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 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. 
S. LeBohec and J. Holder