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: A new mirror-manufacturing process shows promise for producing the strongly aspherical mirrors suited to the next generation of ground-based gamma-ray astronomical telescopes. Imaging atmospheric Cherenkov telescopes (IACTs) are cutting-edge instruments for studying astronomical sources that emit very high energy gamma rays. They operate from the ground using Earth's atmosphere as a calorimeter: energetic photons or charged particles coming from the most distant parts of the Uni-verse hit the atmosphere and interact with it. They lose energy by producing pairs that run faster than light (in air) and emit a bluish light by the Cherenkov effect. This propagates into the atmosphere, generating a so-called shower, whose light pool covers about 30,000m 2 on the ground. These electromagnetic showers are very faint relative to the night-sky background. They are also rare and last for only a few billionths of a second. Showers can be imaged by very large light collectors equipped with proper focal plane instrumentation and fast gigahertz elec-tronics. From these images, both the incoming direction and the energy of the primary photons can be recovered. If many telescopes are used and more than one detects the same event, the angular resolution of the reconstructed incoming direction is improved. Figure 1 shows a sketch of the working principle described so far. Such observations make it possible to under-stand the physics behind the extremely powerful acceleration mechanisms at work in the astronomical sources emitting the primary gamma photons and to gather clues to the origin of the Universe. The Cherenkov Telescope Array is an ambitious worldwide project to build many IACTs whose reflecting area will approach about 10,000m 2 spread over a few square kilometers. 1 Here we examine some aspects of the mirror requirements and related developments in their manufacturing technique. The reflectors used in IACTs are tessellated and typically con-sist of a number of identical segments with spherical profiles. The design requirements for those segments are summarized in Table 1. They must be low-cost, lightweight, robust, and Figure 1. Schematic view of the working principle of imaging at-mospheric Cherenkov telescopes. reliable, and each one must be more than 1m 2 in size. A focal point-spread-function value of a few arcmin per single mirror segment is acceptable. This is a much less stringent requirement than that for optical telescopes. However, IACTs are usually not protected by domes, so the mirrors are permanently exposed to the environment. All these requirements make their fabrication challenging. A recent project at the Brera Astronomical Observatory of the Italian National Institute of Astrophysics (INAF-OAB) was ded-icated to a manufacturing process optimized for this type of mirror. 2 The chosen substrate has a sandwich-type mechanical structure that confers stiffness and low areal density. The process is called cold-glass slumping, and it exploits the concept of repli-cation of a master shape. Figure 2 shows the main manufacturing steps, which are described briefly below.
    SPIENewsroom 05/2011;
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    ABSTRACT: The next generation of Imaging Atmospheric Cherenkov Telescope will explore the uppermost end of the Very High Energy domain up to about few hundreds of TeV with unprecedented sensitivity, angular resolution and imaging quality. To this end, the Italian National Institute of Astrophysics (INAF) is currently developing a scientific and technological telescope prototype for the implementation of the Cherenkov Telescope Array (CTA) observatory. The Italian ASTRI program foresees the full design, development, installation and calibration of a Small Size 4-meter class Telescope, adopting an aplanatic, wide-field, double-reflection optical layout in a Schwarzschild-Couder configuration. In this paper we discuss about the technological solutions adopted for the telescope and for the mirrors. In particular we focus on the structural and electro-mechanical design of the telescope, now under fabrication. The results on the optical performance derived from mirror prototypes are here described, too.
    Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 09/2013

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