The aperture is made from a 20-mm-diameter 3Dprinted disc with a 0.5-mm opaque plastic sheet glued to one face, Fig 6a . A 1-mm hole is drilled in the plastic disc, yielding a better-quality circular hole than could be achieved by 3D-printing. The disc is mounted in a centring ring with three threaded holes at 120º intervals. An M4 thumb-screw is inserted into each hole, allowing the aperture to be securely gripped between the three screw tips. The aperture is centred on the optical axis by adjusting the three thumb-screws. (Note, unless the cage plates are cut by hand, this is the only 3D-printed part that requires drilling).