ray astronomy has considerably been improved or even been established at all. In
this technique Cherenkov light emitted from atmospheric showers is observed by an imaging
atmospheric Cherenkov telescope (IACT) with a focal camera consisting of a larger number of
small size photomultiplier tubes (PMT) most often building an hexagonal array. In order to
interpret the measured data in terms of the characteristic quantities of the primary cosmic
ray simulations have to be performed of all the processes involved. Starting from the chosen
initial properties of the primary the simulation includes interactions within the
atmosphere and the production of secondaries, the emission of Cherenkov light and finally
its detection by the telescope. For the last step of the simulation procedure it is
necessary to have the most precise knowledge possible of the telescope response function.
The operation of a Cherenkov telescope, however, is influenced by various not easily predictable effects, such as accumulation of dust on the mirrors. Therefore, it is not obvious that the response function of the apparatus can be constructed by simply superimposing theoretical functions of the individual components. An experimental absolute calibration which could easily and often be repeated would considerably improve the interpretation of the measured air shower events. Further on, since the Cherenkov light represents the calorimetric component of an electromagnetic shower the integrated absolute flux of the Cherenkov photons is an important parameter the exact knowledge of which leads to a considerable improvement of the interpretation of the measurement.
For an absolute calibration a simple procedure has been developed based on the detection of light emitted by selected stars having a certain spectrum. Some advantages of this procedure are obvious: starlight can always be detected during clear nights, the method is very cheap and only a small amount of operation time is necessary.
As compared to other calibration procedures (Fraß et al. [1997]; Konopelko et al. [1996]; Vacanti et al. [1994]) the advantage of the use of starlight as light source is the possibility to calculate a ADC counts to Cherenkov photons conversion factor that is valid in the whole spectral range of Cherenkov light and not only for preselected wavelengths. Additionally, it can be used for small single IACTs.
As an important experimental precondition for this procedure the PMTs of the camera must have an anode current readout with well known features which was especially designed for IACT#1 (Njoo [1995]).
In the present paper the main results of a feasibility study are presented, All further details can be found in Karschnick ([1996]).
It should be noted that it was not in the scope of this study to develop the method as a mature tool for a routine application. The purpose of the paper is to show that it is worthwhile to invest further research in order to reach this goal.
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