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.

The First G-APD Cherenkov Telescope (FACT) has been monitoring Active Galactic Nuclei (AGN) for the past five years. The use of robust silicon photomultipliers (SiPMs) allows for a continuous, unbiased sampling even during bright-light conditions. This dataset promises insights into the core regions of AGN by investigating the periodicity of the sources. Periodic changes in the flux could indicate a binary nature of the supermassive black holes. A study using the Lomb-Scargle periodogram to find periodicity in monitored AGN is presented. Repeating patterns in the observation times, like moon periods and seasonal effects, affect the analysis by introducing spurious peaks into the periodogram. The zenith-dependence of the observed γ-ray rates further complicate the interpretation. Showing no variability at TeV energies, the γ -ray flux of the Crab Nebula is used to characterize this latter effect, before applying the Lomb-Scargle algorithm.

Abstract. Atmospheric monitoring is a field of special importance for astroparticle physics, especially for Imaging Atmospheric Cherenkov Telescopes (IACTs) as clouds will absorb and scatter the Cherenkov photons of air showers. Conventional tools used for atmospheric monitoring (e.g. LIDAR) are very expensive and monitor only a small part of the sky at once. Therefore, they are not suitable to perform a wide scan of the sky which is necessary to detect clouds in advance. This article gives a short overview about a method that uses an all sky camera with a 180• field of view to identify the cloud distribution by measuring the absorption of star light. It can be used to assign a sky quality rating to single spots, arbitrary regions or the whole sky at once within a 1 min exposure time. A cloud map can be created from the available data that can be used to determine shape and dimension of clouds and to predict their movement. The resulting data can be used by a scheduling algorithm or the operating crew to point the telescope to a different source before the current source gets covered by clouds. The all sky cameras used so far are located on La Palma at the observatory Roque de los Muchachos close to the telescopes FACT and MAGIC and the planned northern CTA site.

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The First G-APD Cherenkov Telescope (FACT) is pioneering the usage of silicon photo multipliers (SIPMs also known as G-APDs) for the imaging atmospheric Cherenkov technique. It is located at the Observatorio Roque de los Muchachos on the Canary island of La Palma. Since first light in October 2011, it is monitoring bright TeV blazars in the northern sky. By now, FACT is the only imaging atmospheric Cherenkov telescope operating with SIPMs on a nightly basis. Over the course of the last five years, FACT has been demonstrating their reliability and excellent performance. Moreover, their robustness allowed for an increase of the duty cycle including nights with strong moon light without the need for UV-filters. In this contribution, we will present the performance of the first Cherenkov telescope using solid state photo sensors, which was determined in analysis of data from Crab Nebula, the so called standard candle in gamma-ray astronomy. The presented analysis chain utilizes modern data mining methods and unfolding techniques to obtain the energy spectrum of this source. The characteristical results of such an analysis will be reported providing, e. g., the angular and energy resolution of FACT, as well as, the energy spectrum of the Crab Nebula. Furthermore, these results are discussed in the context of the performance of coexisting Cherenkov telescopes.