Design of a prototype device to calibrate the Large Size Telescope camera of the Cherenkov Telescope Array

TL;DR

This paper presents a prototype optical calibration device for the Large Size Telescope camera of the Cherenkov Telescope Array, enabling accurate calibration over a wide dynamic range for gamma-ray astronomy observations.

Contribution

It introduces a novel calibration system using a 355 nm laser and neutral density filters, designed specifically for the LST camera to improve calibration accuracy and stability.

Findings

Calibration system achieves wide dynamic range up to 1000 photoelectrons/PMT.

The device demonstrates stable optical and mechanical performance.

Laser stability and ambient conditions are effectively monitored.

Abstract

The Cherenkov Telescope Array is a project that aims to exploring the highest energy region of electromagnetic spectrum. Two arrays, one for each hemisphere, will cover the full sky in a range from few tens of GeV to hundreds of TeV improving the sensitivity and angular resolution of the present operating arrays. A prototype of the Large Size Telescope (LST) for the study of gamma ray astronomy above some tens of GeV will be installed at the Canary Island of La Palma in 2016. The LST camera, made by an array of photomultipliers (PMTs), requires an accurate and systematic calibration over a wide dynamic range. In this contribution, we present an optical calibration system made by a 355 nm wavelength laser with 400 ps pulse width, 1 muJ output energy, up to 4k Hz repetition rate and a set of neutral density filters to obtain a wide range of photon intensities, up to 1000 photoelectrons/PMT, to be sent to the camera plane 28 m away. The number of photons after the diffuser of the calibration box, located in the center of the reflective plane, is monitored by a photodiode. The stability of the laser and the ambient parameters inside this calibration box are checked by a multi-task processor and a trigger signal is sent to the camera data acquisition system. The box frame is designed with special attention to obtain a robust device with stable optical and mechanical features.