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Recent advances in unmanned aerial vehicle (UAV) technology have made UAVs anattractive possibility as an airborne calibration platform for astronomicalfacilities. This is especially true for arrays of telescopes spread over alarge area such as the Cherenkov Telescope Array (CTA). In this paper, thefeasibility of using UAVs to calibrate CTA is investigated. Assuming a UAV at 1km altitude above CTA, operating on astronomically clear nights withstratified, low atmospheric dust content, appropriate thermal protection forthe calibration light source and an onboard photodiode to monitor its absolutelight intensity, inter-calibration of CTA's telescopes of the same size classis found to be achievable with a 6-8 % uncertainty. For cross-calibration ofdifferent telescope size classes, a systematic uncertainty of 8-10 % is foundto be achievable. Importantly, equipping the UAV with a multi-wavelengthcalibration light source affords us the ability to monitor thewavelength-dependent degradation of CTA telescopes' optical system, allowing usto not only maintain this 6-10 % uncertainty after the first few years oftelescope deployment, but also to accurately account for the effect ofmulti-wavelength degradation on the cross-calibration of CTA by othertechniques, namely with images of air showers and local muons. A UAV-basedsystem thus provides CTA with several independent and complementary methods ofcross-calibrating the optical throughput of individual telescopes. Furthermore,housing environmental sensors on the UAV system allows us to not only minimisethe systematic uncertainty associated with the atmospheric transmission of thecalibration signal, it also allows us to map the dust content above CTA as wellas monitor the temperature, humidity and pressure profiles of the firstkilometre of atmosphere above CTA with each UAV flight.

On the prospects of cross-calibrating the Cherenkov Telescope Array with an airborne calibration platform