Optimised FPMs with ten concentric rings, for the narrowband 875 filter on the left, and for the broadband z′ filter on the right. Note that the colourbars have different scales, and the X and Y axes are dependent on central wavelength of the filter. The axes are shown in both λ/D and physical units. Both masks have a radius of 0.75 λ/D, corresponding to 45.3 rm for the 875 filter and 47 µm for the z’ filter.