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The Reflective Surface of the MAGIC Telescope

The Reflective Surface of the MAGIC Telescope. Michele Doro on behalf of the MAGIC Collaboration University of Padova & INFN michele.doro@pd.infn.it 6th RICH - Trieste, Italy 15-20 October, 2007. Overview. Part I: The MAGIC detector Overview on the IACT technique

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The Reflective Surface of the MAGIC Telescope

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  1. The Reflective Surfaceof the MAGIC Telescope Michele Doroon behalf of the MAGIC CollaborationUniversity of Padova & INFN michele.doro@pd.infn.it 6th RICH - Trieste, Italy15-20 October, 2007

  2. Overview • Part I: The MAGIC detector • Overview on the IACT technique • Part II: The Reflective Surface • Demands • Mirror • Tests and measurement • MAGIC I problem • MAGIC II upgrade • Overview on future M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  3. PART 1 The MAGIC Telescope

  4. The MAGIC Telescope • Collaboration of 22 institutes (mostly European) ~150 physicists • Installation completed 2003, fully-operating since fall 2004 • ~50 publications on journals • Currently on III-year cycle of scientific-observations M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  5. The IACT technique Physics of the atmospheric showers: • Cosmic rays (protons, heavier Z, electrons, photons) hit the upper atmosphere • Interactions create cascade of billions of particles: • Electromagnetic shower (e+,e-,) • Hadronic shower (, , e+,e-,) • Charged particles in turn emit Cherenkov light: • Blueish flash • ~2ns duration • ~1º aperture • Cherenkov cone reaches the ground • Circle of ~120m radius • Effective telescope area ~ 104-5 m2 M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  6. Imaging Technique • Light is reflected on a multi-pixel camera (576 total) • 396 central PMT 1 inch • 180 outer PMT 1.5 inch • Image is ellipsoid • Pointing to the centre for gammas • Randomly distributed for hadrons • Study of the image gives information on primary particle The reflective surface must ensure a PSF possibly smaller than the pixel size M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  7. Pulsars Physics program AGNs GRBs SNRs ? Origin of Cosmic Rays cosmological g-Ray Horizon Quantum Gravity effects Cold Dark Matter M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  8. MAGIC II • Currently a second telescope is being built • Structurally a clone of MAGIC I • Each system adopted new enhanced solutions • Better telescope than MAGIC I • Stereoscopic MAGIC I + II will have increased performance: • Increased angular resolution • Increased energy resolution • Increased flux sensitivity • Inauguration 21/09/2008 M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  9. Many challenging solutions • Reflector and mirrors: • World largest dish diameter 17m • Light undercarriage made of CFRP • All aluminium mirrors with sandwich structure and diamond-milled surface • Active mirror control • Drive • Faster repositioning ever achieved • Camera • Lacquer-coated enhances photon conversion of PMT • Operation with moonlight • Signal transmission • Ultra-fast acquisition (2GhZ) • Optical transmission instead of coaxial M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  10. PART 2 MAGIC reflective surface

