The Fluorescence Detector of the Pierre Auger Observatory

The Auger fluorescence detector is optimized to measure showers with energy above 10$^{19}$~eV over an area of 3000 km$^{2}$. The design calls for a system capable of detect a few tens of Watts of ultraviolet light produced more than 25 km away. To achieve that, large aperture mirrors and a extremely sensitive light detector system are required. This paper describes the telescope optics, light detection system, as well as calibration techniques and reconstruction algorithms. Examples of reconstructed events will be shown.