Bastieri, D

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29th International Cosmic Ray Conference Pune (2005) 5, 283286The Mirrors for the MAGIC TelescopesD. Bastieri,2 D. Agguiaro,2 J. Arnold,4 C. Bigongiari,2 F. Dazzi,2 M. Doro,2 N. Galante,3M. Garczarczyk,1 E. Lorenz,1 D. Maniero,2 M. Mariotti,2 R. Mirzoyan,1 A. Moralejo,2D. Pascoli,2 A. Pepato,2 L. Peruzzo,2 M. Rebeschini,2 A. Saggion,2 P. Sartori,2 V. Scalzotto,2D. Tescaro,2 and N. Tonello1 for the MAGIC Collaboration (1) MaxPlanckInstitut f¤ur Physik, F¤ohringer Ring 6, D80805 M¤unchen, Germany.(2) Universita and INFN Padova, Via Marzolo 8, I35131 Padova, Italy.(3) Universita di Siena and INFN Sezione di Pisa, Via F. Buonarroti 2, I56127 Pisa, Italy.(4) LT Ultra GmbH, Wiesenstrasse 9, D-88634 Aftholderberg, Germany.  Updated collaborator list at http://magic.mppmu.mpg.de/collaboration/members/index.html.Presenter: D. Bastieri (bastieri@pd.infn.it), ita-bastieri-D-abs1-og27-posterThe MAGIC Telescope has the largest reflecting surface, among other Cherenkov detectors, in order to have anenergy threshold well below 100GeV. In this contribution, we describe the technology used for the productionand the optical qualities of the surface currently mounted onto the telescope. MAGIC features now 964 squaremirrors, 50 cm by side, each of spherical shape. We present also a new technology that, producing pre-shapedpanels, will finally yield lighter mirrors and can even be applied to make bigger mirrors of 1 m by side.1. IntroductionThe MAGIC telescope[1] features a huge reflecting surface[2] of 236 m2 and overall parabolic shape, whichallows detected photons to keep the correct timing information. The surface was segmented into 964 smaller el-ements (50cm  50cm), each machined to spherical shape with the curvature radius that better fits the requiredparabolic shape. Each element is an aluminium honeycomb core sandwiched between two outer Al-layers us-ing laminating adhesives. The sandwich, called raw blank, is later worked and polished with milling machines.Details can be found in sec. 2, while the optical properties of the mirrors can also be found in sec. 3.2. The MirrorsMAGIC mirrors are mainly a 5-mm thick AlMgSi1.0 plate, pre-machined to spherical shape and polishedwith a milling tool equipped with a diamond tip of large1 m  curvature radius. Plates are glued to anAl-honeycomb inside a thin Al-box making up the raw blank of4 kg of weight. The reflecting surface issubdivided into zones with varying curvature radii34  125  36  625m  to match the parabolic shape of thedish. After diamond milling, front plates are coated with a hard, transparent protective layer against scratchesand aging. Mirrors are then grouped in 3 or 4 onto panels and each panel can be moved and aligned by an activemirror control system. Each mirror is also equipped with a heating system to prevent ice and dew formation.2.1 Raw blank productionRaw blanks assembled in Padova are composed of a 1-mm thick Al 3003 box, 2  5 cm high, containing the Al5052 honeycomb of 2  0 cm of thickness and the heating printed circuit board (PCB). Four small plates, 5-mmthick, are embedded into the honeycomb and glued to the outer box. They host four screws each, to fix thefinished mirror to a panel. The box is then closed with the aluminium plate. Final assembly of the raw blankparts is done using three layers of 3M glue foils between box, honeycomb, PCB and front plate (see fig. 1 left).The gluing procedure consists in a 4-hour cycle. During this time the raw blanks, closed in an evacuated bag,are heated to 120  and stand to 3 atm of pressure. Up to 12 mirrors can be made in the same gluing cycle.284 D. Bastieri et al.Figure 1. Exploded view of the raw-blank structure (left). Sketch of the milling tool and the rotating table (right).About a quarter of the actual mirrors were machined from raw blanks made by MPI, which used a very similarassembling method. Their behaviour is quite similar to the mirrors made using raw blanks produced in Padova.2.2 Premilling and diamond polishingProduction starts with a rough, but quick, premilling of the raw blanks with an accuracy of better than 110 mm.The machining is done by fixing the raw blank onto a rotating plate and using a “T”-shaped tool, the fly-cutter,as shown in fig. 1 (right). The final shape is a spherical surface of radius R 	 rsinϑ where r is the radius of themilling tool and ϑ is the angle between the rotation axis of the milling tool and that of the plate.The diamond milling of the surface is done by the LT Ultra company (Aftholderberg, Germany). After diamondmilling, the roughness of the surface is well below 10 nmrms, as can be seen in fig. 2 (left) for a typical profileanalysed with a commercial surface roughness tester. From the same picture one can also see one step of themilling machine, that can follow the desired profile at a level of the micrometer.After coating, the overall reflectivity of the mirrors is between 85% and 90% in the visible band (see fig. 2right). Once all mirrors are mounted in place, the actual reflectivity of the whole surface can be measured.Different measurement techniques agree, estimating the light collection efficiency to be 77%  4%[1].2.3 Pre-shaped raw blanksTo further reduce production times, we investigated the feasibility of assembling pre-shaped raw blanks. Usingpre-shaped raw-blanks, two major issues could be improved:– the thickness of the Al slab, needed for the milling, was reduced from 5 to 1  2 mm;– premilling could be completely skipped.Pre-shaped mirrors are assembled, as the old one, in an autoclave environment, but are sandwiched betweentwo curved