The first cross-calibration of Imaging Atmospheric Cherenkov Telescopes with a UAV-based airborne calibration platform

The Cherenkov Telescope Array (CTA) aims to have unprecedented accuracy and sensitivity, affording us the ability to understand themysteries of the high energy universe. This unprecedented accuracy forces us to improve current calibration procedures, or indeed pioneer new techniques, to ensure the envisaged CTA performance. CTA will infer the energy of the gamma-rays it detects from the amount of Cherenkov radiation it observes. As such, the optical efficiency of the telescopes needs to be monitored, and its wavelength dependent degradation, which might be different for different telescope types, needs to be determined. Based on the results of a feasibility study, a novel cross-calibration method with an unmanned aerial vehicle (UAV) was tested on the H.E.S.S. telescope array, leading to the first cross-calibration of an Imaging Atmospheric Cherenkov Telescope (IACT) array with a single light source. In this talk, we present the cross-calibration results from a first test campaign in which we determine the relative optical efficiencies of the four HESS-I telescopes by successfully recording light from the UAV-mounted nanosecond pulsed UV light source simultaneously in all four telescopes. In addition, we show that the UAV data can be used to monitor the pointing accuracy at the level of at least tens of arcseconds and we give an outlook on other potential use cases of UAVs such as the monitoring of the atmospheric state.