Calibration and commissioning of 256-pixel SiPM Imaging Camera for 4-Meter Cherenkov Telescope

A 256-pixel Silicon Photomultiplier (SiPM) camera has been developed for a 4-meter diameter atmospheric Cherenkov telescope for ground-based gamma-ray astronomy at energies above 500 GeV. An accurate and stable camera performance requires regular, precise calibration of its readout chain. This paper presents the calibration methodology and results obtained using a dedicated calibration box developed in-house, employing a nanosecond pulsed light flasher operating at 405 nm, which provides a controlled and uniform illumination of the camera at few photoelectrons level. Absolute gain and photoelectron spectra were measured for each pixel under low-light conditions. Calibration measurements were initially performed on a laboratory test bench and subsequent repeated on site during telescope operations. The on-site calibration results show excellent agreement with the laboratory measurements, demonstrating the stability and robustness of the calibration procedure under actual observing conditions. The consistency between laboratory and in situ calibrations will allow the same calibration constants to be applied to both observational data analysis and detector simulations, thereby improving the accuracy of the instrument response and the reliability of the derived astrophysical measurements.