The throughput calibration of the VERITAS telescopes. (arXiv:2111.04676v2 [astro-ph.IM] UPDATED)

The throughput calibration of the VERITAS telescopes. (arXiv:2111.04676v2 [astro-ph.IM] UPDATED)
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Context. The response of imaging atmospheric Cherenkov telescopes to incident
{gamma}-ray-initiated showers in the atmosphere changes as the telescopes age
due to exposure to light and weather. These aging processes affect the
reconstructed energies of the events and {gamma}-ray fluxes. Aims. This work
discusses the implementation of signal calibration methods for the Very
Energetic Radiation Imaging Telescope Array System (VERITAS) to account for
changes in the optical throughput and detector performance over time. Methods.
The total throughput of a Cherenkov telescope is the product of
camera-dependent factors, such as the photomultiplier tube gains and their
quantum efficiencies, and the mirror reflectivity and Winston cone response to
incoming radiation. This document summarizes different methods to determine how
the camera gains and mirror reflectivity have evolved over time and how we can
calibrate this changing throughput in reconstruction pipelines for imaging
atmospheric Cherenkov telescopes. The implementation is validated against seven
years of observations with the VERITAS telescopes of the Crab Nebula, which is
a reference object in very-high-energy astronomy. Results. Regular optical
throughput monitoring and the corresponding signal calibrations are found to be
critical for the reconstruction of extensive air shower images. The proposed
implementation is applied as a correction to the signals of the photomultiplier
tubes in the telescope simulation to produce fine-tuned instrument response
functions. This method is shown to be effective for calibrating the acquired
{gamma}-ray data and for recovering the correct energy of the events and
photon fluxes. At the same time, it keeps the computational effort of
generating Monte Carlo simulations for instrument response functions affordably
low.

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