The dual-mirror Small Size Telescope for the Cherenkov Telescope Array
G. Pareschi, G. Agnetta, L.A. Antonelli, D. Bastieri, G. Bellassai, M. Belluso, C. Bigongiari, S. Billotta, B. Biondo, G. Bonanno, G. Bonnoli, P. Bruno, A. Bulgarelli, R. Canestrari, M. Capalbi, P. Caraveo, A. Carosi, E. Cascone, O. Catalano, M. Cereda, P. Conconi, V. Conforti, G. Cusumano, V. De Caprio, A. De Luca, A. Di Paola, F. Di Pierro, D. Fantinel, M. Fiorini, D. Fugazza, D. Gardiol, M. Ghigo, F. Gianotti, S. Giarrusso, E. Giro, A. Grillo, D. Impiombato, S. Incorvaia, A. La Barbera, N. La Palombara, V. La Parola, G. La Rosa, L. Lessio, G. Leto, S. Lombardi, F. Lucarelli, M.C. Maccarone, G. Malaguti, G. Malaspina, V. Mangano, D. Marano, E. Martinetti, R. Millul, T. Mineo, A. MistÒ, C. Morello, G. Morlino, M.R. Panzera, G. Rodeghiero, P. Romano, F. Russo, B. Sacco, N. Sartore, J. Schwarz, A. Segreto, G. Sironi, G. Sottile, A. Stamerra, E. Strazzeri, L. Stringhetti, G. Tagliaferri, V. Testa, M.C. Timpanaro, G. Toso, G. Tosti, M. Trifoglio, P. Vallania, S. Vercellone, V. Zitelli, J.P. Amans, C. Boisson, C. Costille, J.L. Dournaux, D. Dumas, G. Fasola, O. Hervet, J.M. Huet, P. Laporte, C. Rulten, H. Sol, A. Zech, R. White, J. Hinton, D. Ross, J. Sykes, S. Ohm, J. Schmoll, P. Chadwick, T. Greenshaw, M. Daniel, et al. (14 additional authors not shown)
333 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
IO DE J ANEIRO T HE A STROPARTICLE P HYSICS C ONFERENCE
The dual-mirror Small Size Telescope for the Cherenkov Telescope Array
G. P
ARESCHI , G. A GNETTA , L. A. A NTONELLI , D. B ASTIERI , G. B ELLASSAI , M. B ELLUSO , S. B ILLOTTA , B.B IONDO , G. B ONANNO , G. B ONNOLI , P. B RUNO , A. B ULGARELLI , R. C ANESTRARI , P. C ARAVEO , A.C AROSI , E. C ASCONE , O. C ATALANO , M. C EREDA , P. C ONCONI , V. C ONFORTI , G. C USUMANO , V. D E C APRIO , A. D E L UCA , A. D I P AOLA , F. D I P IERRO , D. F ANTINEL , M. F IORINI , D. F UGAZZA , D. G ARDIOL ,M. G HIGO , F. G IANOTTI , S. G IARRUSSO , E. G IRO , A. G RILLO , D. I MPIOMBATO , S. I NCORVAIA , A. L A B ARBERA , N. L A P ALOMBARA , V. L A P AROLA , G. L A R OSA , L. L ESSIO , G. L ETO , S. L OMBARDI , F.L UCARELLI , M. C. M ACCARONE , G. M ALAGUTI , G. M ALASPINA , A. M ANGANO , V. M ANGANO , D.M ARANO , E. M ARTINETTI , R. M ILLUL , T. M INEO , A. M IST ´ O , C. M ORELLO , M.R. P ANZERA , C. P ERNA , G.R ODEGHIERO , P. R OMANO , F. R USSO , B. S ACCO , N. S ARTORE , J. S CHWARZ , A. S EGRETO , G. S IRONI , G.S OTTILE , E. S TRAZZERI , L. S TRINGHETTI , G. T AGLIAFERRI , V. T ESTA , M. C. T IMPANARO , G. T OSO , G.T OSTI , M. T RIFOGLIO , P. V ALLANIA , S. V ERCELLONE , V. Z ITELLI , D. D UMAS , P. L APORTE , H. S OL , F. DE F RONDAT , J.-M. H UET , J.-L. D OURNAUX , J.-P. A MANS , S. B LANC , G. F ASOLA , R. F LEURISSON , O.H ERVET , I. J EGOUZO -G IROUX , D. M ASSOL , C. R ULTEN , F. S AYEDE , D. S AVOIE , A. Z ECH , C. B OISSON , P.D ELEVOYE , N. O LLIVIER , R. W HITE , J. H INTON , D. R OSS , J. S YKES , S. O HM , S. B LAKE , J. S CHMOLL , P.C HADWICK , T. G REENSHAW , M. D ANIEL , G. C OTTER , G. S. V ARNER , S. F UNK , J. V ANDENBROUCKE , L.S APOZHNIKOV , J. B UCKLEY , P. M OORE , D. W ILLIAMS , S. M ARKOFF , J. V INK , D. B ERGE , N. H IDAKA , A.O KUMURA , H. T AJIMA , FOR THE
CTA
COLLABORATIONS INAF & the ASTRI collaboration Paris Observatory & the GATE collaboration The CHEC collaboration, various affiliations [email protected]
Abstract:
In this paper, the development of the dual mirror Small Size Telescopes (SST) for the CherenkovTelescope Array (CTA) is reviewed. Up to 70 SST, with a primary mirror diameter of ∼ ∼
40 cm in diameter), and low-cost. The camera, which has about 2000 pixels of size 6 × , coversa field of view of ∼ ◦ . The dual mirror telescopes and their cameras are being developed by three consortia,ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana, Italy/INAF), GATE (Gamma-ray TelescopeElements, France/Paris Observ.) and CHEC (Compact High Energy Camera, universities in UK, US and Japan)which are merging their efforts in order to finalize an end-to-end design that will be constructed for CTA. Anumber of prototype structures and cameras are being developed in order to investigate various alternative designs.In this contribution, these designs are presented, along with the technological solutions under study. Keywords:
Cherenkov Telescope Array, SST, dual-mirror telescopes, SiPM, MAPM, structures and mirrors.
