  Atmospheric Cerenkov telescopes are used to detect electromagnetic showers
from primary gamma rays of energy > 300 GeV and to discriminate these from
cascades due to hadrons using the shape and orientation of the Cerenkov images.
The geomagnetic field affects the development of showers and diffuses and
distorts 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.
