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Box 1 Explanation
1.
Box 1 is a map of the sky near the assumed point-like X-ray source shown as a circle in the bottom left hand corner. The instrument axis of rotation is marked with a plus sign at the center of the box. The dark and light bars mark possible positions in the sky where detected X-ray photons could have originated. This is known as back projection since the detected X-ray photons are "back projected" onto the sky using possible arrival directions allowed by the grids. At any given time, there is a high probability that the source of the detected X-ray photons is somewhere in the white areas of the box, a low probability that the source is in the black areas, and an intermediate probability that it is in the gray areas. The bar pattern on the sky arises since X-rays from a source located in the white area could have passed unimpeded through slits in the upper and lower grids. X-rays from a source located in a black area would have had to pass through a slat in either the upper or lower grid in order to reach the detector. Since the slats are made of high density material, they are opaque to X-rays (at least for the lower energy photons). There is little chance that photons could reach the detector if they have to pass through a slat to get there. The gray area represents X-ray photons that have gotten through a slit on the front grid but approach a rear grid whose slats partly overlap the slits on the front grid due to the rotation of the instrument. Part of the photons will be blocked so that the region is neither all white (unblocked) or all black (blocked) but gray (partially blocked). See the section on grid pairs.
