EXPERIENCE WITH THE CONSTRUCTION AND OPERATION WITH THE MAGIC TELESCOPE E. Lorenz, MPI Munich WILL...
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Transcript of EXPERIENCE WITH THE CONSTRUCTION AND OPERATION WITH THE MAGIC TELESCOPE E. Lorenz, MPI Munich WILL...
EXPERIENCE WITH THE CONSTRUCTION ANDOPERATION WITH THE MAGIC TELESCOPE
E. Lorenz, MPI Munich
WILL CONCENTRATE ON ISSUES RELEVANT FOR THE CONSTRUCTION OF POSSIBLEFUTURE LARGE IACTS
A FEW COMMENTS ON THE PAST:IDEA FOR MAGIC WAS ‘BORN’ IN 1994/5
LARGE MIRROR INSPIRED BY A 17 m Ø SOLAR MIRROR OF THE DLR CONSTRUCTEDIN LAMPOLDSHAUSENMECHANICS INSPIRED BY THE DESIGN OF SCHLAICH, BERGEMAN AND SONS
SOON OBVIOUS THAT THE ORIGINAL MIRROR CONCEPT UNSUITED FOR THEREQUIRED PRECISION OF AN IACT (FOCAL SPOT MUCH TOO LARGE)
FUNDING AGENCY SET TIGHT PRICE LIMIT OF 5 MDM (=2.5 M€) IN 1996
SOME ORIGINAL GOALS
• CLOSE THE ENERGY GAP BETWEEN 2. GENERATION IACTS AND EGRET-> THRESHOLD 15 GEV ?NOT POSSIBLE WITH CLASSICAL PMTS ->NEW DEVELOPMENT OF HIGH QE PMTSLOTS OF NEW PHYSICS EXPECTED IN THE ENERGY DOMAIN < 300(150) GEV
• RECOMMENDATION FROM REVIEW COMMITTEE: BUILD MAGIC FIRST WITHCLASSICAL PMTS
ALLOW FAST ROTATION FOR GRB OBSERVATIONS -> LIGHT WEIGHT CONSTRUCTION, RIGID CONSTRUCTION
ALLOW OBSERVATIONS DURING PARTIAL MOON SHINE-> USE PMTS WITH LOW GAIN + VERY GOOD AC COUPLED PREAMPS
INCREASE DAQ RATE TO 500-1000 HZ
BUILD A NEW TECHNOLOGY TELESCOPE, AFTER SUCCESSFUL DEMONSTRATION-> MULTIPLE TELESCOPE (STEREO OR MULTIPLE PARALLEL OBSERVATIONS)
WHY NOT A STEREO SYSTEM FROM THE BEGINNING ?
NO FUNDING AGENCY WOULD HAVE GIVEN MONEY FOR 3-4 NEW TECHNOLOGY TELESCOPES WITHOUT TESTING A LARGE PROTOTYPE
FUNDING AGENCIES WOULD FIRST REQUIRE 1 DEMO AND THAN DECIDE ON THE NEXT ONES.
EVEN IF WE HAD APPLIED FOR NOVEL TELESCOPES WE COULD, WITHIN THE FINANCIAL LIMITS, ASK ONLY FOR SMALL TELESCOPES
ADVANTAGE OF ONE LARGE TELESCOPE FIRST: LOWER THRESHOLDDISADVANTAGE: LOWER SENSITIVITY IN OVERLAP REGION
LESSON TO BE LEARNED: IF THE CONSTRUCTION WILL BE TOO AMBITIOUSONE WILL HAVE TROUBLE TO GET FULL SUPPORT FROM THE COMMUNITY(BMBF WAS MORE POSITIVE ABOUT NEW IDEAS AND NEW TECHNOLOGIESTHAN COMMUNITY)
THE MAGIC I TELESCOPE
17 mtr
NOVEL OR UNCONVENTIONAL TECHNICAL COMPONENTS
ITEM WHY .
