Illustration of the total throughput map for one full frame CCD. The dotted diagonal line shows the distance from the optical axis in degrees (ϑ) and the red dashed lines show the angular position of the stray light mask. We note that the FOV in the focal plane physically extents beyond ϑ_max (to ~19.6° indicated by the red dotted line) due to the effect of optical distortion cf. Sect. 6.5 and is followed by an exponential intensity decay of vignetting (modelled out to 20^° shown by the red solid line).