PSF formation along the “fine” direction. In the top section of the figure, the rays of an off-axis source illuminate the thick mask somewhat sideways and introduce a shadow on one side of the projection of the mask element on the detector (middle blue graph). When cross-correlating with the decoding mask (moving the decoding mask from left to right over the detector; see middle blue graph), this introduces (bottom blue graph) (1) a flat top to the PSF which is off-centered; (2) a lower PSF peak representative of a less sensitive area; and (3) a narrower PSF. The red dots indicate where the discrete cross-correlation would sample the PSF if the sampling is equal to the mask element size. The sampled cross-correlation looks like there is no shadowing, but its integral is actually smaller.