17 m DIAMETER MORE LIGHT -OPTICAL DISTORTIONS
SPACE FRAME - CARBON FIBER-EPOXY TUBES LOW WEIGHT, SEE LIST
SINGLE MAST CAMERA SUPPORT LESS OBSCURATION, WEIGHT
NEW MIRRORS LOW WEIGHT, CHEAPER, SEE LIST
ACTIVE MIRROR CONTROL CHEAPER SPACE FRAME
SPECIAL PMTS MORE SIGNAL, SEE LIST
ANALOG SIGNAL TRANSMISSION OF PMT SIGNALS NO BULKY CABLES, SEE LIST
300 MHZ FADC NSB REDUCTION, NOW STANDARD OR EVEN OUTDATED
MULTI-LEVEL TRIGGERS HIGHER TRIGGER SELECTIVITY
NEW TECHNOLOGIES HAVE ALWAYS SOME RISK. GENERAL: 1 DEVELOPMENT ≈ BREAK EVEN OR FAILURE 2. PRODUCTION: REAL BREAKTHROUGH 3. PRODUCTION: SETS NEW STANDARD
SOME TECHNICAL DETAILS I:
CONCRETE FOUNDATION: CONVENTIONAL, WEIGHT >> TELESCOPE WEIGHT FOR STABILITY REASON: WIND, VIBRATIONS GOOD FOR 15 Y LIFETIME, BUT NOT FOR>>15 Y WE HAVE TO BUILD IN EASY DISMOUNTING
KNOWN PROBLEMS: SHOULD BE WHITE TO PREVENT SUN HEATUP SHOULD BE BLACK TO SUPPRESS NSB REFLECTION
DOME ?? TO EXPENSIVE BUT MAY BE NEW IDEAS AROUND TO COVER ONLY MIRROR
RAIL: SIMPLE RAILWAY RAIL WITH < 1 mm VARIATION IN HEIGHT CURRENT SYSTEM CAN MOVE UNDER THERMAL EXPANSION SLIGHT DEFORMATION -> LIMITED TRACKING PRECISION RAIL SUPPORTED EVERY 60 cm -> SLIGHT DEFORMATION UNDER TELESCOPE LOAD (2 BOGEY WHEELS ≈ 210 CM APART->SOME EQUALIZING OF DEFORMATION BUT NOT ALL FOR 6 BOGEYS)
NOTE: WITH 6 BOGEYS SYSTEM OVERDETERMINED_> SOME INTERNALDEFORMATION OF SUBSTRUCTURE ->NO PERFECT SHAFTENCODERS NEEDED <-> PRECISE TELESCOPE AXIS POSITION MEASUREMENTS IMPOSSIBLE -> 13 BIT ENCODERS OK 0.02° LSB
ALSO AFFECTING TRACKING:WE USE CHAINS LIKE IN A BICYCLE- SOME PLAYNOT PERFECTLY ROUND -> SOFTWARE CORRECTIONSNOT PERFECTLY SMOOTH (LINEAR) -ALSO A REASON WHY A PERFECT HIGH RESOLUTION SHAFTENCODER DOES NOT HELP
NOTE: WE HAVE 1 SHAFTENCODER FOR AZIMUTH POSITION, 2 FORDECLINATION (DEVIATE FROM EACH OTHER- MIRROR DISH DEFORMATIONPROBLEM WITH CULMINATION-> SEPARATE TALK)TELESCOPE CAN TURN > 450° IN AZIMUTH -> MULTITURN SHAFTENCODER
ENCODERS ARE ABSOLUTE ENCODERS -> NO CHANGE AFTER POWER FAILUREENCODER ARE THE HIGHEST MECHANICAL PRECISION ITEMS OF DETECTOR
DRIVE MOTORS: 11 KW SYNCHRO-MOTORS (CURRENTLY LIMITED TO ≈ 70%OF PEAK POWER)MAX TURNING OF TELESCOPE->CAN REACH 180° IN AZIMUTH IN ≈ 20 SEC2 MOTORS FOR AZIMUTH (HAVE TO MOVE 64 TONS)1 MOTOR FOR DECLINATION (HAS TO MOVE 22 TONS
WEIGHT OF TELESCOPE ELEMENTS•BOGEYS 25.2 T•LOWER STRUCTURE 15.8 T•CAMERA 0.75 T•CAMERA MAST RING 1.2 T•MIRRORS 12. T•SPACE FRAME 5.6 T•COUNTER WEIGHT 2. T•AUXILIAR ELEMENTS 1.4 T
