A conceptual Design of an Advanced 23 m Ø IACT

of 50 Tons for Ground-Based Gamma-RayAstronomy

E. Lorenz, MPI Munich and ETH Zurich

On behalf of the 23 m IACT working group

•INTRODUCTION•PHYSICS AND TECHNICAL GOALS•A POSSIBLE CONSTRUCTION•AUXILIARY INFORMATION•CONCLUSIONS

IACT: Imaging Atmospheric Cherenkov Telescope

Introduction

•High energy astroparticle physics is a rapidly expanding field of fundamental physics

•High energy gamma-ray physics is one of the subfields of astroparticle physicsHigh energy gamma-rays are the messengers of processes in the relativistic universeCharged cosmic rays unsuited because of the deflection by the weak galactic and extragalacticMagnetic fields - cannot correlated with sources

•The window of high energy gamma-ray astronomy was opened in 1989 by the observationof TeV gamma-rays from the Crab Nebula (Whipple collaboration) after a > 20 years frustrating search.

•Nearly all discoveries and measurements in the energy (25) 100 GeV andup to nearly 100 TeV were achieved by using so-called imaging air Cherenkov telescopesIACTs

•The currently leading instruments-CangarooIII, H.E.S.S., Magic and Veritas will soon havediscovered 100 cosmic sources and contributed to resolve many fundamental physics questionsnevertheless one ‘scratches just at the surface’ and a large improvement in sensitivity and alower energy threshold is needed

•We need better and cheaper telescopes to build large arrays (for example CTA, AGIS)

Principle of Cherenkov TelescopesPrinciple of Cherenkov Telescopes

THE COMBINATION OF THE ATMOSPHERE AND A POWERFUL LIGHT SYSTEMDETECTING THE CHERENKOV LIGHT FROM AIR SHOWER IS A FULLY ACTIVE CALORIMETER

BUT•NO CHARGED PARTICLE VETO: NO SUPPRESSION OF THE MANY ORDERS OF MAGNITUDE LARGER HADRON BG

•LOW Z MATERIAL, LOW DENSITIY-> AND H SHOWERS NEARLY SAME LENGTH

•DENSITIY VARIES WITH DEPTH AND ANGLE

•NO CONFINING WALLS -> HUGHE BACKGROUND LIGHT

•UNPREDICTABLE CHANGES OF OPTICAL TRANSMISSION DUE TO ATMOSPHERIC CHANGES

•CONSTANTLY CHANGES ITS GEOMETRY DUE TO EARTH ROTATION

•NIGHT SKY LIGHT BG: = 2 1012 photons/m2 sec sterad (300-600 nm) DARK NIGHT•A 1 TEV SHOWER PRODUCES TYPICAL 100 PHOTONS/m2 WITHIN 2-5 nsec

•The real challenge: to discriminate s vs hadrons in such a calorimeter and to build a detector where photon lossesare minimized (most current IACTs have a mean photon-> photoelectron conversion of 10-12%)

•Currently the largest IACTs: MAGIC I (17 mØ, 60 GeV threshold) running since 2004, MAGIC II (17 mØ) currently commissioned. (MAGIC I+II in stereomode < 50 GeV threHESS II (28 mØ, 30-40 GeV threshold) still under construction

DESIGN GOALS FOR A 23 mØ TELESCOPE FOR AN IACT ARRAY

•ARRAY SENSITIVITY BELOW 250 GEV: 6-15 BETTER THAN MAGIC, HESS II BUT ALSO MUCH HIGHER THAN FERMI ABOVE ˜ 30 GEV

•ARRAY THRESHOLD: TRY TO REACH 20 GEV -> GOOD OVERLAP WITH FERMI

•COST PER TELESCOPE ABOUT FACTOR 2 BELOW EXTRAPOLATED PRICE FOR A 23 M Ø IACT WHEN FOLLOWING TECHNIQUE OF MAGIC, HESS II

•RAPID ROTATION TO CATCH REALTIME FLARE OF GRBs-> LOW WEIGHT

•OPERATION DURING PARTIAL MOON LIGHT-> INCREASE OBS. TIME BY 50%

•OPERATION IN SINGLE/ STEREO MODE

•RELIABILITY FOR > 10 YEARS OF OPERATION (NO PROTECTIVE DOME!)

