode.
