Transcript document

EXPERIENCE WITH THE CONSTRUCTION AND
OPERATION WITH THE MAGIC TELESCOPE
E. Lorenz, MPI Munich
WILL CONCENTRATE ON ISSUES RELEVANT FOR THE CONSTRUCTION OF POSSIBLE
FUTURE 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 CONSTRUCTED
IN LAMPOLDSHAUSEN
MECHANICS INSPIRED BY THE DESIGN OF SCHLAICH, BERGEMAN AND SONS
SOON OBVIOUS THAT THE ORIGINAL MIRROR CONCEPT UNSUITED FOR THE
REQUIRED 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 PMTS
LOTS OF NEW PHYSICS EXPECTED IN THE ENERGY DOMAIN < 300(150) GEV
• RECOMMENDATION FROM REVIEW COMMITTEE: BUILD MAGIC FIRST WITH
CLASSICAL 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 THRESHOLD
DISADVANTAGE: LOWER SENSITIVITY IN OVERLAP REGION
LESSON TO BE LEARNED: IF THE CONSTRUCTION WILL BE TOO AMBITIOUS
ONE WILL HAVE TROUBLE TO GET FULL SUPPORT FROM THE COMMUNITY
(BMBF WAS MORE POSITIVE ABOUT NEW IDEAS AND NEW TECHNOLOGIES
THAN 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OR EVEN OUTDATED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 INTERNAL
DEFORMATION 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 PLAY
NOT PERFECTLY ROUND -> SOFTWARE CORRECTIONS
NOT PERFECTLY SMOOTH (LINEAR) -ALSO A REASON WHY A PERFECT
HIGH RESOLUTION SHAFTENCODER DOES NOT HELP
NOTE: WE HAVE 1 SHAFTENCODER FOR AZIMUTH POSITION, 2 FOR
DECLINATION (DEVIATE FROM EACH OTHER- MIRROR DISH DEFORMATION
PROBLEM WITH CULMINATION-> SEPARATE TALK)
TELESCOPE CAN TURN > 450° IN AZIMUTH -> MULTITURN SHAFTENCODER
ENCODERS ARE ABSOLUTE ENCODERS -> NO CHANGE AFTER POWER
FAILURE
ENCODER 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 SEC
2 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
MIRRORS
ALL ALUMINIUM
SANDWICH CONSTRUCTION WITH INTERNAL HEATING
DIAMOND 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, OPTICAL
DISTORTIONS SMALL)
4 MIRRORS MOUNTED ON A PANEL
PANELS MOUNTED TO SPACE FRAM VIA AMC
TOTAL MIRROR AREA 236m**2
MIRRORS HAVE QUARTZ COATING FOR PROTECTION
ADVANTAGES:
LESS WEIGHT THAN GLASS MIRRORS
CHEAPER (WHEN USING WESTERN PRODUCTION COSTS)
CAN HAVE NON-CIRCULAR SHAPE : SQUARES WITH ROUNDED EDGES…
LESS DAMAGE DUE TO CORROSION
EASIER TO PRODUCE WITH DIFFERENT RADII
WILL NOT BREAK
DISADVANTAGES
SLIGHTLY LESS REFLECTIVITY
CAN HAVE AGEING DEFORMATIONS DUE TO SLOW CONTINUING PLASTIC
POLYMERISATION (MPI MIRRORS)
CAN HAVE WATER LEAKS (DELAMINATION WHEN FREEZING)
THE MIRROR
COMPOSED OF 940 ELEMENT
ALL ALUMINIUM CONSTRUCTION
MANY DIFFERENT RADII (PARABOLIC PROFILE)
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Dekompressor “Foto - JPEG”
benötigt.
