Feasibility study of an absolute energy calibration of imaging atmospheric Cherenkov telescopes by starlight

A new method is presented for an absolute energy calibration of imaging atmospheric Cherenkov telescopes as a whole instrument at all relevant wavelengths. For this study the Cherenkov telescope #1 of the HEGRA experiment was used. The calibration was carried out by detecting the light of different selected stars during one night. From the data a simple linear correlation was derived between the anode current of the Cherenkov camera photomultiplier tubes and the incident photon flux. This result is important for the imaging air Cherenkov technique in several aspects: The method enables to determine the response function of Cherenkov telescopes in the energetic range of Cherenkov light quickly and reliably. Therefore it can be used as an atmospheric calorimeter by measuring the absolute integrated flux of Cherenkov photons. Moreover, the signal of a point run can be predicted and a simulation of the apparatus response can be carried out more reliably.