Design and commissioning of the trigger electronics for a novel Geiger-mode avalanche photodiode based camera for Imaging Atmospheric Cherenkov Telescopes

The successful operation of large ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) has opened a new window in astronomy by observing very high-energy (VHE) gamma rays in the energy range between 0.1 – 100 TeV, originating from galactic and extragalactic sources. When entering into the atmosphere, the gamma rays initiate the formation of showers of secondary particles that emit Cherenkov light. These short and faint flashes of light can be measured with IACTs requiring fast and sensitive cameras, which were so far solely based on Photo-Multiplier tubes (PMTs). When silicon-based semiconductor light sensors, so called G-APDs, became commercially available, their application as new photo-sensor in the very demanding field of ground-based gamma ray astronomy, had to be demonstrated. The First G-APD Cherenkov Telescope (FACT) was constructed with the aim to operate a full-scale IACT using a novel camera concept based on G-APDs. To prove that G-APD are a viable alternative for the next generation of IACTs currently under design for the large scale project CTA (Cherenkov Telescope Array, an observatory for ground-based gamma-ray astronomy), it was crucial to get a fully functional system in a rather short time frame. Therefore, it was not possible to extensively test and calibrate the full camera system in the laboratory at ETH in Zurich. In autumn 2011, the FACT camera was shipped to La Palma, Canary Islands (Spain) and installed on a refurbished HEGRA Cherenkov telescope mount located on the Roque de los Muchachos observatory. On 11 October 2011, FACT successfully triggered the first air showers only hours after installation. This thesis describes in detail the design, construction and commissioning of the trigger system’s hardware and firmware. The trigger of FACT is based on the analogue sum of trigger patches consisting of nine adjacent pixels. To each sum, a programmable threshold is applied. Not being able to perform all tests in the laboratory before installation in La Palma required a detailed analysis and optimization of the trigger system during the commissioning and datataking phase. As a remedial measure, methods were developed to investigate the trigger using standard physics data as well as some special runs taken under non-standard operation modes. These analyses fully confirmed that the functionality and performance of the trigger system is according to the expectations. Furthermore it also allowed investigating a few remaining irregularities, which are now well understood. In particular, there are a few trigger patches that suffer from externally induced noise and distortion, although their effect on standard data taking is negligible. While the trigger was highly uniform from the beginning, the analysis allows to even better flatfielding the trigger threshold for different patches without hardware modifications. It could be demonstrated that a simple analogue sum-trigger is sufficient for a G-APD based IACT camera. Furthermore it also could be shown that the problem of after-pulses present in cameras based on PMTs is not an issue for the FACT system.