The recently introduced Spatial Spectral Compressive Spectral Imager (SSCSI) has been proposed as an alternative to carry out spatial and spectral coding using a binary ON-OFF coded aperture. In SSCSI, the pixel pitch size of the coded aperture, as well as its location with respect to the detector array, plays a critical role in the quality of image reconstruction. In this paper, a rigorous discretization model for this architecture is developed, based on a light propagation analysis across the imager.