Layout of four of the FPI systems discussed in this paper, here shown scaled individually to highlight the similarities in their optical designs. The systems shown are (from top to bottom) CHROMIS (overall length 1.579 m, FPI clear aperture diameter 75 mm), CRISP2 (1.824 m, 98 mm), EST-B (4.4-4.7 m, 135 mm), and EST-V (4.5-4.7 m, 180 mm). Symbols used: F1-F3 are focal planes, L1-L4 lenses, P1-P2 pupil planes, PBS polarising beam splitter. The vertical scale of all panels has been expanded 2 × for clarity. For further details about the systems, see Tables 1 and 3.