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

2. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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

3. PART 1 The MAGIC Telescope

4. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 4 The MAGIC Telescope Collaboration of 22 institutes (headed by Germany, Italy, Spain), ~150 physicists Installed 2003, fully-operating since fall 2004 ~50 publications on journals Currently on III-year cycle of observations

5. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 5 Many challenging solutions Reflector and mirrors: World largest dish diameter 17m Light undercarriage made of CFRP All aluminium mirrors with sandwich structure and diamond- grinded surfaces Active mirror control Drive Faster repositioning ever achieved Camera Lacquer-coated enhances conversion PMT Operation with moonlight Signal transmission Ultra-fast acquisition (2GhZ) Optical transmission instead of coaxial

6. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 6 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 (e + e - ) in turn emit Cherenkov light: Blueish flash ~2ns duration ~1º aperture Cherenkov cone reaches the ground Circle of ~100m radius Effective telescope area ~ 10 4-5 m 2

7. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 7 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 Steoreoscopic MAGIC I + II will have increased performance : Increased angular resolution Increased energy resolution Increased flux sensitivity Inauguration 21/09/2008

8. PART 2 MAGIC reflective surface

9. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 9 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 2006- MAGIC II mirrors 1m 2 Aluminium mirrors (INFN) 1m 2 Glass mirrors (INAF) MAGIC IMAGIC II

10. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 10 Technical Demands Lightweight Telescope must rotate fast and then need to be light Shape Profile is spherical Each mirror has different radius of curvature Square, round, hexagonal Rigidity Deformations due to winds Bending during tracking Objects can hit the mirrors Insulation Sometimes strong rains and snows Also high humidity Mounting Coupling with actuators of Active Mirror Control Easy mounting and substitution Optical quality Maximize reflectivity Maximize encircled energy

11. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 11 The Reflector Parabolic profile to preserve temporal structure of the shower Huge dimension demands to tessellate of the surface Radius of curvature changes according to position So-called average radius used (mean of principal radii) 34 to 36.5 metres radius range

12. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 12 Shape Large reflector area (~234m 2 ) requires to tessellate the surface Geometry of the mirror tile Past used solutions: round, hexagonal Solution: MAGIC has square mirrors to minimize empty regions Size Construction reasons Aberrations Solution: MAGIC I has 0.5m side, MAGIC II has 1m 2 mirrors

13. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 13 Materials Established experience with glass mirrors (astronomy) for many IACTs 1-2cm thick glass layer Aluminized for reflectivity Protection drawbacks Usual large weight Difficulty of producing different focal lengths Idea of the full-aluminium sandwich Al-alloy surface AlMgSi0.5 Al-box Hexcell honeycomb structure Diamond-grinding of the surface AlMgSi0.5 plate Hexcell Al-box Mounting and laser

14. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 14 Assembly 1 2 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 3 Result is the raw-blank

15. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 15 Diamond-milling A diamond grind the surface To give spherical shape, rotation on two axis Mirror rotates around optical axis Machine axis rotates tilted and diamong at distance d, then R=d/sin  Adjustable R curvature In the MAGIC reflector around 20 different radius of curvatures are needed 3 The mirror gets the reflective properties

16. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 16 Coating The aluminium must be protected against environment Solutions Diamond chemical vacuum deposition (CVD) Al2O3 anodization SiO2 vacuum deposition Solution adopted: quartz because of price 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

17. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 17 Testing the quality REFLECTIVITY: Perkin-Elmer device (mirror must be cut) Spectro-photometer Reflectivity around 85%, peaked at blue (400nm) SPOT SIZE (PSF): Illuminate the mirror with intense (monochromatic) light at 2f=R distance and observe reflected spot at 2f=R Estimation of R 50 and R 90 MAGIC I mirror PSF~1cm MAGIC II mirrors PSF~0.5cm

18. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 18 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

19. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 19 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 (out of ~1000) Re-designed mirrors Test for large mirrors Now problem seem solved

20. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 20 Reflector performance monitor Reflector performance can be monitored Reflector PSF 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. Garczarczyk Phd Thesis 2007

21. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 21 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

22. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 22 MAGIC II MAGIC II will have 144 m 2 of INFN-Padova mirrors 104 m 2 of INAF-Brera mirrors Upgrade to 1m 2 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

23. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 23 MAGIC II Al-mirrors Basically an extension of MAGIC I-upgraded mirror Increased width=60mm (2x) results in enhanced rigidity Diamond-milling is of higher quality and spot PSF almost is reduced of 1/2 and is 1/6 of the pixel size Best mirrors ever built! Extensive check on insulation LED

24. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 24 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)

25. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 25 Summary and outlook MAGIC II mirrors production is already on the pipe-line Technique gave excellent results in term of light concentration Ageing problems seem solved Main drawback: 2.8k€/m 2 is problem for third generation IACTs Scale production can decrease costs or find other techniques (glass)

26. Back-up slides

27. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 27 Imaging Technique Light is reflected on a multi-pixel camera Image is ellipsoid Pointing to the centre for gammas Randomly distributed for hadrons Study of the image Hillas parameters Size and moments of the image Reconstruction of: direction, energy of the primary gammas Background 99% of events is background Random Forest technique based on comparison with Monte Carlo events Gamma/hadron separation Energy estimation

28. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 28 Physics program SNRs Cold Dark Matter Pulsars GRBs Quantum Gravity effects cosmological  -Ray Horizon AGNs ? Origin of Cosmic Rays

29. 15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 2007 29 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/