A technique to produce mirrors for the Auger Observatory

The Fluorescence Detector of the Pierre Auger Cosmic Ray Observatory will provide a measurement of the parameters of Extensive Air Showers with energy >10¹⁹ eV. The mirror of the Fluorescence Detector prototype has dimension ~3.6×3.6 m² and is composed of 49 trapezoidal segments of approximately 50×50 cm². Each segment is curved using a steel mold in which the glass sags in a controlled environment. The glass is then aluminized under vacuum and coated with oxide (Al₂O₃ ). The oxide thickness is chosen to maximize the reflectivity in the wavelength range of fluorescence light (300 nm <λ<400 nm). The 49 mirror segments plus 10 spare ones are now ready for installation. In this paper we discuss the production technique of the segments for the Torino prototype mirror