The cosmic ray background as a tool for relative calibration of atmospheric Cherenkov telescopes

The atmosphere is an intrinsic part of any ground based Cherenkov γ-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 γ-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 γ-ray rate and for inter-calibration between the elements of an array of Cherenkov telescopes.