The correction of atmospheric Cerenkov images for the effect of the geomagnetic field

Imaging atmospheric Cerenkov telescopes use the differences in shape and or ientation of the images of development of cascades to distinguish between v ery high energy gamma rays and the cosmic ray background. We have previousl y shown that these images may be distorted by the effects of the geomagneti c field, which decreases the sensitivity of a telescope. In this paper we p ropose an analysis technique which corrects for(in first order) the effects of the geomagnetic field on the orientation of gamma ray images. Applying this technique to observations of gamma ray sources, we demonstrate that th ese corrections reduce the effect of the geomagnetic field and improve the sensitivity of the University of Durham Mark 6 telescope.