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

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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/