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Description
Summary
Observation of reference stars is a simple and accurate procedure commonly used for the absolute calibration of optical telescopes, however, Fluorescence telescopes, that are designed to detect very short burst of light, are not able to directly measure the slowly varying night sky background flux.
It is possible, however, to overcome this limitation by the analysis of the statistical fluctuation (variance) of the signal recorded by the telescope detectors that, beeing directly proportional to the photon flux, allows to indirectly measure the star light and use it for calibration purposes. Without requiring any dedicated hardware device, and without any interference with normal telescope operations, the method developed provides an economical and simple way to verify the absolute calibration and to monitor the long-term stability of Fluorescence Telescopes.
In this contribution we describe details of the procedure, in particular on the correction of the star signals from atmospheric attenuation, and then report on results obtained by analysis of several reference stars as seen in the Fluorescence Telescopes of the Pierre Auger Observatory.