•MAXIMIZE REMOTE CONTROL/OPERATION

•TELESCOPE EASY TO ASSEMBLE/DISASSEMBLE

VERY RICH PHYSICS PROGRAM

FUNDAMENTAL PHYSICS ISSUES•SEARCH FOR DARK MATTER

•QUANTUM GRAVITY STUDIES, SEARCHING FOR LORENTZ INVARIANCE VIOLATION

•EBL (EXTRAGALACTIC BACKGROUND LIGHT) STUDIES (INFORMAT ABOUT EARLY STAR FORMATION..)

•CONTRIBUTION TO BLACK HOLE PHYSICS

•SEARCH FOR TOPOLOGICAL DEFECTS, RELIC PARTICLES

EXAMPLES SOURCE SEARCHES

•STUDY OF THE UPPER ENERGY EMISSION OF GRBs

•STUDY OF THE UPPER ENERGY END OF PULSARS

•SEARCH AND STUDY OF HIGH REDSHIFT AGNS

•SEARCH FOR CLUSTERS OF GALAXIES

•STUDY OF FLARING AGNS AND OF AGNS IN THEIR ‚LOW‘ STATE

•STUDY OF UPPER ENERGY END OF ALL TYPES OF STEADY SOURCES IN A RANGE ABOVE 50 GEV

•.

•MULTIWAVELENGTH STUDIES

•.

NOTE I : HIGH REDSHIFT UNIVERSE NOT TRANSPARENT TO s ABOVE 50-100 GEV (interaction with EBL photons)

NOTE II: ABOVE 40 (80) GEV: an array of 23 m IACTs should be 10 (100) times more sensitive than FERMI

THE BASIC CONCEPT FOR A 23 mØ TELESCOPE:

DERIVATIVE OF THE MAGIC IACT DESIGN

23 m Ø parabolic mirror

Cost goal for mechanics including foundation : 1-1.5 M€

Cost goal for the mirrors: 1- 1.5 M€

Cost goals for the camera+ readout: 3 M€

Assumed that mass production results in 25-35% lower costs than that for a

single telescope

IACTs:9 -4Cost goals for a small series production of

Aluminumdouble mast2.5 % shadowing

CAMERA

HIGH STRENGTH CABLES DYNEMA15 times stronger than steel for same weightActive bending control

MIRROR SUPPORT SPACE FRAME CFRP

AZIMUTH STRUCTURECFRP TUBULAR CONSTRUCTION

Mirror panels

VIEWS OF THE 23 mØ IACT

POSSIBLE USE OF ACTIVE BENDING CORRECTION

CCD DETECTING DISPLACEMENT OF CAMERASENDS COMMANDS TO ACTUATORS

ACTUATORS RELEASE TENSION

ACTUATORS PULL

SAGGING DUE TOGRAVITY

A challenge: how to reduce the weight to reach 50 tons for the movable part?

•Move weight from the mobile part to the static part (foundation) whenever possible

•Use carbon fiber reinforced plastic (CFRP) tubes for y the entire structure

•Use stiffer structure

•Use lightweight mirrors

Is it possible to construct substructure with CFRP tubes?

Industry is now able to produce large diameter CFRP tubes of sufficient strength

Requirements for 10 m tube: tensile strength 2000 kNewton: no problemcompression (buckling) resistance 1200 k Newton: yes, but at limitRequirements 3x normal limitThe main problem is the glue joints for the tubes under pulling load

Discussion with two companies: EPSILON,France and EXEL, Finland

Both companies are willing to invest to improve product

BASIC SPACE FRAME ELEMENT MAGIC BASIC ELEMENT FOR LSTCONSIDERABLY STIFFER

15 m Ø radiotelescope Plateau de Bure

Cw ˜ 0.4 (wind from rear)Cw ˜ 1.3 (wind from frontside)

HOW TO MAKE DESIGN SAFERIN STORMS: USE OF COVER ONSPACE FRAME TO REDUCE WIND RESISTANCECw OF NAKED SPACE FRAME: 2.4

COVER SPACE FRAME BY PANELS OR HYPALON FOIL

FOIL CAN STAND 10 YEARS IN SUN SHINE. LOW WEIGTH!- STILL 340 KGWE NEED SOME TEST SOON FOREASY FIXING

:SPECS•Mirror Diameter: 23 m

•Mirror Area: 410 m2

•Focal length: 28 (f/d ˜ 1.2)

•Weight ˜ 50 tons (needed for GRB studies), 50 tons possible for CFRP

•Foundation: Concrete ring with steel I-Beam ring with protection against

wind lift-up of telescopes during storms

•Bogeys: 6 (4 wheels each, similar to version of PETAL)