moulds, that shape the final raw blank with the requested curvature radius, between 34 and 36 m.Let us remind that, in this range, the sagittae vary0  2 mm. Therefore, we can produce all pre-shaped raw-blanks with just one gross curvature radius and let the diamond milling machine refine them by removing justa minimal amount of material. This results in a faster, and less expensive, overall procedure.The Mirrors for the MAGIC Telescopes 285Position: mm0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6nm-80-60-40-2002040Roughness = 4.1 nmnm200 300 400 500 600 700 800 900Reflectivity0.550.60.650.70.750.80.850.90.95Figure 2. Roughness (left) and optical reectivity (right) of a sample taken from a MAGIC mirror after milling and coating.In the roughness diagram, the actual prole (measured in nanometers) is plotted against the position (in millimeters).2.4 Larger raw blanks and mirrorsWorking with thinner, but pre-curved, Al-plates also makes the assembling and machining of larger mirrorseasier: in fact, a 1m  1m spherical mirror of34 m of curvature radius requests that4 mm of materialwould have to be removed from its centre if it were assembled with a flat plate, whereas virtually no material atall is removed from pre-shaped mirrors. Moreover, as MAGIC currently uses panels hosting four fixed mirrorseach for active optics, increasing the mirror size also eliminates the necessity to use back-panels, as the mirrorsthemselves could be controlled with minor refinements to the actual active optics device.Larger mirrors have nevertheless some drawbacks. In fact, MAGIC is made up with many small sphericalmirrors that best fit the desired overall parabolic shape: increasing mirror size makes the fit harder, at least forthe outer mirrors, where the requested paraboloid differs more from a sphere. Astigmatic mirrors can adaptbetter to parabolic shapes, but their production can be quite difficult, and for MAGIC-II[3], if machining ofastigmatic mirrors does not prove to be feasible via the diamond milling technique, it could be envisaged theconstruction of a mixed-size surface, with 1-m mirrors in the inner rings and 50-cm ones outside.3. Optical quality checks of the mirrorsTo check the optical quality, we use an ultrabright blue LED that is reflected by the mirror under study onto awhite screen: the reflected image, the spot, is analysed with a CCD camera. The centre of the screen and theLED are at a distance of40 cm, and are symmetric with respect to the mirror axis. The distance between themirror and the LED (and between the mirror and the screen) is equal to the nominal curvature radius (or twicethe focal length) of the mirror itself, in such a way that a point image is reflected again into a point image.For the quality check we compute the R90, that is the radius of the circle, taken from the centre of gravity ofthe spot, containing 90% of the total, reflected light. As the picture is taken at twice the focal, when focusinglight-rays coming from infinity the spot is actually half the size of the measured one. Looking at fig. 3, theresult is that 90% of the light from a parallel beam will be focused, on average, within a circle of 1 cm ofdiameter, or less than half of MAGIC pixel size (PMTs of 1”  ).The effective radius of curvature is defined operatively as the distance between the spot and the mirror wherethe R90 is minimum. It is the effective radius of curvature that is taken into account for the correct positioningof the mirror onto the parabolic dish, having to match the local mean curvature radius of the paraboloid.286 D. Bastieri et al.cm0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2020406080100120140160180All mirrorsInteg =    964Mean  =  0.980RMS   = 0.2001Minimum R90Figure 3. Distribution of R90 for the actual MAGIC mirrors.4. ConclusionsThe reflecting surface of the MAGIC telescope is operating in open air already for some years. During thisperiod, few percents of it were damaged by water infiltrations inside the sandwich structure. Due to the ex-tremely bad weather conditions of last winter, the continuous changing of state between water and ice had theeffect of detaching some of the aluminium plates from the raw blanks structure. Maintenance and substitutionof damaged mirrors is now on-going. Moreover, an improved sealing has been now devised in order to preventwater creeping inside the mirror structure.On the other hand, even in these extreme weather conditions, with also strong wind blowing and calima (Saharasand particles with 5  10µm  ), the hard surface seems to resist quite well: samples coming from mirrors thathad to be substituted evidenced no change in local reflectivity.Few mirrors made from pre-shaped raw-blanks are already installed on MAGIC and survived safely the lastexceptionally-hard winter. In the near future it is foreseen to install also some larger 1-m mirror, in order totest the technology and adopt it for the construction of MAGIC II.5. AcknowledgementsThe production of the reflecting surface of the MAGIC Telescope was made possible by the financial contri-butions of the Italian INFN and the German BMBF, to whom goes our grateful acknowledgement. We wouldalso like to thank the IAC for the excellent working conditions provided at El Roque de los Muchachos.References[1] J. Cortina et al., “Technical Performance of the MAGIC Telescope”, these proceedings.[2] D. Bastieri et al., Procs. 28th ICRC, Tsukuba (2003), Japanhttp://www-rccn.icrr.u-tokyo.ac.jp/icrc2003/PROCEEDINGS/PDF/721.pdf.[3] M. Teshima et al., “MAGIC Phase II”, these proceedings.

The Mirrors for the MAGIC Telescopes

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