Production of large monolithic mirrors (1.8 m diameter) for dual-mirror Cherenkov telescopes via thermal slumping

The standard configuration for Small-Size (4 m diameter class) Imaging Atmospheric Cherenkov Telescopes (IACTs) is the ASTRI-like dual-mirror telescope adopted for both ASTRI Mini-Array and CTA SST subarray. It provides a wide field of view and a flat optical response. The innovative aplanatic design reduced the plate scale, making it an ideal choice for compact cameras. The introduction of telescopes with secondary mirrors in IACTs marked a significant milestone. However, the challenge of producing large secondary mirrors had to be overcome to bring this optical design to fruition. The ASTRI-secondary (aspherical) mirrors are characterized by a diameter of 1.8 meters, and a hot-slumping replication process was adopted to produce the sizeable monolithic glass substrates. In the context of developing the ASTRI Mini-Array, our Institute (INAF) established a new procurement chain with external companies. This paper presents the manufacturing and characterization results of the prototypal mirrors.