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When a high energetic particle or gamma-ray interacts with the atmosphere, it induces an air shower. Secondary particles of such showers emit dim and short duration flashes of Cherenkov light that can be measured with Imaging Atmospheric Cherenkov Telescopes (IACTs). For this task, very fast and sensitive photosensors are necessary. To be able to statistically distinguish between showers induced by hadrons and gamma-rays, the camera of an IACT typically contains more than 1000 pixels. In the past, all IACT cameras used Photomultiplier Tubes (PMTs) as photosensors. When the first generation of Geiger-mode operated Avalanche Photo Diodes (G-APD) became commercially available in the year 2007, the question raised if such solid-state photosensors could be a viable alternative for future IACTs. The FACT Collaboration was formed in 2008 with the goal to construct a novel camera and operate it in a refurbished 9.5m^2 IACT on the Canary Island La Palma. In October 2011, the camera was installed and data taking started within few hours. Since then, data are successfully taken almost every night.
One concern about using G-APDs is the strong dependence of their gain on the temperature, taking into account that the operation temperature at an IACTs can vary by more than 25 K. FACT has developed a method to self-calibrate the camera without the need of any external calibration device. This results in a very stable and reliable operation, allowing to operate FACT automatically without the need of a shift
crew onsite. When weather conditions allow, the data taking efficiency of FACT exceeds 95%.
While PMTs can be damaged if illuminated by to much light, G-APDs are far more robust. This allows to operate the FACT camera also under strong moonlight conditions. In the past five years, each of the more than 1400 sensors of the FACT camera has collected an unprecedented amount of photons, without any indication of
aging or other problems related to the photo sensors.
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