FACT, the First G-APD Cherenkov Telescope, is an Imaging Air Cherenkov Telescope (IACT)
operating since 2011 at the Observatorio del Roque de los Muchachos on the Canary Island of La
Palma. As typical for IACTs, its reflector is comprised of smaller mirror facets and not protected
by a dome. In the case of FACT, 30 hexagonal facets form a total mirror area of 9.5 m 2 . Hence,
it is crucial to monitor the optical properties of this system and realign the facets if necessary.
Up to now, measuring the Point Spread Function of FACT required human interaction to mount
a screen and an optical camera. In this contribution, a new method to measure the optical Point
Spread Function using directly the Cherenkov camera of the telescope is presented. Inspired by
the method radio telescopes use to determine their resolution, the telescope is pointed towards
a fixed position on the trajectory of a star. During the star’s passage through the field of view,
the camera is read out using a fixed rate. In each event, the pedestal variance is determined for
each pixel. This value is directly correlated with the amount of night sky background light a pixel
received. Translating the time of the measurement to the position of the star in the camera enables
to determine the optical point spread function from this measurement. As the measurement is
done for each pixel along the trajectory of the star, the Point Spread Function can be determined
not only for the camera center but for the entire field of view. In this contribution, the new method
will also be compared with the existing methods of determining the optical Point Spread Function:
direct measurement with an optical camera and the width of Muon ring events.