•SUM ≈ 65 T
LESSON TO BE LEARNED: BOGEYS ARE UNNECESSARY HEAVY, IF A BETTER SOLUTION FOR PROTECTION AGAINST STRONG WINDS CAN BE FOUND(SEE ALSO LATER COMMENT ON ADVERSE ATMOSPHERIC CONDITIONS)
WHY DID WE CHOOSE A SPACE FRAME MADE FROM CARBON FIBER-EOPXY TUBES
•LOW WEIGHT (5.4 TONS FOR MAGIC)•LESS DEFORMATION THAN STEEL OR ALUMINUM (17 MØ DISH QUITE DIFFERENT TO 10 MØ DISH )•SAGGING ABOUT 1/2 OF SAME STEEL CONSTRUCTION•WEIGHT 1/3 OF STEEL CONSTRUCTION•THERMAL EXPANSION NEARLY ZERO•(STEEL EXPANDS BY 3 cm BETWEEN DAY-NIGHT TEMPERATURE CHANGE OF 50°C), WE SEE EFFECT VERY CLEARLY DURING DAY-TIME•HIGH OSCILLATION DAMPING•NO CORROSION-RUST
DISADVANTAGES•HIGHER DEVELOPMENT COSTS (NOT MORE IN FUTURE)•HIGHER PRODUCTIOPN COSTS( NOW MARGINALLY HIGHER)•ORIGINALLY NO LONGTERM EXPERIENCE
•LESSON TO BE LEARNED: RAPID PROGRESS IN CFRP CONSTRUCTION COMPONENTS. ENORMOUS PROGRESS IN YOUNGS MODULUS, MAX BREAKING LIMITS EPOXY MATERIALS
MIRRORSALL ALUMINIUM SANDWICH CONSTRUCTION WITH INTERNAL HEATINGDIAMOND TURNED SURFACES, 10 DIFFERENT RADII (-> APPROXIMATION OF PARABOLA-> ISOCHRONOUS -> NO ADDITIONAL TIME SPREAD FOR SIGNALS - STAGGERING- TIME SPREAD < 0.4 nsec)
WE HAVE 49.5X 49.5 cm MIRROR ELEMENTS (DIAMOND TURNING TECHNIQUE 2000, OPTICALDISTORTIONS SMALL)4 MIRRORS MOUNTED ON A PANELPANELS MOUNTED TO SPACE FRAM VIA AMC TOTAL MIRROR AREA 236m**2MIRRORS HAVE QUARTZ COATING FOR PROTECTION
ADVANTAGES:LESS WEIGHT THAN GLASS MIRRORSCHEAPER (WHEN USING WESTERN PRODUCTION COSTS)CAN HAVE NON-CIRCULAR SHAPE : SQUARES WITH ROUNDED EDGES…LESS DAMAGE DUE TO CORROSIONEASIER TO PRODUCE WITH DIFFERENT RADIIWILL NOT BREAK
DISADVANTAGESSLIGHTLY LESS REFLECTIVITYCAN HAVE AGEING DEFORMATIONS DUE TO SLOW CONTINUING PLASTIC POLYMERISATION (MPI MIRRORS)CAN HAVE WATER LEAKS (DELAMINATION WHEN FREEZING)
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Mirrors quartz coated
THE MIRRORCOMPOSED OF 940 ELEMENTALL ALUMINIUM CONSTRUCTIONMANY DIFFERENT RADII (PARABOLIC PROFILE)
M. GARCZARZYK, USING METHOD PROPOSED. BY RAZMIK MIRZOYAN IN 2003 (SEE ALSO CONTR, PALAUSEAU CHERENKOV MEETING): OBSERVE DIRECT STAR + REFLECTED IMAGE ON CAMERA COVER
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VEGA, OBSERVED BOTH DIECTLY BY STARGUIDER CAMERA AND OVER CAMERA COVER(IMAGES OF VEGA COMPLETELY OVEREXPOSED)
THE ACTIVE MIRROR CONTROL
FOR LARGE DISHES DEFORMATIONS A PROBLEM
