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Developments for coating, testing, and aligning Cherenkov Telescope Array mirrors in Tübingen  [PDF]
A. Bonardi,J. Dick,E. Kendziorra,G. Pühlhofer,A. Santangelo for the CTA consortium
Physics , 2013,
Abstract: The Cherenkov Telescope Array (CTA) is the next generation very-high energy gamma-ray air-shower Cherenkov observatory. CTA will consist of many segmented-mirror telescopes of three different diameters, placed in two arrays, one in the Northern hemisphere and one in the South, thus covering the whole sky. The total number of mirror tiles will be of the order of 10000, corresponding to a reflective area of ~10^4 m^2. The Institute for Astronomy and Astrophysics in T\"ubingen is developing procedures to coat glass-substrate-based mirror tiles, is participating to the CTA mirror prototype testing, and is prototyping Active Mirror Control alignment mechanics, electronics and software. We will present the current status of our work and plans for future developments.
The Formation of Condensation on Cherenkov Telescope Mirrors  [PDF]
P. M. Chadwick,S. A. Cleaver,M. Dyrda,A. F?rster,J. Michalowski,J. Niemiec,C. Schultz,M. Stodulski,for the CTA Consortium.
Physics , 2013,
Abstract: The mirrors of imaging atmospheric Cherenkov telescopes are different from those of conventional astronomical telescopes in several ways, not least in that they are exposed to the elements. One of the issues which may arise is condensation forming on the mirrors during observing under certain atmospheric conditions, which has important consequences for the operation of the telescopes. This contribution discusses why telescope mirrors suffer condensation and describes the atmospheric conditions and mirror designs which are likely to be problematic.
An outdoor test facility for the Cherenkov Telescope Array mirrors  [PDF]
M. C. Medina,B. Garcia,J. Maya,A. Mancilla,J. J. Larrarte,E. Rasztocky,M. Benitez,J. Dipold,M. Platino,for the CTA Consortium
Physics , 2013,
Abstract: The Cherenkov Telescopes Array (CTA) is planned to be an Observatory for very high energy gamma ray astronomy and will consist of several tens of telescopes which account for a reflective surface of more than 10000 m$^2$. The mirrors of these telescopes will be formed by a set of facets. Different technological solutions, for a fast and cost efficient production of light-weight mirror facets are under test inside the CTA Consortium. Most of them involve composite structures whose behavior under real observing conditions is not yet fully tested. An outdoor test facility has been built in one of the candidate sites for CTA, in Argentina (San Antonio de los Cobres [SAC], 3600m a.s.l) in order to monitor the optical and mechanical properties of these facets exposed to the local atmospheric conditions for a given period of time. In this work we present the preliminary results of the first Middle Size Telescope (MST) mirror-monitoring campaign, started in 2013.
Open-structure composite mirrors for the Cherenkov Telescope Array  [PDF]
Michal Dyrda,Jerzy Michalowski,Jacek Niemiec,Marek Stodulski
Physics , 2013,
Abstract: The Cherenkov Telescope Array (CTA) Observatory for high-energy gamma-ray astronomy will comprise several tens of imaging atmospheric Cherenkov telescopes (IACTs) of different size with a total reflective area of about 10,000 m$^2$. Here we present a new technology for the production of IACT mirrors that has been developed in the Institute of Nuclear Physics PAS in Krakow, Poland. An open-structure composite mirror consists of a rigid flat sandwich support structure and cast-in-mould spherical epoxy resin layer. To this layer a thin glass sheet complete with optical coating is cold-slumped to provide the spherical reflective layer of the mirror. The main components of the sandwich support structure are two flat float glass panels inter spaced with V-shape aluminum spacers of equal length. The sandwich support structure is open, thus enabling good cooling and ventilation of the mirror. A special arrangement of the aluminum spacers also prohibits water being trapped inside. The open-structure technology thus represents a novel cost-efficient approach that does not require the perfect sealing needed in closed-type mirrors. In addition, the technology enables the application of a dielectric coating. Full-size prototype mirrors designed for the medium-size CTA telescope will be presented together with the results of recent optical tests.
