The aperure, sensitivity and precision of the Auger Fluorescence Detector

Detecting the fluorescence yield of light emitted by Nitrogen when a high energy Cosmic Ray shower devel- ops through the atmosphere, it is possible to perform a pre- cise calorimetric measurement of the shower energy profile. Fluorescence detectors were used successfully in the past. In the Auger hybrid system they complement the information coming from the large Surface Detector Array providing an important validation of the methods used to determine the energy and the identity of the primary Cosmic Ray. The de- sign parameters and the expected performance of the Auger Fluorescence Detector are discussed, with an overview of the factors determining their sensitivity and the precision of the