3D CAD (computer-aided design) rendering of the instrument cavity. In this orientation the telescope is below the assembly and observing towards the bottom. For clarity, we have added the principal light path and optical components to the rendering. The similarity with the actual flight hardware shown in Fig. B.2 is striking. It is evident from this figure that accurate models of water-ice contamination in the instrument cavity would need to account for a complex geometry of optical surfaces and water emitters. The NISP instrument begins immediately after the dichroic element and is completely covered in its own MLI; water in NISP will be mostly trapped and just redistributed during thermal decontamination. The large white structure to the right of NISP is its outward-facing radiator. It can be clearly seen in the photographs shown in Figs. B.4 and B.5. A high-resolution image is available at https://sci.esa.int/web/euclid/-/61034-euclid-payload-module. Copyright of the rendering: Airbus Defence and Space - Toulouse.