  11. MAGIC I MAGIC II • 2006- MAGIC II mirrors • 1m2 Aluminium mirrors (INFN) • 1m2 Glass mirrors (INAF) Brief History • 2001-2004 MAGIC I mirrors are designed, tested and installed • 732 INFN mirrors (76%) • 224 MPI mirrors (24%) • 2005-06 MAGIC I • Upgrade of the design • Substitution of damaged mirrors M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  12. Mirror Technical Demands M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  13. The Reflector • Parabolic profile • preserve temporal structure of the shower • slightly increased aberrations • Huge dimension demands tessellation of the surface • Radius of curvature changes according to position • So-called average radius used (mean of paraboloid principal radii ) • 34 to 36.5 meters radius range M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  14. Shape • Large reflector area (~234m2) requires to tessellate the surface • Geometry of the mirror tile • Past used solutions: round, hexagonal • Solution: MAGIC has square mirrors to minimize empty regions and easier production • Size • Construction reasons • Aberrations • Solution: MAGIC I has 0.5m side, MAGIC II has 1mside (2x) mirrors M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  15. Materials • Established experience with glass mirrors (astronomy) for other IACTs • 1-2cm thick glass layer • Aluminized for reflectivity • Protection some drawbacks • Usual large weight • Difficulty of producing different focal lengths AlMgSi0.5 plate Hexcell Al-box • Idea of the full-aluminium sandwich • Al-alloy surface • Al-box • Hexcell honeycomb structure • Diamond-milling of the surface Mounting and laser M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  16. MAGIC II 3 Result is the raw-blank 35m Assembly • The sandwich is assembled with the use of the aeronautic glue 3M™ AF163-2K • The sandwich in then inserted between two very-stiff aluminum-moulds • Plane for MAGIC I • Already curved for MAGIC II and everything is put into a plastic vacuum-bag • Autoclave curing • 5 bar pressure • 120º temperature 1 2 M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  17. Diamond-milling • A diamond mills the surface • To give spherical shape, rotation on two axis • Mirror rotates around optical axis • Machine axis rotates tilted and diamong at distance d • Adjustable R curvature • In the MAGIC reflector around 20 different bins of radius of curvatures are needed 3 The mirror gets the reflective properties M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  18. Coating • The aluminium must be protected against environment • Solutions • Diamond chemical vacuum deposition (CVD) • Al2O3 anodization • SiO2 vacuum deposition • Solution adopted: quartz because of costs and transparency in 300-700nm • The width must be optimized for positive interference in the wavelength where Cherenkov light is peaked (blue) • Width ~ 100 nm • Measurement of the roughness gives 4nm on average M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  19. Testing the quality - Reflectivity • The reflectivity was measured in the Cherenkov range (200-800nm) using: • Perkin-Elmer device (mirror must be cut and put inside the machine) • Spectro-photometer (local measure on 3x3cm2 of surface) • MAGIC I: • Reflectivity is correctly peaked at 400nm (close to peak of the Cherenkov spectrum) • Average reflectivity around 80-85% • MAGIC II: • Mantained the same qualities M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  20. MAGIC II mirrors • d90 ~ 5mm • Better of factor 2 • BEST RESULT! LED MAGIC I mirror • d90 ~ 10mm at the camera distance LED Testing the quality - Spot Size • Mirror is put at twice the focal length (~35m) and illuminated with point-like bright source • Analysis of the CCD image: • So-called “d90”= diameter containing 90% of the focused light • d90 = spot size • It is not a measure of reflectivity due to difficulties in estimating scattered light M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  21. Panel & AMC • Major difference between MI and MII • mirrors are grouped into panels of 4 (3 in some cases) • Panel is also Al-sandwich (20kg) • Inter-alignment and fixing • Single mirror host AMC for MII • The back of the panel hosts the actuators for the Active Mirror Control • AMC moves panel to re-adjust the focussing to correct small bending during the tracking • Use of laser M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  22. MAGIC I experience • Main problem with mirrors installed in MAGIC I after two years from installation • Humidity was entering from edge of top plate of the mirrors • Condensation into water • Ice formation and bubbles • Due to strong rigidity, deformation is local and mirror maintains reflective area • Substituted around 100 mirrors (~10%) • Re-designed mirrors for insulation • Test for large mirrors • Now problem seems solved M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  23. SPOT Inner PMT Reflector performance monitor • Reflector performance can be monitored • Reflector PSF, now stable <11mm • Single mirror abs.reflectivity • SBIG CCD at the centre of the reflector, observe a star and the camera at the same time • All mirror focussed:PSF • One mirror focussed and others defocussed: single mirror reflectivity M. GarczarczykPhd Thesis 2007 ~11mm M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  24. Upgrade on MAGIC I • Improved design • Thinner skin and pre-shaped box: the mould is not spherical and the raw-blank comes out already with ~35m curvature • Larger top-plate and gluing of the edge with 3M™ DP190 • External heater to avoid coupling between plastic and aluminium M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  25. MAGIC II Al-mirrors • Basically an extension of MAGIC I-upgraded mirror: • Larger upper plate with respect to box • Use of aeronautic glue DP190 for insulation • No internal heater • Curvable box • Increased honeycomb width resulted in increased rigidity: • Best spot size due to more accurate diamond milling of the surface MAGIC I MAGIC II M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  26. MAGIC II • MAGIC II will have • 144 m2 of INFN-Padova mirrors • 104 m2 of INAF-Brera mirrors • Upgrade to 1m2 facet • Technologically achievable • Less number of items • Decreased weight and direct coupling to active mirror control • No-need of inter-alignment • Drawback: • Less approximating the parabola: increased aberrations, nevertheless the coma aberration dominates for tilted incidence M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  27. MAGIC II Glass mirrors • A thin glass sheet (1-2 mm) is elastically deformed so to retain the shape imparted by a mould having convex profile. If the radius of curvature is large, the sheet can be pressed against the mould using the vacuum suction. • On the deformed glass sheet (under vacuum force) is glued an honeycomb structure that provide the structural rigidity. • Then a second glass sheet is glued on the top to create a sandwich. • After releasing the vacuum, on the concave side is deposited a reflecting design for their mirrors coating (Aluminum) and a thin protective coating (Quartz) M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  28. Summary and outlook • MAGIC II mirrors production is already on the production-line • Technique gave excellent results in term of light concentration • Insulating problems seem solved • Price is decreased wrt to MAGIC I, nevertheless is still main drawback: 2.8k€/m2 can be a problem for third generation IACTs • Scale production can decrease costs or find other techniques (glass) M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

  29. Back-up slides

  30. Results • Around 50 publications on journals • ~21 VHE source observed (6 MAGIC discoveries!) • 7 new analysis techniques • 23 technical papers • Observed sources: • 12 = extragalactic • 9 = galactic http://tevcat.uchicago.edu/ M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007

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