A)EITHER BILD A STIFF BUT HEAVY CONSTRUCTIONB) OR A LOW WEIGHT CONSTRUCTION AND CORRECTION OF BENDING EFFECTS
LARGE MODERN OPTICAL TELESCOPES FOLLOW B)
MAGIC: THE FIRST TEST OF AMC FOR IACTS NO PREVIOUS EXPERIENCE INDUSTRY HAD NO SUITABLE UNITS AND WE HAD TO DEVELOP IT INHOUSE HARD ROAD TO GET IT WORKING PROBLEMS TO UNDERSTAND THE PRINCIPLE HARDWARE PROBLEMS WITH WATER TIGHTNESS IP67 NOT ENOUGH MONEY TO EQUIPP EVERY MIRROR WITH AMCBUT FINALLY IT WORKED WELL
THE ACTIVE MIRROR CONTROLCOUNTERACTS SOME SMALL DEFORMATIONS OF MIRROR SUPPORT FRAME
EXAMPLE OF MIRROR FOCUSSED TO A LIGHTSOURCE 1000mtr AWAY
PSF 0.03° AFTER MANUALADJUSTMENT (0.02° POSSIBLE)WILL DEGRADE DURINGRUNS IF NOT FREQUENTLYREADJUSTED
0.1°
pedestal
Slide 3
Lasers of the Automatic Mirror Control on a foggy evening …Lasers of the Automatic Mirror Control on a foggy evening …
The camera
Two sections: Inner part: 0.10 PMTs Outer part: 0.20 PMTs
Plate of Winston conesPlate of Winston cones Active camera area Active camera area 100%100%
Matrix of 577 PMTsMatrix of 577 PMTs
SEE SEPARATE TALK FLORIAN GOEBEL
A FREQUENTLY ASKED QUESTION
WHY DID WE NOT BUILD A UNIFORM CAMERA
TWO DIFFERENT SIZES OF PIXELS VERY INCONVENIENT MC, CALIBRATION, UNIFORMITY
MAIN REASON WAS COSTS
1 PIXEL PRICE ≈ 2000€
AN ALL FINE PIXEL CAMERA WOULD BE NEARLY TWICE AS EXPENSIVE
FINE PIXELS NOT JUSTIFIED FOR OUTER AREA DUE TO ABERRATIONS
-> A POINT IMAGE BLURRED BY ABOUT 0.2° DUE TO f/D ≈ 1 + ALSO PARABOLIC PROFILE
LESSON TO BE LEARNED: USE f/D > 1.2 AND AT LEAST 5° FOW CAMERA WITH
UNIFORM PIXELS IF MONEY IS NOT THE MAJOR CONSTRAINT
A PARABOLIC PROFILE IS NEARLY A MUST
WE USE HEMISPHERICAL PMTS WITH 6 DYNODES
ADVANTAGES•BETTER TRANSIT TIME SPREAD•OPTIMAL FOR LIGHT CATCHERS•SOME DOUBLE CHANCE OF PHOTON-PHOTOELECTRON CONVERSION•LOW GAIN-> LOW ANODE CURRENT DUE TO NSB, MOON SHINE->•LESS AGING, LESS DAMAGE, CAN WORK DURING MOON SHINE•LESS POWER FOR HT, LOWER HT, LESS BLEEDER CURRENT-LOWER HEATING IN CAMERA•FASTER•HIGHER GAIN FOR PANEQUE LACQUER ->ABOUT 13-15% PHOTON-PHOTOELECTRON CONVERSION POSSIBLE
DISADVANTAGESHIGH QUALITY PREAMP NEEDEDCAN HAVE MORE PICKUP, NOISE ON SIGNAL
OTHER DISADVANTAGE SPECIFIC TO SELECTED PMTHIGH AFTERPULSE RATE AND LARGE AFTERPULSES FOR HIGH PE COLLECTION EFF.TUBE VACUUM
WE HAVE A CAMERA WINDOW THAT CAUSES 8% LIGHT LOSSWE HAVE SOME PROBLEMS WITH TEMPERATURE STABILIZATION, HUMIDITY
ONE GENERAL ISSUE IF ONE USES PMTS:WHAT IS THE PHOTOELECTRON COLLECTION EFF.
