Calibration and performance of the photon sensor response of FACT — the first G-APD Cherenkov telescope

A Biland ,S Einecke ,D Dorner ,W Lustermann ,D Eisenacher ,Dominik Neise ,J Thaele ,D Hildebrand ,M L Knoetig ,Jens Buß ,A Overkemping ,J.-P Stucki ,U Horisberger ,Jan Freiwald ,K Mannheim ,V Commichau ,Fabian Temme ,C Haller ,T Steinbring ,T Bretz ,W Rhode ,U Röser ,K Meier ,G Hughes ,P Vogler ,F Pauß ,L Djambazov ,R Walter ,T Krähenbühl ,O Grimm ,S Mueller ,Christina Hempfling ,Aleksander Paravac ,E Lyard ,H Von Gunten ,Q Weitzel

doi:10.1088/1748-0221/9/10/p10012

Abstract

The First G-APD Cherenkov Telescope (FACT) is the first in-operation test of the performance of silicon photo detectors in Cherenkov Astronomy. For more than two years it is operated on La Palma, Canary Islands (Spain), for the purpose of long-term monitoring of astrophysical sources. For this, the performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken for their temperature and voltage dependence implementing a correction method to keep their properties stable. Several measurements have been carried out to monitor the performance. The measurements and their results are shown, demonstrating the stability of the gain below the percent level. The resulting stability of the whole system is discussed, nicely demonstrating that silicon photo detectors are perfectly suited for the usage in Cherenkov telescopes, especially for long-term monitoring purpose.

Highlights

The First Geiger-mode avalanche photo diodes (G-APDs) Cherenkov telescope (FACT) is the first Imaging air-Cherenkov telescope to use silicon based sensors for photo detection
The resulting stability of the whole system is discussed, nicely demonstrating that silicon photo detectors are perfectly suited for the usage in Cherenkov telescopes, especially for long-term monitoring purpose
Due to the excellent single-p.e. resolution of G-APDs, most of their properties can be extracted from their dark count spectrum

Summary

Introduction

The First G-APD Cherenkov telescope (FACT) is the first Imaging air-Cherenkov telescope to use silicon based sensors for photo detection. The telescope is dedicated to the monitoring of the brightest known gamma-ray sources, mainly Active Galactic Nuclei (AGN) with their highly variable flux. Much more sensitive instruments are currently available as the H.E.S.S., MAGIC and VERITAS telescopes, their high discovery potential is best used for the detection of new sources at TeV energies and precision studies of known sources. For long-term and especially continous monitoring the observation time of those instruments is too expensive which suggests the construction of an inexpensive instrument with an adapted sensitivity, cf [2]. The application of silicon based photo sensors promised the increase of available observation time due to their robustness against light exposure as string moon light

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Conclusion