A new coating campaign for the secondary mirrors of the ASTRI mini-array

The ASTRI Mini-Array is an international collaboration, led by the Italian National Institute for Astrophysics (INAF), devoted to the construction, deployment and operation of a set of nine identical dual-mirror Cherenkov telescopes. They will be located at the Teide Astronomical Observatory, operated by IAC, in the Canary island of Tenerife. The secondary mirrors of the ASTRI telescopes were realized already at the beginning of the ASTRI Project. After a few years, some of them revealed a clear degradation of the surface reflective coating. Therefore, it was necessary to look for a qualified industrial supplier able to perform a new coating of these mirrors. To this aim, the ASTRI Collaboration identified the French company CILAS as the best option. In this paper, we present the activities performed by CILAS on the mirrors. We first describe the coating approach adopted by CILAS and its tuning to the case of the ASTRI M2 mirrors. Then, we describe the qualification activities of the coating process, the problems arised and the remedial actions which were adopted. Finally, we report the obtained results from the reflectivity and homogeneity points of view.