SIGMA - GRANAT

The SIGMA imaging principle
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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.

.Image projected on the detector
image:images/image.before.decoding.gif[]

.Image reconstructed
image:images/image.after.decoding.gif[]