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 eight spare ones, are now ready for installation. The authors discuss the production technique of the segments for the Torino prototype mirror