The forthcoming Cherenkov Telescope Array (CTA)[1, 2], with its innovative approach based on the useof three different sizes of telescopes, will obtain aone-order-of-magnitude improvement with respect tothe current Cherenkov telescope performance (achievedwithin HESS, MAGIC and VERITAS). CTA aims toprovide global coverage of the sky from two observatorysites: a Southern array, implemented in particular for theexploration of both the Galactic plane and the extragalacticsky, and a Northern array, mainly devoted to the study ofextragalactic sources. In the atmospheric showers originatedby a gamma–ray primary, the Cherenkov light intensityis almost proportional to the gamma–ray energy (see e.g.[3]). In general, large–diameter mirrors are needed totrigger low energy gamma-rays, while small mirrors are sufficient enough to trigger high energy events. Moreover,due to the very low gamma-ray fluxes at high energy, futureCherenkov telescopes like CTA must be able to catch eventsreaching the ground very far ( ∼ −
500 m) from thetelescope position, thus achieving effective areas of theorder of 10 m ; to trigger far showers, imaged at largeoff-axis angles, Cherenkov telescopes must be providedwith sufficiently large fields of view. Optical dish diametersand field of view are the first parameters to be consideredbefore analyzing other specific aspects such as opticaldesign, mirror structure, focal plane sensors and electronics.In this respect, due to the forward direction of Cherenkovlight emission in air and the resulting Cherenkov lightpool of fairly uniform illumination of about 200–250mdiameter, for CTA an inter-telescope spacing of about100m is needed at threshold energies to provide images a r X i v : . [ a s t r o - ph . I M ] J u l he dual mirror Small Size Telescope for CTA33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
IO DE J ANEIRO in multiple telescopes. Well above this threshold, showerscan be detected from outside the light pool, if the field ofview is large enough. In CTA a small number (4 units forboth northern and southern sites) of Large Size Telescopes(LST) of 23 m diameter will be deployed close to the centreof the array with (cid:39)
100 m spacing. A larger number (up to61 in the southern site, 15 in the northern site) of MediumSize Telescopes (MST) will cover a larger area, with aninter-telescope spacing of (cid:39)
150 m. The southern site willinclude at least 25 single-mirror telescopes of 12 m and upto 36 Schwarzschild-Couder Telescopes of 9.5 m diameter.Above a few TeV the Cherenkov light intensity is suchthat showers can be detected even well outside the lightpool by telescopes significantly smaller than the MST. Toachieve the required sensitivity at high energies, a verylarge area on the ground needs to be covered by the SmallSize Telescopes with a resolution of (cid:39) ◦ and a fieldof view of (cid:39) ◦ . The SST sub–array can therefore beaccomplished by 70 telescopes with a mirror area of 5–10m and (cid:39)
300 m spacing, distributed across an area of 10km and within a radius of about 3 km.The SST array will be implemented just on the southernsite for reason of costs and taking into account that thevery high energy emission can be observed just for galacticsources, unless non standard processes are invoked. In orderto allow the implementation of the large number ( ∼ ≤
500 kEuro per unit is essential. It shouldbe noted that classical parabolic or Davies-Cotton (DC)single–mirror configurations have been used so far forCherenkov telescopes, and they are adopted also for theCTA LST and MST telescopes respectively. Howeverthey are dominated by the cost of the camera, which isbased on classical large-size photo-multipliers and theydo not seem ideal for making the wide-field SST units.A possible solution is to implement the classical DCsolution with Winston cone light guides, to squeeze thelight with an aggressive concentration ratio, but with thecrucial drawbacks of a difficult implementation and limitednumber of pixels [4]. But a particularly attractive solution torealize the SST telescopes is the use of a dual-mirror (2M)solution, adopting the so called Schwarzschild–Couder(SC) configuration. This enables good angular resolutionacross the entire field of view, almost 10 ◦ in diameter,and also reduces the effective focal length and camerasize [5]. As has previously been demonstrated, 2M SCtelescopes allow better correction of aberrations at largefield angles and hence the construction of telescopes with asmaller focal ratio. This implies that, for a given primarymirror and angular pixel size, the physical pixels aresmaller. This approach allows