•Substructure: similar like HESS/MAGIC, but CFRP frame with some steel components

•Dish spaceframe: 4 layer space frame, with tetraeders as basic elements

•Space frame material: CFRP (high strength fibers) + Al knots

•Reduction of wind resistance: cover of space frame by panels as in radio telescopes

•Tetraeder elements: rods of 150 cm, detailed length following mirror profile

•Mirror profile: main curvature: parabolic, locally with deviations up to 2-3 cm

•Gross mirror shape: hexagonal

•Mirror elements: hexagonal, 155 cm width (width across flats)

•Production technique similar to MAGIC 1x1 m2, central hole

•allow for a small zone of imperfection (change of diamond)

•individual mirror elements: 1.9-2 m2

•Area of individual mirrors: ˜ 2 m2

•# of mirrors: ˜ 220, weight < 30(40) kg/ mirror

•Mirrors with dielectric coating for high reflectivity ??

R> 95% between 300 and 550 nm,

R > 85% between 550-650 nm

•PSF: < 1cm FWHM, > 90% of light within 1 cm radius

•Active mirror control: permanent, fast response. IR lasers (not disturbing PMTs)

project a spot on a screen outside camera, viewed by IR CCD

alternatively: 1 CCD camera per mirror

viewing an LED at the camera position.Inclinometer?

AMC will be a key element to cut costs (allows a softer frame, cheaper)

•Camera support by 2 masts.

•Reasonable limit of camera weight: 2 tons (1 ton preferred)

•Motors: 2 for azimuth (10 KW /motor), 1 for declination (10 KW/motor),

••ANGULAR RANGE: > 540ANGULAR RANGE: > 540°° IN AZIMUTH, + 110IN AZIMUTH, + 110°° TO TO --9090°° IN DECLINATIONIN DECLINATION

••SURVIVEABILTY: = 180 kmh WIND SPEEDSURVIVEABILTY: = 180 kmh WIND SPEED

••TRACKING PRECISION: 0.005TRACKING PRECISION: 0.005°°, SHAFT ENCODERS 16 BIT, STAR GUIDER, SHAFT ENCODERS 16 BIT, STAR GUIDER

••Peak power consumption from Buffer < 20 kWPeak power consumption from Buffer < 20 kWmean power needs from grid < 5 kWmean power needs from grid < 5 kW

First estimate of weight of moving part

Bogeys and wheels (8-12t) 12 tSubstructure (CF+ Steel)(10-14 t) 12 tSupport dish (7-9t) 8 tMirrors and AMC 10 tCamera masts, declination drive ring camera support frame 6 tCamera 2 tAuxiliary stuff ?? ?

50 ± x tons

HESS I (12 mØ) : 68 tonsMAGIC (17mØ) : 70 tonsHESS II (28mØ) : 560 tons

Raw costs for CF tubes for dish ˜ 250 k€Raw costs for CF tubes for substructure ˜ 270 k€Mirror costs /m**2 ˜ 3000€ / m**2

MIRROR

Parabolic mirror profile220 (240) elements 400 (420) m2 areaObscuration < 3% Mirror elements: hexagonal, lightweight sandwich construction

either all aluminum diamond turned ˜ 18-20kg/m**2 (PADOVA,MPI DEV.)or cold slumped glass sandwich mirrors ˜ 12-15 kg/m**2(INAF DEV.)

Diamond turned mirrors: at least 30% more expensive but little aging Test of MAGIC prototype mirrors: drop in reflectivity = 1 %/year

Total weight of mirror surface include. AMC and link elements ˜ 10 tons

MIRROR FIXING POINTS, CLOSE TO IDEAL POSITION, 2 ACTUATORS

MIRROR (BLUE) AND TOP LAYER SECTION OF SPACE FRAME (BLACK)

THE ACTIVE MIRROR CONTROL

SCREEN ON BOTH SIDES OF CAMERA

IR laser beams not interferingwith photosensors

IR sensitive

COUNTERACTS SOME SMALL DEFORMATIONS OF MIRROR SUPPORT FRAME

EXAMPLE OF MIRROR FOCUSSED TO A LIGHTSOURCE 1000mtr AWAY

AFTER MANUALADJUST. (0.02° AUTOMATIC)WILL DEGRADE DURINGRUNS IF NOT FREQUENTLYREADJUSTED

0.1°

pedestal

PSF 0.03°

CAMERA AND DAQ

•Field of view : 4 degree•Pixels: 0.066 degree diameter•# of pixels: ˜ 2500•Sensors first generation : hemispherical PMTs, superbialkali cathode (34% peak QE), 6 dynodes