Developments of a new mirror technology for the Cherenkov Telescope Array  [PDF]
Jerzy Micha?owski,Michal Dyrda,Jacek Niemiec,Maciej Sowiński,Marek Stodulski
Physics , 2015,
Abstract: The Cherenkov Telescope Array (CTA) observatory for very high-energy gamma rays will consist of about a hundred of imaging atmospheric Cherenkov telescopes (IACTs) of different size with a total reflective area of about 10,000 m$^2$. Here we present a novel technology for the production of IACT mirrors that has been developed in the Institute of Nuclear Physics PAS in Krakow, Poland. The mirrors are made by cold-slumping of the front reflecting aluminium-coated panel and the rear panel interspaced with aluminium spacers. Each panel is built of two glass panels laminated with a layer of a fibreglass tissue in between for reinforcement of the structure against mechanical damage. The mirror structure is open and does not require a perfect sealing needed in closed-type designs. It prohibits water to be trapped inside and enables a proper ventilation of the mirror. Full-size hexagonal prototype mirrors produced for the medium-sized CTA telescopes will be presented together with the results of recent comprehensive optical and durability tests. Their design will be compared to the earlier technology developed at INP PAS that used a rigid flat open support structure with a reflective layer made by cold-slumping of the coated glass panel to the cast-in-mould spherical epoxy resin layer.
Status of the technologies for the production of the Cherenkov Telescope Array (CTA) mirrors  [PDF]
G. Pareschi,T. Armstrong,H. Baba,J. B?hr,A. Bonardi,G. Bonnoli,P. Brun,R. Canestrari,P. Chadwick,M. Chikawa,P. -H. Carton,V. de Souza,J. Dipold,M. Doro,D. Durand,M. Dyrda,A. F?rster,M. Garczarczyk,E. Giro,J. -F. Glicenstein,Y. Hanabata,M. Hayashida,M. Hrabovski,C. Jeanney,M. Kagaya,H. Katagiri,L. Lessio,D. Mandat,M. Mariotti,C. Medina,J. Micha?owski,P. Micolon,D. Nakajima,J. Niemiec,A. Nozato,M. Palatka,M. Pech,B. Peyaud,G. Pühlhofer,M. Rataj,G. Rodeghiero,G. Rojas,J. Rousselle,R. Sakonaka,P. Schovanek,K. Seweryn,C. Schultz,S. Shu,F. Stinzing,M. Stodulski,M. Teshima,P. Travniczek,C. van Eldik,V. Vassiliev,?. Wi?niewski,A. W?rnlein,T. Yoshida
Physics , 2013, DOI: 10.1117/12.2025476
Abstract: 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.
The Optical System for the Large Size Telescope of the Cherenkov Telescope Array  [PDF]
M. Hayashida,K. Noda,M. Teshima,U. Barres de Almeida,M. Chikawa,N. Cho,S. Fukami,A. Gadola,Y. Hanabata,D. Horns,C. Jablonski,H. Katagiri,M. Kagaya,M. Ogino,A. Okumura,T. Saito,R. Stadler,S. Steiner,U. Straumann,A. Vollhardt,H. Wetteskind,T. Yamamoto,T. Yoshida,for the CTA Consortium
Physics , 2015,
Abstract: The Large Size Telescope (LST) of the Cherenkov Telescope Array (CTA) is designed to achieve a threshold energy of 20 GeV. The LST optics is composed of one parabolic primary mirror 23 m in diameter and 28 m focal length. The reflector dish is segmented in 198 hexagonal, 1.51 m flat to flat mirrors. The total effective reflective area, taking into account the shadow of the mechanical structure, is about 368 m$^2$. The mirrors have a sandwich structure consisting of a glass sheet of 2.7 mm thickness, aluminum honeycomb of 60 mm thickness, and another glass sheet on the rear, and have a total weight about 47 kg. The mirror surface is produced using a sputtering deposition technique to apply a 5-layer coating, and the mirrors reach a reflectivity of $\sim$94% at peak. The mirror facets are actively aligned during operations by an active mirror control system, using actuators, CMOS cameras and a reference laser. Each mirror facet carries a CMOS camera, which measures the position of the light spot of the optical axis reference laser on the target of the telescope camera. The two actuators and the universal joint of each mirror facet are respectively fixed to three neighboring joints of the dish space frame, via specially designed interface plate.