the use of low-cost, compact,and low power consumption cameras, based on siliconphotomultipliers (SiPM) or multi-anode photomultipliers(MAPM), commercially available sensors with typicalpixel size of ∼ × . The trigger threshold of 1 TeVimplies primary mirror diameter D (cid:39) The Figure 1 shows the 3-dimensional drawings of thetelescope structures under development by the ASTRIand GATE SST collaborations. Matching the physicalsize of the pixels offered by MAPM or SiPM sensors (afew millimetres) to the required angular pixel size of theSST implies that the focal length of the telescope F is (cid:39) ◦ , an angularpixel size of approximately 0.17 ◦ and an equivalent focallength of 2150 mm. This setup delivers a corrected field ofview up to 9.6 ◦ in diameter. Concerning the throughput, amean value of the effective area of about 6 m is achieved,taking into account: the segmentation of the primary mirror,the obscuration of the secondary mirror, the obscurationof the camera, the reflectivity of the optical surfaces as afunction of the wavelength and incident angle, the lossesdue to the camera’s protection window and finally theefficiency of the detector as a function of the incidentangles (ranging from 25 ◦ to 72 ◦ ). The resulting telescope iscompact, having a primary mirror (M1) diameter of 4 mand a primary-to-secondary distance of 3 m.Both the ASTRI and GATE telescopes adopt analt-azimuthal design in which the azimuth axis will permita rotation range of ± ◦ . The mirror dish is mounted onthe azimuth fork which allows rotation around the elevationaxis from -5 ◦ to +95 ◦ . The mast structure that supports thesecondary mirror and the camera has different solutionsbetween the two projects. In order to balance the torquedue to the overhang of the optical tube assembly withrespect to the horizontal rotation axis, counterweights arealso envisaged. For the ASTRI telescope it is proposed toconstruct the primary mirror as a set of 18 hexagonal-shapedpanels having 850 mm face-to-face dimension. Threedifferent types of mirror profiles are necessary to reproducethe M1 profile. GATE will exploit petal-shaped segmentsmade in polished Aluminum. For both telescopes, the aimis to build a monolithic secondary mirror. The ASTRI design.
The top of the column is interfaced to the azimuth fork, thisinterface being the azimuth bearing. The fork supports theelevation assembly (dish, counterweight and the quadrupodthat is the structure supporting the secondary mirror andconnected to the dish) through a linear actuator and bearingsat its upper ends. The fork is composed of welded steel he dual mirror Small Size Telescope for CTA33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
IO DE J ANEIRO box sections. The M1 dish structure is attached to theazimuth fork using two preloaded tapered roller bearings,one on each of the fork’s arms. The dish is a ribbed steelplate of about 40 cm thickness. To this are attached thesupports for the mirror segments, each of which includesa single and a double axis actuator and a bearing, whichallows steering of the segments for alignment purposes. Theeccentric quadrupod legs counteract the lateral deformationsof the mast structure due to gravitational and wind loads,while the central tube increases the torsional stiffness ofthe structure. At the upper end of the mast is located thestructure for supporting the monolithic secondary mirror.This consists of three actuators attached to the mirror viawhiffletrees to ensure the load is spread over a sufficientlylarge area. In addition, three lateral arms support thetransverse components of the secondary mirror’s weightas they vary with the orientation of the telescope. Allstructures are made of steel and protected against corrosionby paint. The azimuth drive is located at the base of thecolumn and is composed of two pinions, driven by electricmotors, that couple with a rim gear. Axially pre-loadedball bearings complete the azimuth assembly. The linearactuator, which drives the elevation, consists of a preloadedball screw driven through a gearbox by an electric motor.The orientation of the telescope is determined using absoluteencoders located on each of the azimuth and altitude axes.Finite Element Analysis (FEA) has been used to evaluatethe performance of the system. The lowest frequencyeigenmode of the oscillations of the structure is 4.5 Hz.FEA has also been carried out to determine the effects onthe telescope of temperature gradients. More details on theASTRI design are presented by Canestrari et al. in [6].
