Fig. 3 shows the focal plane, in which the PHT-P and -S apertures and sizes of the PHT-C detector pixels are projected on the unvignetted field of view from the pyramidal mirror. Note that the gaps between the individual PHT-C detector pixels are real. These gaps may have influence on the observing mode to be chosen, in particular if the flux of a source or its spatial flux distribution has to be measured accurately.