The elements of the coded mask have a transparency 0 or 1. The position detector receives the flux from the sky modulated by the mask. So a point-like source situed at infinity projects a gamma-ray light and shadow pattern on the detector which is characteristic of the source position in the sky
The mask is built with a 31x29 Uniformly Redundant Array : The mask is limited to 53x49 in order to avoid ambiguities.
The part of the sky that projects the full mask pattern is named completely coded field of view and its dimension is 4 degrees 45' x 4 degrees 20'.
Beyond there is the partially coded field of view. Its dimension is 18 degrees x 16 degrees. A point-like source projects an incomplete mask pattern on the detector. The sensitivity becomes weaker as the source is further from the axis of the telescope.
The position of a source is determined by comparing the observed pattern with all possible projections.
A pattern projected by a source in a given direction is completely different from the one projected by any other source. So it is possible to reconstruct mathematically the image of a source from the pattern projected on the detector.