FACT -- Operation of the First G-APD Cherenkov Telescope
T. Bretz, A. Biland, J. Bu\ss, V. Commichau, L. Djambazov, D. Dorner,, S. Einecke, D. Eisenacher, J. Freiwald, O. Grimm, H. von Gunten, C. Haller,, C. Hempfling, D. Hildebrand, G. Hughes, U. Horisberger, M. L. Knoetig, T., Kr\"ahenb\"uhl, W. Lustermann, E. Lyard, K. Mannheim

TL;DR
The FACT telescope successfully operates using G-APD photon counters, demonstrating real-time calibration, high data throughput, and effective data compression, advancing Cherenkov astronomy technology and monitoring capabilities.
Contribution
This paper presents the first operational implementation of a G-APD based Cherenkov telescope with real-time calibration, high data rate handling, and efficient data processing, marking a significant technological advancement.
Findings
Stable G-APD operation under varying environmental conditions.
Real-time data compression outperforms gzip in speed and ratio.
Effective temperature and current feedback systems maintain detector stability.
Abstract
Since more than two years, the First G-APD Cherenkov Telescope (FACT) is operating successfully at the Canary Island of La Palma. Apart from its purpose to serve as a monitoring facility for the brightest TeV blazars, it was built as a major step to establish solid state photon counters as detectors in Cherenkov astronomy. The camera of the First G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes (G-APD aka. MPPC or SiPM) for photon detection. Since properties as the gain of G-APDs depend on temperature and the applied voltage, a real-time feedback system has been developed and implemented. To correct for the change introduced by temperature, several sensors have been placed close to the photon detectors. Their read out is used to calculate a corresponding voltage offset. In addition to temperature changes, changing current introduces a voltage drop in the…
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