Night Sky Background Analysis for the Cherenkov Telescope Array using the Atmoscope instrument

TL;DR

This study analyzes night sky brightness at nine candidate Cherenkov Telescope Array sites using Atmoscope sensors, modeling starlight contributions to improve calibration and distinguish various light pollution sources.

Contribution

It introduces a detailed model of starlight contribution for calibration, enabling better comparison of site sky brightness and pollution sources.

Findings

Starlight modeling reduces measurement uncertainty to less than 15%.

Residual sky brightness can be decomposed into zodiacal light, airglow, and anthropogenic pollution.

Cross-calibration improves consistency among different Atmoscope sensors.

Abstract

The site selection group for the future Cherenkov Telescope Array (CTA) has deployed sensitive light sensors at 9 candidate sites, 5 of them in the Southern and 4 in the Northern hemisphere. The sensors are equipped with a PIN diode and a calibrated V-filter, and a blue/UV filter matching the spectral response of the photomultipliers to be employed in the CTA cameras. All sensor installations, denominated "Atmoscopes", have been cross-calibrated before deployment, and their sensitivity is monitored every two to five months. We show that a thoroughly developed model of the integral contribution of starlight to the overall light measure serves as an additional cross-calibration for each device during each night, reducing the systematic uncertainty of this measurement to less than 15%. The starlight can then be subtracted from the measurements, and the residuals compared among the different sites. We show that in most cases a decomposition into the contributions from zodiacal light, airglow and anthropogenic light pollution is possible.