The image on the left shows the coded mask of the ECLAIRs telescope and its portion (in yellow) that is projected onto the detector. The projected portion is different for each position of the image in the sky. This makes it possible to unambiguously find the position of the source from the detector image made of the photons from the source that have passed through the mask apertures (right).
The ECLAIRs telescope also receives a background signal called noise. In the same way as for the source, the image below shows the recording of the background photons that have passed through the mask apertures, in addition to the source photons.