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AREA RATIO OFWinston Cone/PMT=1.91
PM GAIN ≈ 10 000PM type 9116ANOTE: NON-NEGL. SENSITIVITY TO NSBLIGHT SCATTERED FROM GROUNG
0
0,5
1
1,5
2
2,5
3
-60 -40 -20 0 20 40 60
Gain: Wiston cone + Laquer coating
RELATIVE GAIN/cos
Θ
, 0° NORMALIZED TO INCIDENCE OF PLAIN PMT
ANGLE OF INCIDENCE
----- 3 + cone lined with M dielectric foil laquer coating----- + cone lined with aluminised Mylar laquer coating----- , 22 plain PMT diaphragm mm
<- ->LIMIT ANGULAR ACCEPTANCE
CAMERA
CONNECTION BETWEEN CAMERA AND DAQ
MAGIC USES OPTICAL FIBERS FOR THE ANALOG SIGNAL TRANSFERFROM CAMERA PMTS TO THE 100 M AWAY COUNTING HOUSE
PROS: LOW WEIGHT OF CAMERA (OSCILLATIONS REDUCED) REDUCED ELECTRONICS IN CAMERA -> LESS DOWNTIME DUE TO DEFECTS LOW POWER CONSUMPTION IN CAMERA EASIER TO MODIFY ELECTRONICS (TRIGGER…) EASIER TO IMPLEMENT TRIGGER FOR MULTITELESCOPESYSTEM
CONS: NEW PRINCIPLE, NO EXPERIENCE EXTRA COSTS
USE OF VCSELS AS LIGHT EMITTER (CAN DRIVE TO 200 MA PULSE CURRENT FORLOW DUTY CYCLE. HIGH BANDWIDTH (> 1 GHZ POSSIBLE, NOW 230 MHZ BW)NO ATTENUATION OVER 165 M LENGTH
ACHIEVED DYNAMIC RANGE ≈ 800 (LINEAR), > 1000 WITH SOME NONLINEARITY
WE ARE WORKING ON NEXT GENERATION ‚MONOMODE‘ VCSELS. -> HIGHER STABILITY, LOWER NOISE, LOWER POWER
EXPERIENCE WITH ADVERSE ATMOSPHERIC CONDITIONSHIGH WIND SPEED, GUSTY WINDS
•NO PROBLEM TO OBSERVE AT WINDSPEEDS > 10, BUT < 15 m/SEC WIND•CAMERA OSCILLATIONS VERY SMALL•CFRP TUBES HIGH OSCILLATION DAMPING (PROBLEMS WITH LONG METAL TUBES)•USE OF HEAVY BOGEYS: A SAFETY ISSUE BUT BETTER SOLUTIONS WITH MUCH LESS WEIGHT POSSIBLE•HIGHEST WIND SPEED AT SITE UP TO NOW ≈ 140 KMH -> NO PROBLEMS
HUMIDITY: SERIOUS PROBLEMS BECAUSE NOW MORE ELECTRONICS, ELECTRICAL COMPONENTS… A LESSON TO BE LEARNED: COMPONENTS MUST BE IP67 TESTED FOR LONG DURATION AND STRONG SUN LIGHT EXPOSURE
ICE, DEW: LESS SERIOUS THAN ANTICIPATED: HEATING OF MIRRORS WORKS BUTPOWER CONSUMPTION MIGHT BE A PROBLEM
UV-LIGHT OF SUN (CAN BE VERY DESTRUCTIVE AT HIGH ALTITUDE FOR PLASTICS)NO SERIOUS PROBLEMS ENCOUNTERED (EX CABLE BINDERS…)
FAILURES OF ELECTRICAL POWER CAN BE A PROBLEM
FOREST FIRES CAN BE A SERIOUS PROBLEMLIGHTNING PROTECTION
PM, 6 DYNODES
PREAMP
OPTICAL FIBER160 mtr
PIN PHOTODIODE
AMPL.
