  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.
