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.