The throughput calibration of the VERITAS telescopes

TL;DR

This paper introduces a throughput calibration method for VERITAS telescopes that accounts for weather-induced changes in optical and detector performance, improving data accuracy.

Contribution

The paper presents a new calibration approach that tracks optical and detector response variations over time in VERITAS telescopes.

Findings

Calibration improves energy threshold estimation.

Validation confirms accuracy with Crab Nebula observations.

Method accounts for mirror and PMT performance changes.

Abstract

Imaging atmospheric Cherenkov telescopes are continuously exposed to varying weather conditions that have short and long-term effects on their response to Cherenkov light from extensive air showers. This work presents the implementation of a throughput calibration method for the VERITAS telescopes taking into account changes in the optical response and detector performance over time. Different methods to measure the total throughput of the instrument, which depend on mirror reflectivites and PMT camera gain and efficiency, are discussed as well as the effect of its evolution on energy thresholds, effective collection areas, and energy reconstruction. The application of this calibration in the VERITAS data analysis chain is discussed, including the validation using Monte Carlo simulations and observations of the Crab Nebula.