Measurement of a visibility. Two panels on the top-left: front/rear pair of grids for one subcollimator. Close inspection shows that they have slightly different pitch and orientation. The combined transmission for an on-axis point source yields a Moire pattern (bottom-left panel). A simulation for an off-axis source shows the location of detected photons (top-right panel), as seen by a detector with four large pixels (blue). Accumulated counts in the four pixels are shown in the center-right panel. Differences among these counts (bottom right) yield the real and imaginary components of the Fourier component independent of background. The sum over the four pixels yields the flux (+background), independent of the visibility. There is also a cross check that is independent of the visibility, flux, and background.