The Reflective Surface of the MAGIC Telescope Michele Doro on behalf of the MAGIC Collaboration University of Padova & INFN 6th RICH - Trieste, Italy October, 2007

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

PART 1 The MAGIC Telescope

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

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

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

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

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

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

PART 2 MAGIC reflective surface

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH Brief History MAGIC I mirrors are designed, tested and installed 732 INFN mirrors (76%) 224 MPI mirrors (24%) MAGIC IMAGIC II MAGIC I Upgrade of the design Substitution of damaged mirrors MAGIC II mirrors 1m 2 Aluminium mirrors (INFN) 1m 2 Glass mirrors (INAF)

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH Mirror Technical Demands Lightweight Telescope must rotate fast and then mirrors need to be as light as possible Mirror Shape Mirrors profile is spherical Each mirror has different radius of curvature because reflector profile is parabolic (f=17m) Rigidity Avoid oscillations due to wind Avoid bending during tracking Insulation Sometimes strong rains and snows Also high humidity Strong UV light Mounting Coupling with actuators of Active Mirror Control Easy mounting and substitution Optical quality Maximize reflectivity Minimize reflected spot size

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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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 and easier production Size Construction reasons Aberrations Solution: MAGIC I has 0.5m side, MAGIC II has 1m side (2x) mirrors

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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 Mounting and laser Idea of the full-aluminium sandwich Al-alloy surface Al-box Hexcell honeycomb structure Diamond-milling of the surface

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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 35m MAGIC II

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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH Coating The aluminium must be protected against environment Solutions Diamond chemical vacuum deposition (CVD) Al 2 O 3 anodization SiO 2 vacuum deposition Solution adopted: quartz because of costs and transparency in nm 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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH Testing the quality - Reflectivity The reflectivity was measured in the Cherenkov range ( nm) using: Perkin-Elmer device (mirror must be cut and put inside the machine) Spectro-photometer (local measure on 3x3cm 2 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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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 MAGIC I mirror d90 ~ 10mm at the camera distance LED MAGIC II mirrors d90 ~ 5mm Better of factor 2 BEST RESULT!

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

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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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. Garczarczyk Phd Thesis 2007 ~11mm Inner PMT SPOT

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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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 MAGIC I MAGIC II Increased honeycomb width resulted in increased rigidity: Best spot size due to more accurate diamond milling of the surface

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

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

15-20 Oct 2007M.Doro - The Reflective Surface of the MAGIC Telescope - RICH 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€/m 2 can be a problem for third generation IACTs Scale production can decrease costs or find other techniques (glass)

Back-up slides

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