UVSiPM: a light auxiliary detector to measure the night sky background seen by the ASTRI mini-array Cherenkov telescopes at the Observatorio del Teide.

The ASTRI Mini-Array is an international project led by the Italian National Institute for Astrophysics (INAF) aiming at building and operating an array of nine Imaging Atmospheric Cherenkov Telescopes (IACTs) at the Observatorio del Teide in Tenerife (Canary Islands, Spain). UVSiPM, a calibrated small photon counter working in the 280-900 nm wavelength range, is one of the auxiliary instruments of the ASTRI Mini-Array. UVSiPM is mainly devoted to measure the level of night sky background during the ASTRI Mini-Array observations in the same energy range of the ASTRI cameras. It is composed of one single multi-pixel SiPM sensor (the same model adopted in the ASTRI Mini-Array Cherenkov cameras) coupled to an electronic chain working in single photon counting mode. The design of the optical system foresees a pin-hole mask equipped with a collimator to regulate the UVSiPM field of view. UVSiPM will be mounted on the external structure of one of the ASTRI Mini-Array telescopes and co-aligned with its camera. In addition, it will be used as a support instrument for the absolute end-to-end calibration of the ASTRI Mini-Array telescopes performed with the illuminator, a further auxiliary device devoted to perform the optical throughput calibration of each telescope of the array. Last but not least, UVSiPM can be used as diagnostic tool for the camera functionalities. In this contribution we present the overall design of the UVSiPM instrument and some preliminary results of its performance based on simulations.