Schematic representation of a typical model. The z axis of the model is shown as the black vertical arrow. In this example, the line of sight is rotated by an inclination of i = 35° relative to the z axis. The dotted lines show the extent of the largest aperture with radius a_max, while the dashed lines show the extent of the slab with half-width d inside which photons are taken into account for the SED. To ensure that the models account for all the emission in the intersection of the largest aperture and the slab (the dark shaded area), the edge of the grid (the thick solid circle) needs to have a radius of .