Design and geometry of the NIRSpec IFU aperture. Left: image of the metal lid (the circular structure on the left) that is mounted on the MSA magnet arm and used to block or unblock the IFU entrance aperture (the square cutout in the center). We note that the picture was taken at ambient temperature, which is why the image slicer does not appear centered in the IFU entrance aperture. Right: schematic of the image slicer, with slice numbers matching those in Fig. 1.