(gain few 104 ) passively enhanced QE by light catcher geometry (dual pass of cathode +scatter coating)We aim for a system PDE (photons -> photoelectrons, 290-650 nm) of 15-17%most current IACTs : 10-12 %

•Long term plan for second generation photosensors: G-APDs (SiPM.......)Potential to increase system PDE to nearly 30 % if industry solves some problemslower gain (only 105 at = 4 V overvoltage), large cells (100x100 ), p-on-n structureprice substantially lower than now

DAQ: the soon becoming available switched capacitor arrays, e.g. DRS 4, can do the job300 MHz bandwidth, 2 GHz sampling, 1024 cells in ring mode, 50 mW/ channel

Trigger: simple ‘Sum’ trigger with readout of multiple segments at 10-20 kHz followed by event builderwriting to a large storage system followed by a multiple step software selectionNote that data are only taken during night time

Minimizing light losses: use of dielectric mirror foil for light catcher liningCurrent foil ESR2 from 3 M has 98-99% reflectivity between 385 and 750 nmNeed to extend down to 285 nm: overcoat with UV reflecting film

0

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300 350 400 450 500 550 600 650

Cgapd-pdeRTCCBa-pdeRTCCSba-pdeRTCCETrub-pdeRTCCUba-pdeRTC

Second generation photosensors: use of G-APD(SiPm.......)Cherenkov light was already observed with a small G-APD array on MAGIC telescope : Biland A. et al., 595 (2008) 165About 1.8 times more PHE than with PMTs of Magic camera

Nevertheless technology not yet mature: gain of G-APDs too high -> optical cross talkOvervoltage (Ubias -Ubreakdown) not high enough -> PDE much lower than QE

QE of some sensors But one has to use PDE, folded by tel. opt. parameters

CE(G-apd) 0.65CE(st.Ba PMT): 0.9CE(Sba PMT): 0.9CE(ET RbCsPMTOn Magic): 0.95CE(Uba,PMTMesh dynodes): 0.65

EXPECT THAT IT WILL TAKE 2-4 YEARS UNTIL G-APDS WILL BE SUITABLE FORLARGE SCALE USE AND PRICE LOW ENOUGH

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300 350 400 450 500 550 600 650

QE-Sba Class BialQEcoat.ETRubUBAG-APD

C Q

E c

oll.

eff.

RT

C

wavelength [nm]

Fig.of merit, FM----------------------Ba 1.0Sba 1.26ET-RbCs 1.36Uba 1.38G-APD 1.93

QE

wavelength [nm]

NIM A

SENSITIVITY ESTIMATE

EXPECTED SENSITIVITYFOR A 9 IACT SYSTEM OF 23 M TELESCOPES± 30% ERROR

CONCLUSIONS:Lightweight construction (weight partition: 60 %CFRP, 20 % Steel, 20 %Aluminium)of 50 tons in total, include mirrors and a 2 ton camera. main mirror composed of 2(1.9) m2 mirror panels (diamond turned aluminium or cold slumped aluminized glass mirrors).

Baseline camera with PMTs of superbialkali type cathode and passive QE enhancement for mean PDE between 290 and 650 nm of 15-17% (peak QE of 34 %)

MAGIC I : mean PDE ˜ 14% (290-650 nm)Classical IACTs: mean PDF ˜ 10-12% (290-650 nm).

Readout : Digital ring sampler (DRS 4..), 300 Mhz BW, GHz sampling, > 10 kHz Trig.Rate

Price: 6-8 M € for single T, (Split 3 M€ for telescope structure and mirrors, rest cameratrigger and readout)Construction time: 2-3 years (1 IACT), 4-6 years small series of 4-9 IACTs

Threshold: trigger <20 GeV, physics analysis threshold at 30 ° Zenith angle: 30- 40 GeV.Sensitivity of array at 60-100 GeV: ˜ 10x higher than MAGIC I (assuming 30 % gain from extended observation time including moon shine)

CONSTRUCTION AND PRICE GOALS ARE IN REACHWE NOW SOLVE THE MANY INDIVIDUAL CONSTRUCTION PROBLEMS