Test results of a prototype device to calibrate the Large Size Telescope camera proposed for the Cherenkov Telescope Array

TL;DR

This paper describes the development and testing of a calibration system for the Large Size Telescope camera in the Cherenkov Telescope Array, ensuring precise, uniform calibration over a large dynamic range.

Contribution

It introduces a novel optical calibration system with a pulsed laser, filters, and a diffusing sphere, specifically designed for the CTA LST camera.

Findings

Photon density at the camera plane was successfully evaluated.

System demonstrated effective isolation from external environment.

Signal shape as detected by PMTs was characterized.

Abstract

A Large Size air Cherenkov Telescope (LST) prototype, proposed for the Cherenkov Telescope Array (CTA), is under construction at the Canary Island of La Palma (Spain) this year. The LST camera, which comprises an array of about 500 photomultipliers (PMTs), requires a precise and regular calibration over a large dynamic range, up to $10^3$ photo-electrons (pe's), for each PMT. We present a system built to provide the optical calibration of the camera consisting of a pulsed laser (355 nm wavelength, 400 ps pulse width), a set of filters to guarantee a large dynamic range of photons on the sensors, and a diffusing sphere to uniformly spread the laser light, with flat fielding within 3%, over the camera focal plane 28 m away. The prototype of the system developed at INFN is hermetically closed and filled with dry air to make the system completely isolated from the external environment. In the paper we present the results of the tests for the evaluation of the photon density at the camera plane, the system isolation from the environment, and the shape of the signal as detected by the PMTs. The description of the communication of the system with the rest of detector is also given.