The GATE design
The design philosophy was to split the telescope intofunctions so that each one becomes as independent aspossible to alleviate the constraints on the design. TheGATE structure is based on a shallow fork structure forwhich one counterweight is needed, mounted internallyto the fork. The azimuth is mounted on the tower andthe fork holds the elevation. The boss-head connects theelevation subsystems to both the counterweight and theoptical structure. The mast and truss structure is composedof the support of the M1 dish, tubes in a Serrurier-likehexapod configuration, the support of the M2 dish andarms holding the camera. Aluminum and carbon fiber havealso been considered, but finally steel has been selected tooptimize costs and easiness of manufacturing and mounting.The alt-azimuth system is the same for the azimuth andelevation and is composed of slewing bearing with wormgear (1 for azimuth, 2 for elevation), motors and absoluteencoders. FEA has determined that the lowest frequencyeigenmodes of the oscillations of the telescope involvetransverse motion of the secondary with respect to theprimary and are about 5 Hz. Torsional eigenmodes ofoscillation have lowest frequencies of about 12 Hz. Thesevalues refer to a preliminary design not yet optimized. Moredetails are presented by Zech et al. in [7].
Figure 2 shows the 3-dimensional drawings of the camerastructures under development by the ASTRI and CHECcollaborations. Details are hereafter reported.
The ASTRI design. Figure 1 : General view of the ASTRI and GATE telescopestructures and electro-mechanical subsystems.The ASTRI camera [8] adopts SiPM as photosensors. Eachpixel contains 3600 microcells, each being an avalanchephotodiode operated in quenched Geiger mode. These cellsare 50 × µ m wide, giving a fill factor of 70%. Theparticular device chosen is the Hamamatsu S11828-334monolithic multi-pixel SiPM, consisting of 4x4 pixels ofroughly 3x3 mm each. Four Hamamatsu pixels are groupedtogether to form one camera pixel with a physical size6.2 × , matching the required angular size. Fourof the Hamamatsu devices are put together to form aunit. Four such units then form a module called a PhotonDetection Module (PDM). This module is composed of 16Hamamatsu devices and has dimensions of 56 ×
56 mm .The PDM are constructed by plugging the Hamamatsudevices into connectors attached to a printed circuit board(PCB). Under each unit on the PCB there is a smalltemperature sensor, allowing the temperature of the SiPMto be monitored, providing a route through which thetemperature dependent SiPM gain can be stabilized. TheFront End Electronics (FEE) boards supply the power forthe SiPM, perform the readout and form the first triggersignals. The EASIROC (Extended Analogue SiPM ReadoutChip) ASIC will be adopted. A range of simulation tools hasbeen produced to aid the design of the readout and trigger.With the presently available Hamamatsu SiPM devicesthere are small gaps between the sensors when they aremounted to form units and PDMs. Truncated pyramidallight guides, made of high refractive index glass (2.5 mmthick) to guarantee a large acceptance angle, are finallyimplemented in order to reduce the light losses due to deadareas. In addition to the FEE mentioned above, design ofthe Back-End Electronics (BEE) is underway. This willuse a Field Programmable Gate Array (FPGA) and localmemory to provide interfaces to the CTA data acquisition,to the camera controller and to the CTA clock. There willalso be circuitry to provide the various DC voltages needed he dual mirror Small Size Telescope for CTA33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
IO DE J ANEIRO to power the elements of the camera. A preliminary designof the chassis of the complete camera is also shown infigure 2 (upper panel). The total height of the camera isabout 30 cm. The camera lid can be seen in this picture.This subsystem can be closed to protect the sensors fromthe elements. As the focal plane of the 2M telescopes isconvex, with a radius of curvature of 1 m, the PDM mustbe attached to a precisely machined curved plate. Belowthe sensor plane is the support structure to which furtherelectronics boards and the cooling system can be attached.
The CHEC camera.
