Ray tracing of the NI-OA. Shown are the obstructed beams for three sources located at the centre of the focal plane and at two opposing edges. The NI-OA consists of four lenses; the filter also acts as a lens, albeit only weakly. The dichroic element is located in the pupil plane of the telescope and is not part of NISP. We notice how the beam paths cover different parts of the filter surface, because – contrary to the dichroic – the filter is located outside the pupil plane. This leads to 10–23% of the passband variations in the focal plane due to local variations in the filter transmission. The AOI on the filter changes noticeably for the three sources; this is the dominant cause for passband variations.