One problem with gratings is that their efficiency characteristic depends on the polarisation of the incident light. In general a given grating has its maximum efficiency at a different wavelength for the two polarisations. Thus `optimum' operation at a chosen wavelength involves a compromise.
One version of the PNS eliminates this problem by exploiting the fact that two images are required anyway. If a polariser is used to split the light then the two gratings can be optimised for a single polarisation:
In view "A" one of the spectrograph arms is clearly visible. The second (not shown) is projected towards the viewer.
View "B" shows the polarising beamsplitter edge-on (a glass plate is included to improve the match between the two arms, though this may not be necessary). The two gratings, one facing the viewer and one facing away, can be seen -- for clarity, the rest of both spectrographs is omitted in this view.
View "C" shows the instrument as seen from the telescope.