Conference Paper

Development of cold-slumping glass mirrors for imaging Cherenkov telescopes

DOI: 10.1117/12.790631 Conference: Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series

ABSTRACT The development of lightweight glass mirrors manufactured via cold-slumping technique for Imaging Atmospheric Cherenkov Telescope is presented. The mirror elements have a sandwich-like structure where the reflecting and backing facets are composed by glass sheets with an interposed honeycomb aluminum core. The reflecting coating is deposited in high vacuum by means of physical vapor deposition and consists of aluminum with an additional protective layer of SiO2. The mirror fabrication and environmental qualification by accelerated ageing, thermal cycling and coating adhesion are presented together with the optical performances measured as angular resolution and reflectivity obtained on spherical, 1 squared meter mirror prototypes.

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    ABSTRACT: The Astronomical Observatory of Brera (INAF-OAB) is investigating a novel slumping technique for the manufacturing of stiff and lightweight optical segments. We propose a 2-steps technique: initially the procedure foresees to manufacture a mirror segment using the hot slumping technique. This step produces a thin and floppy Borofloat glass shell using the slumping of a glass sheet onto a ceramic mould that has a surface with a high optical quality. After this step, this curved shell is assembled and glued to a stiff substrate made in foamed and pre-shaped material. On the back of the substrate it is also glued a flat sheet of the same glass. This procedure combine the good optical performances achievable on optics produced by means of the hot slumping technique with the lightweight and stiffness of the foamed material and, finally, the good structural properties achievable in sandwich-like structures. This approach could be in principle used for the production of mirrors for a number of applications, from the primary segmented mirror for large telescopes to the mirrors for the future Cherenkov telescopes nowadays under development. This paper describes the process of production of a prototype optical segment and the status of the investigation.
    Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 07/2008
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    ABSTRACT: Motivated by the recent challenging results from TeV astronomy, the VHE INAF community asked a group of them to write this White Paper to summarize the status and future of Cherenkov telescopes for gamma-ray astronomy and the INAF perspectives in this field. This document wants to review both the scientific topics and potential developments of the field as well as to point out both the interests and the capacities (scientific and technical) of the VHE astrophysics community in INAF. It is aimed at identifying the scientific and technological areas where INAF should focus its efforts and resources so that Italian researchers can achieve (or maintain) a leading position in this field. Comment: 54 pages, presented to the President of INAF and to the Chiefs of INAF Departments
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    ABSTRACT: We report on the production and implementation of 100 square panels 1 m x 1 m, based on the innovative approach of cold slumping of thin glass sheets. The more than 100 segments will cover around one half of the 240 m-square reflecting surface of the MAGIC II, a clone of the atmospheric Cherenkov telescope MAGIC I (with a single-dish 17 m diameter mirror) which is already operating since late 2003 at La Palma. The MAGIC II telescope will be completed by the end of 2008 and will operate in stereoscopic mode with MAGIC I. While the central part of the of the reflector is composed of by diamond milled Aluminum of 1m2 area panels (following a design similar to that already used for MAGIC I), the outer coronas will be made of sandwiched glass segments. The glass panel production foresees the following steps: a) a thin glass sheet (1-2mm) is elastically deformed so as to retain the shape imparted by a master with convex profile - the radius of curvature is large, the sheet can be pressed against the master using vacuum suction -; b) on the deformed glass sheet a honeycomb structure that provides the needed rigidity is glued ; c) then a second glass sheet is glued on the top in order to obtain a sandwich; d) after on the concave side a reflecting coating (Aluminum) and a thin protective coating (Quartz) are deposited. The typical weight of each panel is about 12 kg and its resolution is better than 1 mrad at a level of diameter that contains the 90% of the energy reflected by the mirror; the areal cost of glass panels is ~2 k per 1m2. The technology based on cold slumping is a good candidate for the production of the primary mirrors of the telescopes forming the Cherenkov Telescope Array (CTA), the future large TeV observatory currently being studied in Europe. Details on the realization of MAGIC II new mirrors based on cold slumping glass will be presented.
    Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 07/2008

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