A facility to evaluate the focusing performance of mirrors for Cherenkov Telescopes  [PDF]
Rodolfo Canestrari,Enrico Giro,Giacomo Bonnoli,Giancarlo Farisato,Luigi Lessio,Gabriele Rodeghiero,Rossella Spiga,Giorgio Toso,Giovanni Pareschi
Physics , 2015, DOI: 10.1016/j.nima.2015.09.092
Abstract: With the advent of the imaging atmospheric Cherenkov technique in late 1980's, ground-based observations of Very High-Energy gamma rays came into reality. Since the first source detected at TeV energies in 1989 by Whipple, the number of high energy gamma-ray sources has rapidly grown up to more than 150 thanks to the second generation experiments like MAGIC, H.E.S.S. and VERITAS. The Cherenkov Telescope Array observatory is the next generation of Imaging Atmospheric Cherenkov Telescopes, with at least 10 times higher sensitivity than current instruments. Cherenkov Telescopes have to be equipped with optical dishes of large diameter -- in general based on segmented mirrors -- with typical angular resolution of a few arc-minutes. To evaluate the mirror's quality specific metrological systems are required that possibly take into account the environmental conditions in which typically Cherenkov telescopes operate (in open air without dome protection). For this purpose a new facility for the characterization of mirrors has been developed at the labs of the Osservatorio Astronomico di Brera of the Italian National Institute of Astrophysics. The facility allows the precise measurement of the radius of curvature and the distribution of the concentred light in terms of focused and scattered components and it works in open air. In this paper we describe the facility and report some examples of its measuring capabilities from recent tests campaigns carried out performed on mirrors devoted to Cherenkov telescopes.
4 m Davies-Cotton telescope for the Cherenkov Telescope Array  [PDF]
R. Moderski,J. A. Aguilar,A. Barnacka,A. Basili,V. Boccone,L. Bogacz,F. Cadoux,A. Christov,M. Della Volpe,M. Dyrda,A. Frankowski,M. Grudzińska,M. Janiak,M. Karczewski,J. Kasperek,W. Kochański,P. Korohoda,J. Kozio?,P. Lubiński,J. Ludwin,E. Lyard,A. Marsza?ek,J. Micha?owski,T. Montaruli,J. Nicolau-Kukliński,J. Niemiec,M. Ostrowski,?. P?atos,P. J. Rajda,M. Rameez,W. Romaszkan,M. Rupiński,K. Seweryn,M. Stodulska,M. Stodulski,R. Walter,K. Winiarski,?. Wi?niewski,A. Zagdański,K. Zietara,P. Zió?kowski,P. ?ychowski,for the the CTA Consortium
Physics , 2013,
Abstract: The Cherenkov Telescope Array (CTA) is the next generation very high energy gamma-ray observatory. It will consist of three classes of telescopes, of large, medium and small sizes. The small telescopes, of 4 m diameter, will be dedicated to the observations of the highest energy gamma-rays, above several TeV. We present the technical characteristics of a single mirror, 4 m diameter, Davies-Cotton telescope for the CTA and the performance of the sub-array consisting of the telescopes of this type. The telescope will be equipped with a fully digital camera based on custom made, hexagonal Geiger-mode avalanche photodiodes. The development of cameras based on such devices is an RnD since traditionally photomultipliers are used. The photodiodes are now being characterized at various institutions of the CTA Consortium. Glass mirrors will be used, although an alternative is being considered: composite mirrors that could be adopted if they meet the project requirements. We present a design of the telescope structure, its components and results of the numerical simulations of the telescope performance.
The small size telescope projects for the Cherenkov Telescope Array  [PDF]
Teresa Montaruli,Giovanni Pareschi,Tim Greenshaw
Physics , 2015,
Abstract: The small size telescopes (SSTs), spread over an area of several square km, dominate the CTA sensitivity in the photon energy range from a few TeV to over 100 TeV, enabling for the detailed exploration of the very high energy gamma-ray sky. The proposed telescopes are innovative designs providing a wide field of view. Two of them, the ASTRI (Astrophysics con Specchi a Tecnologia Replicante Italiana) and the GCT (Gamma-ray Cherenkov Telescope) telescopes, are based on dual mirror Schwarzschild-Couder optics, with primary mirror diameters of 4 m. The third, SST-1M, is a Davies-Cotton design with a 4 m diameter mirror. Progress with the construction and testing of prototypes of these telescopes is presented. The SST cameras use silicon photomultipliers, with preamplifier and readout/trigger electronics designed to optimize the performance of these sensors for (atmospheric) Cherenkov light. The status of the camera developments is discussed. The SST sub-array will consist of about 70 telescopes at the CTA southern site. Current plans for the implementation of the array are presented.
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