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 m^2 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 lightweight 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 in the phase of optimizing the parameters
of the system and the procedures. The outcome which we are aiming for is
to reduce the effort for the procedure without diminishing the quality
of the measurement.
First measurements with this setup have been already performed and the
measurements of the geometrical and optical properties of the
light source are an ongoing activity. We will present our calibration
scheme and the first, preliminary results.