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

Imaging atmospheric Cerenkov telescopes use the differences in shape and orientation of the images of development of cascades to distinguish between very high energy gamma rays and the cosmic ray background. We have previously shown that these images may be distorted by the effects of the geomagnetic field, which decreases the sensitivity of a telescope. In this paper we propose 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 these corrections reduce the effect of the geomagnetic field and improve the sensitivity of the University of Durham Mark 6 telescope. (author). Letter-to-the-editor