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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 - PowerPoint PPT Presentation
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The Reflective Surfaceof the MAGIC Telescope
Michele Doro on behalf of the MAGIC CollaborationUniversity of Padova & INFN michele.doro@pd.infn.it
6th RICH - Trieste, Italy15-20 October, 2007
15-20 Oct 2007 M.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
PART 1
The MAGIC Telescope
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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
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The IACT techniquePhysics 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
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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
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The reflective surface must ensure a PSF possibly smaller than the pixel size
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Physics program QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
SNRsSNRs
Cold Dark Cold Dark MatterMatter
PulsarsPulsars
GRBsGRBs
Quantum Gravity Quantum Gravity effectseffects
cosmologicalcosmological-Ray Horizon-Ray Horizon
AGNsAGNs
??Origin of Origin of Cosmic Cosmic RaysRays
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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
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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
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Brief History 2001-2004 MAGIC I
mirrors are designed, tested and installed
732 INFN mirrors (76%) 224 MPI mirrors (24%)
MAGIC I MAGIC II
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2005-06 MAGIC I Upgrade of the designSubstitution of damaged mirrors
2006- MAGIC II mirrors1m2 Aluminium mirrors (INFN)1m2 Glass mirrors (INAF)
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Mirror Technical DemandsQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
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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
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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
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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 1m side (2x) mirrors
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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
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AlMgSi0.5 plate
Hexcell
Al-box
Mounting and laser
Idea of the full-aluminium sandwichAl-alloy surfaceAl-boxHexcell honeycomb structureDiamond-milling of the surface
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Assembly QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
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
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3Result is
the raw-blank
35m
MAGIC II
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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
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3
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The mirror gets the reflective properties
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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
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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)
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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
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Testing the quality - Spot SizeQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
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
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MAGIC I mirror d90 ~ 10mm at the
camera distance
LED LED MAGIC II mirrors d90 ~ 5mm Better of factor 2 BEST RESULT!
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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
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AMC moves panel to re-adjust the focussing to correct small bending during the tracking
Use of laser
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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
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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
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M. GarczarczykPhd Thesis 2007
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~11mm
Inner PMT
SPOT
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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
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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
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MAGIC I
MAGIC II
Increased honeycomb width resulted in increased rigidity:
Best spot size due to more accurate diamond milling of the surface
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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
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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)
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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)
Back-up slides
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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
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http://tevcat.uchicago.edu/
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