With the calibration procedure the response of the whole IACT was found to be approximated very well by a linear function. The errors of the method are sufficiently and can be improved by further studies.
An estimate for one flash of Cherenkov light using Eq. (2) leads to approximately 10 photons from the night sky background. Taking mean values for the mirror and cone reflectivity, quantum efficiency and photomultiplier gain this corresponds to 1 photoelectron. This figure matches very well with other considerations (Krennrich ).
The small offset of the calibration function is not fully understood yet. It may be based on the night sky background and its fluctuation. This contribution could not be included in the background subtraction described in Sect. 4 because it is not measured (threshold effect). Including this into the data analysis would perfectly remove the offset.
The main result (Eq. 4) can be compared with the measured photoelectron to ADC-count conversation factor from Mirzoyan et al. (). With his result (i.e. 1 photoelectron corresponds to 1.39() ADC channels) using mean values for the mirror reflectivity (85%), the quantum efficiency (12%) and the loss at the aluminum cones of the photomultiplier (11%) the absolute calibration is compatible. A vulnerable proof of the calibration would be if this method and another independent one is applied to the same telescope.
The results achieved here can reach more accuracy by measuring various stellar spectra and the atmospheric extinction parallel to the calibration measurements. Nevertheless, these measurements show the possibility of absolute calibration by means of a very simple method which, therefore, is planned to be used in new IACT-projects (Aharonian et al. ).
We would like to thank all members of the HEGRA collaboration for their support during that work - especially H.G. Börst, G. Heinzelmann and C.A. Wiedner for fruitful discussions as well as M. Hemberger, A. Konopelko and R. Mirzoyan who realized the measurements with the Cherenkov telescope #1. H.J.G. acknowledges the kind hospitality received by IEAP during his stay at the University of Kiel.
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