Design and development of the reflecting panels for the large size telescopes at the southern site of the Cherenkov Telescope Array Observatory (CTAO)

In the context of the Cherenkov Telescope Array gamma-ray Observatory (CTAO), two large-size telescopes, each with a diameter of 23 m, will be installed at the southern site in Paranal, Chile. They are referred to as Large Size Telescope South, LST-S (it should be noted that 4 Large Size Telescopes, with a similar optical configuration, are already being installed at the CTAO northern side of La Palma, Canary Islands, Spain). INAF oversees the coordination of the implementation effort related to the LST-S telescopes. They will use a single mirror parabolic shape to capture images with moderate angular resolution. To achieve this shape, 198 hexagonal reflecting panels will be assembled into the telescope structure. Each panel is roughly 150 cm side-to-side in size and weighs less than 50 kg. It comprises two solid glass plates bonded to a lightweight honeycomb structure of an aluminum alloy core. The panels are spherical and distributed in three coronas with different curvature radii to achieve the desired shape. They will be exposed to the open air for several years and must withstand mechanical stresses, wind impact, and possible strong earthquake solicitations. The panels are the elements of the telescope's segmented primary mirror. The development activities for such large panels performed to optimize the mirror design and the results after the production of prototypes are summarized in this paper.