A novel method for the absolute end-to-end calibration of the Auger fluorescence telescopes

The fluorescence detector technique is using the atmosphere as a calorimeter. Besides the precise monitoring of the parameters of the atmosphere a proper knowledge of the optical properties in the UV range of all optical components involved in the measurements of the fluorescence light is vital. Until now, the end-to-end calibration was performed with a 4.5 m² large, uniformly lit light source attached to the aperture of the telescopes. To improve the maintainability we propose an alternative setup where a small and low weight light source of known optical properties re-samples the measurement of the big light source piece by piece. This will be achieved by moving the light source based on an integrating sphere in two dimensions in front of the aperture. A prototype setup has been installed and we are currently optimizing the parameters of the system and the procedures. The aim is to reduce the effort for the procedure without diminishing the quality of the measurement. First measurements with this setup have already been performed and the measurements of the geometrical and optical properties of the light source are an ongoing activity. We present our calibration scheme and the first, preliminary results. (authors)