  The atmosphere is an intrinsic part of any ground based Cherenkov gamma-ray
telescope, and the telescope response is therefore sensitive to unpredictable
changes in the atmospheric transparency which are difficult to measure and
interpret in the absence of a calibrated beam of high energy gamma-rays. In
this paper, we use the detector response to Cherenkov emission from cosmic ray
initiated air showers to obtain a relative calibration for data obtained under
different instrumental and atmospheric conditions as well as over a range of
source angles to the Zenith. We show that such a relative calibration is useful
and efficient for data selection, for correcting the measured gamma-ray rate
and for inter-calibration between the elements of an array of Cherenkov
telescopes.
