Optical efficiency from muons vs. ring center positions in the camera (left) and impact points on the mirror (right) for two selected H.E.S.S.-I telescopes. The gradients visible may be due to photon detector efficiencies or mirror reflectances due to abrasion by sand particles (Mitchell 2016). The right panel displays also the effects of an asymmetrically opening camera lid (blue area) and the camera support structure (green shadows, visually supported by black lines), which had not been included in the muon model. Figure reproduced from Bolz (2004).