The CHEC collaboration will develop two cameras, onebased on MAPM and the other on SiPM sensors. The firstprototype will be the one based on MAPM sensors [9]and is hereafter described. The default photosensor for theCompact High-Energy Camera is the Hamamatsu H10966MAPM. This consists of 64 pixels each of size 6 × , ina unit of dimensions 52 ×
52 mm . Thirty-two MAPM canbe used to cover the focal plane, providing a field of viewof about 9 ◦ . Each FEE module provides the high voltagesupply needed by a MAPM, samples the signals producedby its 64 channels at a frequency of about 500 MHz, formsa first level trigger by applying thresholds to sums of fourpixels and outputs a digitized waveform for each of theMAPM channels. CHEC will make use of modules designedat SLAC both for the SC MSTs and for CHEC based on theTARGET ASIC. Minor modifications to this are needed andthese will be carried out in collaboration with SLAC. TheTARGET modules [10], as they are, cannot be connecteddirectly to the MAPM in the CHEC because the curvatureof the focal plane would then require large gaps between theMAPM. The MAPM is instead attached to a preamplifierboard and then a structure containing a twisted lengthof ribbon cable, allowing bending in two planes, whichcarries the signals from the MAPM to the electronics. Thepreamplifier board behind the MAPM allows the PMT tobe operated at low gain (typically 10 ), important given thehigh counting rate they will experience due to backgroundphotons. The preamplifier also allows shaping of the MAPMsignal and hence optimization of the performance of theFEE. Further, if it becomes clear that SiPM will offerbetter performance per unit cost than MAPM, the MAPMphotosensor plane with its preamplifiers can be replacedwith a SiPM-based system with new preamplifiers ensuringthe correct signal shape enters the TARGET module. Amechanical frame that provides the required rigid supporthas been designed. Prototype mechanical structures haveshown that this system functions as hoped, allowing theMAPM to be placed on a curved surface. The triggersignals provided by the FEE modules must be combinedand examined to select candidate Cherenkov events againstthe night-sky background. This camera trigger systemforms part of the BEE, which is also responsible forprocessing data from the FEE modules and distributingclock signals with the required level of precision to theFEE modules. Recent developments mean that it is feasibleto process trigger signals with nanosecond accuracy andsub-nanosecond delay correction from all FEE modules inone or two FPGA (Field Programmable Gate Array) devices.The mechanical aspects of the camera include a supportmatrix for the MAPM, internal elements for support ofthe electronics, a cooling system and an external structure,which includes a lid and the interface to the telescope. TheMAPM support matrix must ensure precise focal-planepositioning. The internal structure must allow adequate cooling of the electronics and remain stable on decadetimescales under repeated camera movements. Thermalmodeling of the camera will be done during the cameradesign to assess the cooling and control requirements, whichwill be implemented during the mechanical prototyping.The external structure must be weather-proof and minimizedust ingress, and provide minimal additional shadowing ofthe primary mirror. Figure 2 : General view of the ASTRI and CHEC cameras.
The dual mirror telescopes and their cameras are beingdeveloped by three consortia, ASTRI (Italy/INAF), GATE(France/Paris Observ.) and CHEC (universities in UK, USand Japan) which are merging their efforts in order tofinalize an end-to-end design that will be constructed forCTA. A number of prototype structures and cameras arebeing developed in order to investigate various alternativedesigns and a tradeoff study will be performed to determinethe final design.
Acknowledgment:
We gratefully acknowledge financialsupport from the following agencies and organizations: Ministeriode Ciencia, Tecnolog´ıa e Innovaci´on Productiva (MinCyT),Comisi´on Nacional de Energ´ıa At´omica (CNEA), ConsejoNacional de Investigaciones Cient´ıficas y T´ecnicas (CONICET),Argentina; State Committee of Science of Armenia, Armenia;FAPESP (Fundac¸˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo,Brasil; Ministry of Education, Youth and Sports, MEYS LE13012,7AMB12AR013, Czech Republic; Ministry of Higher Educationand Research, CNRS-INSU and CNRS-IN2P3, CEA-Irfu,ANR, Regional Council Ile de France, Labex ENIGMASS,OSUG2020 and OCEVU, France; Max Planck Society, BMBF,DESY, Helmholtz Association, Germany; Istituto Nazionaledi Astrofisica (INAF), MIUR, Italy; ICRR, The University ofTokyo, JSPS, Japan; Netherlands Research School for Astronomy(NOVA), Netherlands Organization for Scientific Research(NWO), Netherlands; The Bergen Research Foundation, Norway;Ministry of Science and Higher Education, the National Centrehe dual mirror Small Size Telescope for CTA33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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