The First G-APD Cherenkov telescope (FACT) is the first telescope using silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise a higher photon detection efficiency, more robustness and higher precision than photo-multiplier tubes. Since the properties of G-APDs depend on auxiliary parameters like temperature, a feedback system adapting the applied voltage accordingly is mandatory. In this presentation, the feedback system, developed and in operation for FACT, is presented. Using the extraction of a single photon-equivalent (pe) spectrum as a reference, it can be proven that the sensors can be operated with very high precision. The extraction of the single-pe, its spectrum up to 10\,pe, its properties and their precision, as well as their long-term behavior during operation are discussed. As a by product a single pulse template is obtained. It is shown that with the presented method, an additional external calibration device can be omitted. The presented method is essential for the application of G-APDs in future projects in Cherenkov astronomy and is supposed to result in a more stable and precise operation than possible with photo-multiplier tubes.
@article{arxiv.1308.1515,
title = {FACT - How stable are the silicon photon detectors?},
author = {T. Bretz and A. Biland and J. Buß and D. Dorner and S. Einecke and D. Eisenacher and D. Hildebrand and M. L. Knoetig and T. Krähenbühl and W. Lustermann and K. Mannheim and K. Meier and D. Neise and A. -K. Overkemping and A. Paravac and F. Pauss and W. Rhode and M. Ribordy and T. Steinbring and F. Temme and J. Thaele and P. Vogler and R. Walter and Q. Weitzel and M. Zänglein},
journal= {arXiv preprint arXiv:1308.1515},
year = {2013}
}