Performance of the INFN Camera calibration device of the first Large Size Telescope in the Cherenkov Telescope Array

Abstract

At the end of 2018, the prototype of the first Large Size atmospheric Cherenkov Telescope (LST-1) proposed for the Cherenkov Telescope Array (CTA) observatory has been completed at the Observatorio del Roque de Los Muchachos in La Palma (Canary Islands, Spain). The LST-1 camera, with more than 1800 photomultipliers (PMTs), requires a precise and regular flat field photo-electron (p.e.) calibration. We present the characteristics and performance of the system (Camera Calibration Box, CaliBox), installed at the center of the LST-1 dish, which provides the optical calibration for the camera PMTs, over a large dynamic range up to $10^3$ p.e. per pixel (1 pixel = 1 PMT). The CaliBox consists of a pulsed UV laser with a wavelength of 355 nm (corresponding to the maximum of the PMT quantum efficiency) and a 400 ps pulse width, a set of filters to guarantee a large dynamic range of photons on each pixel, and a diffusing Ulbricht sphere to integrate and uniformly spread the laser light over the camera plane 28 m away. The CaliBox is designed to be hermetically closed and filled with dry air to make the system completely isolated from the external environment and maintain a controlled low internal relative humidity. An additional inter-calibration system provides the absolute (and relative) number of p.e. for each filter combination in addition to a self-diagnose mechanism. The system is completely managed by an internal ODROID-C1+ board , which can be remotely controlled via an Open Platform Communication's Unified Architecture (OPC-UA) protocol.