  Atmospheric 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 the 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.
