Characteristics and brightness of Cerenkov shower images for gamma-ray astronomy near 1 TeV

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Abstract
The appearance of Cerenkov light signals from high-altitude air showers has been calculated and typical differences between the images of showers produced by conventional gamma rays from a point or diffuse source and by background cosmic rays are illustrated by examples of high and low resolution images which show individual photo-electrons, and indicate the advantage that higher resolution offers. A geometrical perspective view of the shower in space mainly determines what is seen when a shower is viewed from more than one location in the light pool. When the emitting particles are far from the observer, the intensity as a function of radius from the axis is dominated by a concave plateau rising to a Cerenkov ring near 140 m. When near, it is humped in the centre and, especially in hadronic showers, ragged, as penetrating muons produce local light peaks. The effects of high and low elevation angles, altitude, filters and field of view are considered. The median energies of the emitting particles are approximately 145 MeV overall or approximately 420 MeV for the 'bright images'. The height of most emission is approximately 9 km (depth approximately 300 g cm-2) for vertical showers.