the reflective surface of the magic telescope michele doro on behalf of the magic collaboration...

The Reflective Surfaceof the MAGIC Telescope

Michele Doro on behalf of the MAGIC CollaborationUniversity of Padova & INFN [email protected]

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


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

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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-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


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 (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 ~ 104-5 m2



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


PART 2

MAGIC reflective surface


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

1m2 Aluminium mirrors (INFN)

1m2 Glass mirrors (INAF)

MAGIC I MAGIC II



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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


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


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

Size Construction reasons Aberrations Solution: MAGIC I has 0.5m side,

MAGIC II has 1m2 mirrors





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


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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



3Result is

the raw-blank


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


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







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=REstimation of R50 and R90

MAGIC I mirror PSF~1cmMAGIC II mirrors PSF~0.5cm



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


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





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. GarczarczykPhd Thesis 2007




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



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 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


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)


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€/m2 is problem for third generation IACTs

Scale production can decrease costs or find other techniques (glass)

Back-up slides


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


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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


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/

the reflective surface of the magic telescope michele doro on behalf of the magic collaboration...

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