Muon efficiency of the H.E.S.S. telescope
  The H.E.S.S. cameras require a precise and regular calibration over time, to reconstruct the gamma-ray characteristics. The different sub-systems used to determine the gain and the uniformity of the PMTs and their evolution with time are presented. Then, we focus on the absolute energy scale calibration, by using a full reconstruction of isolated muons recorded during normal observation. The method and the evolution of the absolute overall light collection efficiency are shown. 
R. Chalme-Calvet, M. de Naurois, J.-P. Tavernet (for the H.E.S.S.   Collaboration)
astro-ph.IM