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

Abstract

Recent advances in unmanned aerial vehicle (UAV) technology have made UAVs an attractive possibility as an airborne calibration platform for astronomical facilities. This is especially true for arrays of telescopes spread over a large area such as the Cherenkov Telescope Array (CTA). In this paper, the feasibility of using UAVs to calibrate CTA is investigated. Assuming a UAV at 1km altitude above CTA, operating on astronomically clear nights with stratified, low atmospheric dust content, appropriate thermal protection for the calibration light source and an onboard photodiode to monitor its absolute light intensity, intercalibration of CTA's telescopes of the same size class is found to be achievable with a 6 − 8% uncertainty. For cross-calibration of different telescope size classes, a systematic uncertainty of 8 − 10% is found to be achievable. Importantly, equipping the UAV with a multi-wavelength calibration light source affords us the ability to monitor the wavelength-dependent degradation of CTA telescopes' optical system, allowing us to not only maintain this 6 − 10% uncertainty after the first few years of telescope deployment, but also to accurately account for the effect of multi-wavelength degradation on the cross-calibration of CTA by other techniques, namely with images of air showers and local muons. A UAV-based system thus provides CTA with several independent and complementary methods of cross-calibrating the optical throughput of individual telescopes. Furthermore, housing environmental sensors on the UAV system allows us to not only minimise the systematic uncertainty associated with the atmospheric transmission of the calibration signal, it also allows us to map the dust content above CTA as well as monitor the temperature, humidity and pressure profiles of the first kilometre of atmosphere above CTA with each UAV flight. either single or dual mirror optics configuration, will extend the high-energy reach of CTA, predominantly observing γ-rays with energies from a few TeV up to 100 TeV and beyond. To allow all-sky coverage, the CTA observatory will consist of two arrays, one in each hemisphere. The northern array is intended to contain approximately 20 telescopes (LSTs and MSTs) spread over about 1 km 2 , while the southern array is intended to contain ∼ 100 telescopes (LSTs, MSTs and SSTs) spread over an area of approximately four square kilometres. More than two thirds of the southern array telescopes are foreseen to be SSTs. Furthermore, the enrichment of the southern array is envisaged with the addition of medium-sized dualmirror Schwarzschild-Couder Telescopes (SCTs; [9]), which have the advantage of a better angular resolution, smaller point spread function and a larger field of view compared to the traditional single-mirror Cherenkov telescopes.CTA will herald a new era for ground-based γ-ray astronomy, with the emphasis shifting from source discovery, to population studies and precision measurements. This change in emphasis requires a reduction in the systematic uncertainties of the ob...

Using Muon Rings for the Calibration of the Cherenkov Telescope Array: A Systematic Review of the Method and Its Potential Accuracy

“…We find that after such a carefully prepared calibration scheme, the optical bandwidth of the full telescope can be determined with a resolution of <1% (LST and MST) and <5% (SST) on time scales of better than a minute and an accuracy of better than 4% (LST and MST) and 5% (SST). This is considerably better than other proposed methods so far (Gaug 2014;Brown 2018;Stefanik et al 2019) and only slightly worse than the cross-calibration scheme proposed by Mitchell et al (2016b), which allows, however, only for a relative calibration between telescopes or telescope types. Other direct methods (Segreto et al 2016) perform even better and show more flexibility but require a dedicated setup and cannot be carried out at the same time as the telescopes perform science observations.…”

Atmospheric monitoring using the Cherenkov Transparency Coefficient for the Cherenkov Telescope Array

“…where 0 ≤ ε i ≤ 1 refers to the percentage of the nominal optical efficiency of a telescope i andR 0 i j is the trigger rate estimate for ε i = ε j = 1. Note that the optical efficiencies of telescopes have to be estimated in an independent procedure [11][12][13][14]. A further influence of the telescope hardware is the possibility of adapting the camera trigger thresholds to the measured rates of individual camera pixels.…”

Inter-calibration of atmospheric Cherenkov telescopes with UAV-based airborne calibration system

“…Advances in UAV technology have made them an attractive possibility as airborne calibration platforms for astronomical facilities (e.g. [1,2,3,4,5]). This is especially true for telescope arrays, where the maneuverability, flexibility and versatility of a UAV-based calibration system allows us to rapidily calibrate the numerous detector elements spread over a large area.…”

“…The physical separation between individual telescope elements adds another level of complexity to these additional calibration requirements. Furthermore, if the telescope elements are spread over a large enough area, additional uncertainties, such as spatially dependent environmental factors, need to be characterised and calibrated [5].…”

“…The atmosphere is a critical part of the ground-based γ-ray detection process; a scientific payload of environmental sensors such as a nephelometer, temperature, humidity and pressure sensors, allows us to quantify the atmosphere above the telescopes, thus allowing us to constrain the uncertainty of atmospheric extinction of the Cherenkov radiation created by the EAS. A UAV-based calibration system also has a unique ability to characterise the optical throughput for all telescopes in the array [5]. This procedure entails placing a well-understood calibration light source, capable of pulsed illumination on nanosecond timescales, at altitude above the IACT array, and simultaneously illuminating numerous telescopes [5].…”