Minimal Stereoscopic Analysis for Imaging Atmospheric Cherenkov   Telescope Arrays
  The trajectory of a primary gamma-ray detected with an array of at least four atmospheric Cherenkov imaging telescopes can be reconstructed from the shower image centroid positions and geometrical considerations independent of the primary energy. Using only the image centroid positions some cosmic-ray discrimination is also possible. This minimal approach opens the possibility of pushing the analysis threshold to lower values, close to the hardware threshold. 
S. LeBohec, C. Duke and P. Jordan