| Abstract |
This dissertation presents a study on the influence of the night sky background (NSB) on the performance of the First G-APD Cherenkov Telescope (FACT), which is the first imaging atmospheric Cherenkov telescope (IACT) with a silicon photomultiplier (SiPM) camera. The up till now state-of-the-art, photomultiplier tubes (PMTs), can be easily and severely damaged by bright moonlight. SiPMs are an alternative robust photon detector for IACTs allowing for maximization of observation time by extending towards extreme NSB conditions, e.g., direct full moonlight. The performance has been determined on observations of the Crab Nebula from winter 2015/16 for all observed NSB levels. Dedicated Monte Carlo simulations have been tailored to the light conditions of the data sets by a new approach superimposing NSB measurements and simulated extensive air showers. The used analysis chain features machine-learning and unfolding techniques to reconstruct the energy spectrum and has been optimized in the course of this study for various NSB levels. The dedicated Monte Carlo simulations are used to train machine-learning models and allow for evaluation of their performance and dependency on the NSB. In preparation for this analysis, a procedure to select optimum cleaning levels for the observed light conditions needed to be improved as introduced in this thesis. With these enhancements, the typical performance metrics for an IACT are evaluated. The Crab Nebula has been detected with significances of ≈( 5 σ)/√h up to an NSB level twelve times brighter than the darkest nights. At even higher NSB level, the source could still be detected with a significance of ≈ (3.4 σ)/√h at NSB levels corresponding to direct moonlight at a 60% lunar phase. Furthermore, it has been shown that the integral sensitivity of FACT degrades with the NSB level from 10% to 20% of the Crab flux necessary for detection with 5𝜎 significance in 50h effective observation time. The main effect of rising NSB has been identified as an increase of the energy threshold, which has also been evident in a shifted low-energy edge of the effective collection area as well as its general decline with the NSB. The Crab Nebula energy spectrum has been successfully reconstructed in an energy range of 450 GeV to 30 TeV for various NSB levels up to these light conditions. According to the rising energy threshold, the lower edge of the unfolded energy range had to be increased up to 600 GeV in order to unfold the energy spectrum at higher NSB levels. Other than the above-mentioned findings, no significant indication for systematic effects on the unfolded spectra have been found. This work shows furthermore that with these adjustments the spectra are still in good agreement with each other and also with reference spectra from MAGIC and First G-APD Cherenkov Telescope (FACT). In summary, the performance values show promising results for observations with FACT at increased NSB levels. They underline the use of SiPMs as a potential alternative to other approaches for extending IACT observation times to bright light conditions.
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