R. Canestrari

Glass mirrors by cold slumping to cover 100 m2 of the MAGIC II Cherenkov telescope reflecting surface

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.

Cold-shaping of thin glass foils as a method for mirror processing: from basic concepts to mass production of mirrors

Abstract. We present a method for the production of segmented optics. It is a process developed at INAF-Osservatorio Astronomico di Brera (INAF-OAB) employing commercial of-the-shelf materials. It is based on the shaping of thin glass foils by means of forced bending that occurs at room temperature [cold-shaping (CS)]. The glass is then assembled into a sandwich structure for retaining the imposed shape. The principal mechanical features of the mirrors are their low weight, rigidity and environmental robustness. The cost and production time also are competitive. We sum up the results achieved during research and development performed in the past years. We have investigated the theoretical limits of the structural components by means of parametric finite elements analyses; we also discuss the effects caused by the most common structural loads. Finally, the process implementation, the more significant validation tests and the mass production at the industry are described.

Analysis of microroughness evolution in x-ray astronomical multilayer mirrors by surface topography with the MPES program and by x-ray scattering

Future hard X-ray telescopes (e.g. SIMBOL-X and Constellation-X) will make use of hard X-ray optics with multilayer coatings, with angular resolutions comparable to the achieved ones in the soft X-rays. One of the crucial points in X-ray optics, indeed, is multilayer interfacial microroughness that causes effective area reduction and X-Ray Scattering (XRS). The latter, in particular, is responsible for image quality degradation. Interfacial smoothness deterioration in multilayer deposition processes is commonly observed as a result of substrate profile replication and intrinsic random deposition noise. For this reason, roughness growth should be carefully investigated by surface topographic analysis, X-ray reflectivity and XRS measurements. It is convenient to express the roughness evolution in terms of interface Power Spectral Densities (PSD), that are directly related to XRS and, in turn, in affecting the optic HEW (Half Energy Width). In order to interpret roughness amplification and to help us to predict the imaging performance of hard X-ray optics, we have implemented a well known kinetic continuum equation model in a IDL language program (MPES, Multilayer PSDs Evolution Simulator), allowing us the determination of characteristic growth parameters in multilayer coatings. In this paper we present some results from analysis we performed on several samples coated with hard X-ray multilayers (W/Si, Pt/C, Mo/Si) using different deposition techniques. We show also the XRS predictions resulting from the obtained modelizations, in comparison to the experimental XRS measurements performed at the energy of 8.05 keV.

Qualification and Testing of a Large Hot Slumped Secondary Mirror for Schwarzschild–Couder Imaging Air Cherenkov Telescopes

Dual-mirror Schwarzschild–Couder (SC) telescopes are based on highly aspherical optics, and they represent a novel design in the world of very high energy astrophysics. This work addresses the realization and the qualification of the secondary mirror for an SC telescope, named ASTRI, developed in the context of the Cherenkov Telescope Array Observatory. The discussion surveys the overall development from the early design concept to the final acceptance optical tests.

Status of the technologies for the production of the Cherenkov telescope array (CTA) mirrors

The Cherenkov Telescope Array (CTA) is the next generation very high-energy gamma-ray observatory, with at least 10 times higher sensitivity than current instruments. CTA will comprise several tens of Imaging Atmospheric Cherenkov Telescopes (IACTs) operated in array-mode and divided into three size classes: large, medium and small telescopes. The total reflective surface could be up to 10,000 m2 requiring unprecedented technological efforts. The properties of the reflector directly influence the telescope performance and thus constitute a fundamental ingredient to improve and maintain the sensitivity. The R&D status of lightweight, reliable and cost-effective mirror facets for the CTA telescope reflectors for the different classes of telescopes is reviewed in this paper.

Mirror development for CTA

The Cherenkov Telescope Array (CTA), currently in its early design phase, is a proposed new project for groundbased gamma-ray astronomy with at least 10 times higher sensitivity than current instruments. CTA is planned to consist of several tens of large Imaging Atmospheric Cherenkov Telescopes (IACTs) with a combined reflective surface of up to 10,000 m2. The challenge for the future CTA array is to develop lightweight and cost efficient mirrors with high production rates, good longterm durability and adequate optical properties. The technologies currently under investigation comprise different methods of carbon fibre/epoxy based substrates, sandwich concepts with cold-slumped surfaces made of thin float glass and different structural materials like aluminum honeycomb, glass foam or PU foam inside, and aluminum sandwich structures with either diamond milled surfaces or reflective foils. The current status of the mirror development for CTA will be summarized together with investigations on the improvement of the reflective surfaces and their protection against degradation.

