Design and characterization of a single photoelectron calibration system for the NectarCAM camera of the medium-sized telescopes of the Cherenkov Telescope Array

TL;DR

This paper presents the design and characterization of a single photoelectron calibration system for NectarCAM, enabling precise in-situ gain measurements of 1855 PMTs to improve gamma-ray energy reconstruction in CTA.

Contribution

It introduces a novel in-situ calibration system with a movable screen for NectarCAM, enhancing accuracy in photo-detection gain measurement for CTA's MSTs.

Findings

High-precision gain measurement capability demonstrated

System allows in-situ calibration of all PMTs in NectarCAM

Calibration reduces systematic uncertainties in gamma-ray energy estimation

Abstract

In this work, we describe the optical properties of the single photoelectron (SPE) calibration system designed for NectarCAM, a camera proposed for the Medium Sized Telescopes (MST) of the Cherenkov Telescope Array (CTA). One of the goals of the SPE system, as integral part of the NectarCAM camera, consists in measuring with high accuracy the gain of its photo-detection chain. The SPE system is based on a white painted screen where light pulses are injected through a fishtail light guide from a dedicated flasher. The screen - placed 15 mm away from the focal plane - is mounted on an XY motorization that allows movements over all the camera plane. This allows in-situ measurements of the SPE spectra via a complete scan of the 1855 photo-multiplier tubes (PMTs) of NectarCAM. This calibration process will enable the reduction of the systematic uncertainties on the energy reconstruction of $\gamma$-rays coming from distant astronomical sources and detected by CTA.