Item

Abstract

Since one year all 4 H.E.S.S. telescopes are fully operational in Namibia and have contributed to the discovery of new sources of cosmic rays. In near future another bigger telescope with a mirror area of 600 m2 of will be built. The 2F-setup for measuring the reflectivity of single mirrors was upgraded with an optical chopper and a Lock-In amplifier, so that it is possible to measure in daylight and in a greater distance. Since the construction of the first telescope, end of 2001, the mirrors have been exposed to the environmental factors. In order to quantify this effect on the reflectivity of the mirrors, two setups were established to measure the reflectivity of each telescope. In the first method, called 2F-setup, the telescope is illuminated by a LED in 30 m distance to the telescope and the reflected light is measured with a photodiode mounted on a scan table. Using the second method, called 20F-setup, the light of a LED in 300 m distance is reflected onto the camera lid and is measured with a photodiode, the telescope scans over the light spot. Both methods need no devices like calibrated photodiodes, lightsources and mirrors with a known reflectivity. This thesis describes in detail both methods and the systematic tests. The measured reflectivity is in the range of (69-75)%. The 2Fsetup will be used to measure routinely the reflectivity of the H.E.S.S telescopes.