Measurement of the radial distribution of Cherenkov light generated by   TeV gamma-ray air showers
  Using air showers induced by TeV gamma-rays from Mrk 501, the radial distribution of Cherenkov light is investigated. The shower geometry is reconstructed from the stereoscopic shower images obtained with the telescopes of the HEGRA IACT system. We observe a change in the shape of the light pool with shower energy, and with zenith angle. Data are well reproduced by Monte-Carlo air shower simulations. 
HEGRA Collaboration, F. Aharonian et al