FAST PM SIGNAL TRANSMISSION BY OPTICAL FIBER SYSTEMWORKING IN ANALOG MODE
VCSEL Input pulse ≈ 2.5 nsecOutput pulse at optical fiber system, 160mtrOutput pulse after RG 58C cable, 156 mtr
•VERY LOW FAILURE RATE AFTER ≈ 1 YEAR
•VERY LOW CROSSTALK, NO PICKUP
•LARGE DYNAMIC RANGE (>60 db)
•SOME NONLINEARITY
•SOME GAIN SHIFT AND MODE HOPPING
•NEEDED TO SELECT VCSELS
TO TRIGGER LOGIC
TO FADC
SUMMARY
MANY TECHNOLOGIES USED IN MAGIC WERE VERY AMBITIOUS IN 2000 BUT NOW NEARLY ‘CONSERVATIVE ‘
CAMERA AND ELECTRONICS ARE OUTDATED MOSTLY AFTER 4-6 YEARS(WE HAVE PROBLEMS TO GET SOME SPARE ELECTRONICS CHIPS)EXAMPLES 300 MHZ FADCs, FAST COMPARATORS
THERE ARE ALSO NEW SOLUTIONS FOR HARDWARE STRUCTURE AVAILABLEFOR LARGE DISHES FOR REASONABLE COSTS
HIGH WIND SPEED MIGHT BECOME A PROBLEM
MORE AND MORE ELECTRONICS COMPONENTS - PROBLEMS WITH WATERTIGHTNESSERVICING AND REPAIR
VERY GOOD EXPERIENCE (EXCEPTIONS CONFIRM THE RULES) WITH DIAMOND TURNED MIRRORS
AMC A VIABLE SOLUTION TO BUILD LOW WEIGHT DISHES
The Calibration: hardware
• Nishia single quantum-well LEDs • Light pulses in 3 colors (UV, blue and green)• Different intensities over dynamic range of factor 300 • Match pulse width of cosmics• Trigger available over wide range of frequences: E.g.: - Calibration run: 500 Hz - Data run with interlaced calibration events: 50Hz - Test runs up to: 4 KHz
WE HAVE A TEMPERATURE
STABILITY PROBLEM
SOME KEY MAGIC PARAMETERS:•MIRROR AREA 242 m2 f/D = 1 MIRROR Ø: 17 m MIRROR PROFILE: PARABOLIC (ISOCHRONOUS)
•MAIN MIRROR COMPOSED OF 940 ELEMENTS,
•ALL ALUMINIUM SANDWICH UNITS, DIAMOND TURNED, LIGHTWEIGHT, HEATEABLE
•ACTIVE MIRROR CONTROL TO COUNTERACT SMALL DISH DEFORMATIONS
•574 PIXEL CAMERA, 3.5 ° Ø, INNER PIXELS 0.1°Ø, OUTER PIXELS 0.2° Ø, SPECIAL LIGHT CATCHERS
•HEMISPHERICAL PMTS (ET 9116,9117, 6 STAGTES) TREATED FOR ENHANCED QE (30-34% AT PEAK )
• LOW LOSS, HIGH BW (> 250 MHz) SIGNAL TRANSMISSION BY OPTICAL FIBER SYSTEM
•THREE LEVEL TRIGGER (REJECTS ALREADY A FRACTION OF HADRONS ON TRIGGER LEVEL)
•TRIGGER AREA ≈ 2° Ø, TRIGGER RATE 250-300 Hz ( TECHNICAL LIMIT 1 KHz)
•SIGNAL DIGITISATION: DUAL RANGE FADC (≈ 58 dB DYN. RANGE) , DIGITISATION FREQUENCY 300
MHz
•MAX ROTATION SPEED: 23(20) sec FOR 180° TURN -> FOR RAPID RESPONSE TO GRBS(CONTROLLED
•TESTS), CURRENTLY NO REVERSE MODE -> SOMETIMES 360° TURNS NECESSARY, ALSO SLOWER
•BECAUSE OF SAFETY REASONS
SOME KEY MAGIC PARAMETERS II:
TRACKING PRECISION ≈ 0.02 (WITHOUT STARGUIDER CAMERA) EXPECT TO REACH 0.005 WITH STAR GUIDER CAMERA
PSF: 0.03° EXPECT TO REACH 0.02° FOR OPTIMISED AMC
CURRENT THRESHOLD: AROUND 50 GeV (TRIGGER) AROUND 80-90 GEV FOR PHYSICS ANALYSIS
->30-40 GEV FOR SPECIAL PULSAR TRIGGER
CURRENT SENSITIVITY: STILL ABOUT FACTOR 2-3 WORSE THAN PREDICTED (USE OF CLASSICAL IMAGE ANALYSIS NOT SO EFFICIENT < 150 GeV)
PMT
Maximizing Double-Hits in a PMT
Not yet optimized for the milky PMT coating
Significant additional improvements to come
PHOTOCATHODE
OLD TRICK OF ASTRONOMERS TO INCREASE QE
SMALL PRISM
FRACTION OF LIGHT SCATTERED OUT
MOST LIGHT TRAPPEDINSIDE HIGH n MATEREIAL
GRADED DIFUSSE SCATTER LAYERWITH MICROSCOPICVOIDS
PHOTOCATHODE
A TOY MODEL