The Atmospheric Cherenkov Imaging Technique for Very High Energy Gamma-ray Astronomy

One of the last frontiers of the gamma-ray sky is that characterized by the distribution of TeV photons. These photons can be detected relatively easily with ground-based detectors (constituting a TeV ”window” in the atmosphere) and thus the detection of sources did not have to await the availability of space platforms. In practice although the technology was available at an early date, it required the impetus of gamma-ray space astronomy to justify a major effort in a new discipline. Since it concerns the highest energy photons with which it is yet feasible to map the sky, it is of particular interest to high energy astrophysics. Any source of TeV photons must be associated with a cosmic particle accelerator and of inherent interest to high energy particle physicists as well as students of the cosmic radiation. To date almost all the observational results in the energy interval 100 GeV 100 TeV have come from observations using the so-called ”Atmospheric Cherenkov Imaging Technique (ACIT)” . Although considerable effort has been applied to the development of alternative techniques, they are not yet competitive and will not be considered here. In this description of the Atmospheric Cherenkov Imaging Technique there will be four sections: a historical review of the ACIT with emphasis on the