Advanced modelling of the night sky background light for imaging atmospheric Cherenkov telescopes

¹ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Nikolaus-Fiebiger-Str. 2, 91058 Erlangen, Germany
² Department of Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK

^★ Corresponding author: gerrit.roellinghoff@fau.de

Received: 14 March 2025
Accepted: 15 May 2025

Abstract

Aims. A significant source of noise for imaging atmospheric Cherenkov telescopes (IACTs), which are designed to measure air showers caused by astrophysical gamma rays, is optical light emitted from the night sky. This night sky background (NSB) influences IACT operating times and their sensitivity. Thus, for scheduling observations and instrument simulation, an accurate estimate of the NSB is important.

Methods. A physics-driven approach to simulating wavelength-dependent, per-photomultiplier-pixel NSB was developed. It included contributions from scattered moonlight, starlight, diffuse galactic light, zodiacal light, and airglow emission. It also accounted for the absorption and scattering of optical light in the atmosphere and telescope-specific factors such as mirror reflectivity, photon detection efficiency, and focal length. The simulated results were corrected for pointing inaccuracies and individual pixel sensitivities and were compared to data from the high energy stereoscopic system (H.E.S.S.) IACT array. The software package developed for this analysis will be made publicly available.

Results. Validation against H.E.S.S. data shows small deviations from the prediction, which are attributable to airglow and atmospheric variability. Per-pixel predictions provide a good match to the data, with a relative 90% error range of [‒21%, 19%]. Compared to the existing standard modelling approach of assuming a constant background, which yields a relative 90% error range of [−64%, 48%], this represents a significant improvement.