Monitoring the optical quality of the FACT Cherenkov Telescope

TL;DR

This paper presents a method to monitor the optical quality of the FACT Cherenkov Telescope by analyzing muon ring images to infer the PSF, enabling ongoing quality assessment without additional observations.

Contribution

It introduces a novel approach using muon ring fuzziness to measure the PSF and tracks its evolution over time, improving optical quality monitoring.

Findings

The method accurately estimates the PSF from muon rings.

PSF evolution over time reveals optical performance changes.

No extra observations or human intervention are needed.

Abstract

In gamma ray astronomy muon events have a distinct feature of casting ring-like images on the sensor plane, thus forming a well known signal class for Cherenkov telescopes. These ring-like images can then be used to deduce the optical point spread function (PSF) which is an important measure of the optical quality of the imaging-reflector. In this thesis the observed 'fuzziness' of muon rings is used as a measure to infer the PSF. However to have a good estimate for this 'fuzziness' parameter, the reconstruction of the ring center and ring radius itself needs to be accurate, so different methods of ring feature extraction are studied. To check for the accuracy of the methods a simulation and analysis is performed. Measuring the evolution of the PSF over time allows to identify its effects and take them into account for the reconstruction of gamma-rays postliminary. As a further benefit of the methods presented here no additional observations are needed to measure the PSF nor any human activity on site is required. The accuracy of the method, and the PSF of FACT vs. time are presented.