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.
The Cherenkov Telescope Array (CTA) is the next ground-based gamma-ray observatory in the TeV γ-ray spectral region operating with the Imaging Atmospheric Cherenkov Technique. It is based on almost 70 telescopes of different class diameters—Large-Sized Telescope, Medium-Sized Telescope (MST), and Small-Sized Telescope (SST) of 23, 12, and 4 m, respectively—to be installed in two sites in the two hemispheres (at La Palma, Canary Islands, and near Paranal, Chile). Several thousands of reflecting mirror tiles larger than 1 m2 will be produced for realizing the segmented primary mirrors of a so large number of telescopes. Almost in parallel, the ASTRI mini-array (MA) is being implemented in Tenerife (Canary Islands), composed of nine 4-m diameter dual-mirror Cherenkov telescopes (very similar to the SSTs). We completed the mirror production for all nine telescopes of the ASTRI MA and two MSTs (400 segments in total) using the cold glass slumping replication technology. The results related to the quality achieved with a large-scale production are presented, also discussing the adopted testing methods and approaches. They will be very useful for the adoption and optimization of the quality assurance process for the huge production (almost 3000 m2 of reflecting surface) of the MST and SST CTA telescopes.
Current generation Imaging Atmospheric Cherenkov Telescopes (IACTs) like HESS, MAGIC and VERITAS, operated in array-mode, have opened a new astronomical window in high energy gamma-ray band from a few tens GeV to a hundred TeV, allowing to observe the most energetic phenomena on going in our Universe. New generation arrays like ASTRI, with its 9 small-class telescopes operated at Tenerife, Canary Islands, and the very ambitious Cherenkov Telescope Array Observatory (CTAO - with its two sites at the northern and southern hemispheres and about 70 telescopes of different classes) will need to fabricate a few thousand square meters of reflective segments of about 1 m2 area and different shapes for making the primary mirrors. Different low-cost technologies have been used so far, like the direct figuring and polishing of glass slabs, the diamond turning of pre-formed aluminum sandwiched panels and the replica of thin glass foils for making composite sandwiched mirrors. In this paper we will present a further alternative solution, based on the replica of aluminum foils - precoated with a high reflectivity and durability multilayer film - for making sandwiched mirrors. This approach will simplify a lot the production chain, allowing us to make low-cost panels entirely made of aluminum. The method of production of the aluminum replicated panels is discussed and the preliminary performance results obtained with a prototype mirror presented.
The Cherenkov Telescope Array (CTA) will be the next generation ground-based observatory for gamma-ray astronomy at very-high energies. It will consist of over a hundred telescopes of different sizes (small, medium, and large) located in the northern and southern hemispheres. The Italian National Institute of Astrophysics (INAF) contributes to CTA through the ASTRI project (Astrofisica con Specchi a Tecnologia Replicante Italiana), whose main aim is to provide a series of dual-mirror small-sized telescopes (SST-2M ASTRI) and the mirrors for the single-mirror design of the medium-sized telescopes (MST). Both the primary mirror of the SST-2M ASTRI and the mirror of the MST are segmented, and such segments are realized with cold-slumping technology already used for the mirror facets of MAGIC, a system of two Cherenkov telescopes operating on the Canary Island of La Palma. On the other hand, the secondary mirror of the SST-2M ASTRI is monolithic and is realized with hot-slumping technology. Currently, we have completed the mirror production for nine SST-2M ASTRI telescopes, which will form the so-called ASTRI Mini-Array. Moreover, we have almost completed also the production of mirrors for two MSTs. In this paper, we present the mirror designs and describe the qualification activities that were performed to assess and consolidate the production process. Moreover, we report on the quality assurance approach we adopted to monitor and verify the production reliability. Finally, we present the performance of the produced mirrors and discuss their compliance with the CTA requirements.
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