Fluorescence detectors (FD) measure the longitudinal profile of an air shower in a calorimetric way. The FD of the Auger Observatory is composed of four observation sites, called `"eyes'". Their names are Los Leones, Los Morados, Loma Amarilla, and Coiheuco. Each eye consists of six independent telescopes. The optical system is composed of a filter at the entrance window of each FD telescope, a circular aperture, a corrector ring, a mirror and a camera with photomultipliers.
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