<mono>Zemax</mono> ray tracing. Top panel: nominal light path through the NISP optical elements. We note that the dichroic is actually a part of the telescope and not of the NISP. The different colours show the light paths for three different sources in the field of view. For details of the optical layout and components (see Euclid Collaboration: Jahnke et al. 2025). Bottom panel: selecting the middle (green) rays of the top panel, we show how the light is internally reflected inside the dichroic to produce a ghost that is offset from the source position. A similar double reflection happens in the filter to produce the filter ghost (suppressed in this plot for clarity).