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.
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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'.
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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.
