The Atmospheric Cherenkov Imaging Technique

VHE gamma-ray telescopes have an inherent advantage over space telescopes because of their very large collection areas (>40,000m² compared with <1m²). However, no anti-coincidence shield can be used in air shower detectors, so to reject the large background of cosmic rays, it is necessary to exploit the differences between the gamma-ray showers and the hadronic showers which constitute the major background. The atmospheric Cherenkov imaging technique was proposed in 1977 for this purpose ([Weekes & Turver 1977]). The basis of the technique is the use of an array of photomultiplier tubes (PMTs) in the focal plane of a large optical reflector to record the Cherenkov image of an air shower. The geometry of the detection is shown in Figure 10.

  

Figure 10: The imaging technique. The cone of acceptance of the camera intercepts the core of the air shower. The elliptical contour of a typical Cherenkov light image as seen in the focal plane of the Whipple Observatory camera (diameter 3.5$^\circ$) is shown on the right. Images of gamma-ray showers coming from a source parallel to the optic axis are narrow and point towards the center. Images from background cosmic rays are broader and have no preferred pointing direction.

There are basic differences in the appearance of the Cherenkov light image of a gamma-ray shower (from a source centered in the field of view) and that of a typical background hadron shower coming from a random direction. The differences arise for two reasons: a) physics: the smaller transverse momentum in electromagnetic interactions compared with hadronic interactions means that the Cherenkov radiating particles in the gamma-ray shower are, on average, closer to the direction of the primary; also there is no penetrating component in electromagnetic showers, so the local contribution to the light is small and the fluctuations in the shower image are far less; b) geometry: the orientation of the roughly elliptical image of a shower depends on the angle it makes with the optic axis of the telescope; a shower whose axis coincides with the optic axis (the trivial case) gives a round image that is centered on the axis; showers which are parallel to the axis but fall up to 120m away have elliptical images whose major axis intersect the optic axis; this holds true for gamma-ray and hadron showers but it is easier to characterize the axis of a gamma-ray shower since its image is more compact.

The power of the imaging technique was demonstrated using an array of photomultiplier tubes (PMTs) in the focal plane of the Whipple Observatory 10m optical reflector; the detection of the Crab Nebula was the first major success of the technique ([Weekes et al. 1989]; [Vacanti et al. 1991]). The background was rejected with 99.7% efficiency and the source location was accurate to 0.05$^\circ$.