Geomagnetic effects on atmospheric Cerenkov images

Atmospheric Cerenkov telescopes are used to detect electromagnetic showers from primary gamma-rays of energy similar to 300 GeV up to approximate to 1 0 TeV and to discriminate these from cascades due to hadrons using the Cere nkov images. The geomagnetic field affects the development of showers and i s shown to diffuse and distort the images. When the component of the field normal to the shower axis is sufficiently large (> 0.4 G) the performance o f gamma-ray telescopes may be affected, although corrections should be poss ible.