The image slicer is the most critical element of the PACS optics, in the figures below the effect of diffraction/vignetting by the entrance field stop and Lyot stop have been included. For the Lyot stop a worst-case loss of 10% is used. For the losses in the spectrometer the fraction of power arriving at the detector is shown in Figure 4.1.

Figure 4.1.  Diffraction throughput of the spectrometer optics; the diffraction losses mainly occur in the image slicer.