Imaging Air Cherenkov Telescopes (IACT) measure the faint flashes of Cherenkov light emitted
by air-showers that are produced when charged particles or gamma rays hit the atmosphere.
Therefore, the atmosphere above the IACT is an integral part of the detector. Variations in the
performance of the IACT itself, but also changes in the absorption and scattering of Cherenkov
light due to clouds or dust affect the interpretation of measured signals. Therefore, information
about the status of the full system is crucial to combine measurements from different time periods.
The First G-APD Cherenkov Telescope (FACT) is using for the first time solid state photosensors
(so-called G-APDs or SiPM) to measure the flashes of Cherenkov light. Based on the stability of
these sensors, we showed in the past that it is possible to identify the existence of strong clouds or
calima when measuring the intrinsically constant flux of cosmic ray particles at different trigger
levels. This necessitated dedicated measurements, preventing normal data taking in parallel. We
have now improved the method to use instead those cosmic ray events that are recorded during
normal data taking as dominant background. By applying a fixed virtual trigger threshold in
software, we measure the rate of charged cosmic ray particles. A deviation from the expected flux
allows to identify data sets with reduced performance of the complete system in quasi real-time,
without the need for any additional device.
Applying the method to a data set when one of the 30 mirror tiles of FACT was missing, we show
that a change of total yield of the Cherenkov light by few percent can be identified within few
minutes of standard data taking. This nicely demonstrates that the hadron rate determined from
standard data taking with FACT can be used for monitoring of the data quality.