Mean AOI, , computed from the AOI on the front and back filter side, for different footprint positions on the filter surface. The coordinate squares represent the largest area on the filter within which the footprint can move: footprints displayed at the grid corners correspond to the corners of the FPA. The linear obstructions by the three spider arms – holding the secondary mirror – are too thin to be seen in this representation. We actually worked with a 9 × 9 grid, but for this figure every second row and column was omitted for clarity.