The fluorescence detector prototype for the Auger project: mechanical structure, optical system and filter

The Pierre Auger Observatory will study cosmic rays with energy exceeding 10¹⁹ eV. The experiment will measure properties of Extensive Air Showers with a hybrid detector consisting of a surface array and an atmospheric fluorescence telescope. This paper presents the characteristics and performance of the optical system for the fluorescence detector prototype. The system adopts a Schmidt camera design with a diaphragm to remove coma aberration. A large spherical mirror (approximately 3.6 m×3.6 m, with curvature radius 3.47 m), segmented in 49 trapezoidal elements, is supported by a mechanical structure, where segments are positioned on the meridians of the spherical cap. At the diaphragm, an absorption filter is installed to preferentially transmit the nitrogen fluorescence light. The construction technique, image characteristics and results of tests performed on the system are discussed