Optimizing the angular resolution of the HEGRA telescope system to study   the emission region of VHE gamma rays in the Crab Nebula
  The HEGRA system of imaging atmospheric Cherenkov telescopes provides for specially selected classes of events an angular resolution of better than 3'. By comparing the measured angular distribution of TeV gamma rays from the Crab Nebula with the distribution expected on the basis of Monte Carlo simulations, and with measurements of gamma rays from the point source Mrk 501, we conclude that the rms size of the VHE gamma-ray emission region in the Crab Nebula is less than 1.5'. 
HEGRA Collaboration, F. Aharonian, et al