2 v 1 7 A pr 1 99 9 Geomagnetic effects on atmospheric Čerenkov images

Atmospherič Cerenkov telescopes are used to detect electromagnetic showers from primary gamma rays of energy ∼ 300 GeV – ∼ 10 TeV and to discriminate these from cascades due to hadrons using thě Cerenkov images. The geomagnetic field affects the development of showers and is 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 of gamma ray telescopes may be affected, although corrections should be possible.