Development of lightweight optical segments for adaptive optics

The large telescopes nowadays under development will have the adaptive optics systems fully integrated from the beginning of the project. These optics are in fact an essential component that is necessary for the full exploitation of the performances obtainable from the large optics foreseen in these instruments. Due to the large reflecting areas of these telescopes their adaptive optics systems will use probably thin segmented mirrors, assembled to create a single surface, placed along the optical train. Today, a number of telescopes (MMT, LBT, etc) have the monolithic secondary mirror of the instrument used as a component of the adaptive optic system. The technique used for the production of these single pieces thin mirror shells, typically having convex shape, is not well suited for the manufacturing of the large number of segments necessaries for the future telescopes. Infact, the procedure foresees the thinning of conventional thick mirrors, a technique expensive and time consuming. It is hence necessary to find a better approach able to produce thin optical segments in a cost effective way and with short delivery time. In this study, financed in the frame of OPTICON-FP6, the Astronomical Observatory of Brera (INAF-OAB) is investigating a technique for the manufacturing of these optical components that has the potential to fulfill these requirements. The curved optical segments that are under development will have a thickness of few mm and will be made in BorofloatTM glass. The technique foresees the thermal slumping of thin glass segments using a high quality ceramic mold as a master to impart a precise shape to the glass. The initially flat glass segment is placed onto the mold and then, by means of a suitable thermal cycle, the material is softened so to copy the master shape. If necessary, at the end of the slumping is foreseen the correction of the eventual remaining errors using the Ion Beam Figuring technique. This paper describes the process of production of the optical segments and the status of the investigation.

Multilayer coatings for x-ray mirrors: extraction of stack parameters from x-ray reflectivity scans and comparison with transmission electron microscopy results

The reflectance effectiveness of a multilayer depends strongly on the stack properties thickness, roughness, and density of each layer and can be directly tested by means of x-ray reflectivity scans at definite photon energies. The reflectivity curves are also a pow- erful tool for the in-depth, nondestructive characterization of the stack structure: The complex task of extracting the stack parameters from re- flectivity curves can be achieved via a suitable best-fitting computer code based on a global automatic optimization procedure. We present the computer-assisted layer-by-layer analysis of the characteristics of Ni/C, Pt/C, and W/Si multilayers, based on x-ray reflectivity scans performed at 8.05 and 17.45 keV. In order to verify the correctness of the code predictions, we present also a comparison of the computer model with the transmission electron microscope profiles of the same multilayer samples.

An x-ray polarimeter for hard x-ray optics

Development of multi-layer optics makes feasible the use of X-ray telescope at energy up to 60-80 keV: in this paper we discuss the extension of photoelectric polarimeter based on Micro Pattern Gas Chamber to high energy X-rays. We calculated the sensitivity with Neon and Argon based mixtures at high pressure with thick absorption gap: placing the MPGC at focus of a next generation multi-layer optics, galatic and extragalactic X-ray polarimetry can be done up till 30 keV.

The ASTRI camera for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) foresees, in its southern site (Chile), the implementation of up to 70 small-sized telescopes (SSTs), which will extend the energy coverage up to hundreds of TeV. It has been proposed that one of the first set of CTA SSTs will be represented by the ASTRI mini-array, which includes (at least) nine ASTRI telescopes. The endto-end prototype of such telescopes, named the ASTRI SST-2M, is installed in Italy and it is now completing the overall commissioning and entering the science verification phase. ASTRI telescopes are characterized by an optical system based on a dual-mirror Schwarzschild-Couder design and a camera at the focal plane composed of silicon photomultiplier sensors managed by a fast read-out electronics specifically designed. Based on a custom peak-detector mode, the ASTRI camera electronics is designed to perform Cherenkov signal detection, trigger generation, digital conversion of the signals and data transmission to the camera server. In this contribution we will describe the main features of the ASTRI camera, its performance and results obtained during the commissioning phase of the ASTRI SST-2M prototype in view of the ASTRI mini-array implementation.