Mirrors quartz coated
M. GARCZARZYK, USING METHOD PROPOSED. BY RAZMIK MIRZOYAN IN 2003 (SEE ALSO CONTR,
PALAUSEAU CHERENKOV MEETING): OBSERVE DIRECT STAR + REFLECTED IMAGE ON CAMERA COVER
VEGA, OBSERVED BOTH DIECTLY BY STARGUIDER CAMERA AND OVER CAMERA COVER
(IMAGES OF VEGA COMPLETELY OVEREXPOSED)
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
THE ACTIVE MIRROR CONTROL
FOR LARGE DISHES DEFORMATIONS A PROBLEM
A)EITHER BILD A STIFF BUT HEAVY CONSTRUCTION
B) 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 AMC
BUT FINALLY IT WORKED WELL
THE ACTIVE MIRROR CONTROL
COUNTERACTS SOME SMALL DEFORMATIONS
OF MIRROR SUPPORT FRAME
EXAMPLE OF MIRROR FOCUSSED TO A LIGHT
SOURCE 1000mtr AWAY
PSF 0.03° AFTER MANUAL
ADJUSTMENT (0.02° POSSIBLE)
WILL DEGRADE DURING
RUNS IF NOT FREQUENTLY
READJUSTED
pedestal
0.1°
Slide 3
Lasers of the Automatic Mirror Control on a foggy evening …
The camera


Matrix of 577 PMTs
Two sections:
 Inner part: 0.10 PMTs
 Outer part: 0.20 PMTs
Plate of Winston cones 
Active camera area 100%
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
DISADVANTAGES
HIGH QUALITY PREAMP NEEDED
CAN HAVE MORE PICKUP, NOISE ON SIGNAL
OTHER DISADVANTAGE SPECIFIC TO SELECTED PMT
HIGH AFTERPULSE RATE AND LARGE AFTERPULSES FOR HIGH PE COLLECTION EFF.
TUBE VACUUM
WE HAVE A CAMERA WINDOW THAT CAUSES 8% LIGHT LOSS
WE HAVE SOME PROBLEMS WITH TEMPERATURE STABILIZATION, HUMIDITY
ONE GENERAL ISSUE IF ONE USES PMTS:WHAT IS THE PHOTOELECTRON COLLECTION EFF.
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CAMERA
Gain: Wiston cone + Laquer coating
----- cone lined with 3M dielectric foil + laquer coating
----- cone lined with aluminised Mylar + laquer coating
----- plain PMT, diaphragm 22 mm
RELATIVE GAIN/cos
, NORMALIZED TO 0° INCIDENCE OF PLAIN PMT
3
<- LIMIT ANGULAR ACCEPTANCE ->
2,5
2
1,5
AREA RATIO OF
Winston Cone/PMT=1.91
1
0,5
0
-60
-40
-20
0
20
ANGLE OF INCIDENCE
40
60
PM GAIN ≈ 10 000
PM type 9116A
NOTE: NON-NEGL.
SENSITIVITY TO NSB
LIGHT SCATTERED
FROM GROUNG
CONNECTION BETWEEN CAMERA AND DAQ
MAGIC USES OPTICAL FIBERS FOR THE ANALOG SIGNAL TRANSFER
FROM 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 FOR
LOW 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 CONDITIONS
HIGH 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 BUT
POWER 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 PROBLEM
LIGHTNING PROTECTION
FAST PM SIGNAL TRANSMISSION BY OPTICAL FIBER SYSTEM
WORKING IN ANALOG MODE
VCSEL
PM,
6 DYNODES
PIN PHOTODIODE
OPTICAL FIBER
160 mtr
TO TRIGGER LOGIC
TO FADC
.
AMPL
PREAMP
•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
Input pulse ≈ 2.5 nsec
Output pulse at optical fiber system, 160mtr
Output pulse after RG 58C cable, 156 mtr
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 AVAILABLE
FOR LARGE DISHES FOR REASONABLE COSTS
HIGH WIND SPEED MIGHT BECOME A PROBLEM
MORE AND MORE ELECTRONICS COMPONENTS - PROBLEMS WITH WATERTIGHTNES
SERVICING 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 l)
• 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)
Maximizing Double-Hits in a PMT
PMT
Not yet optimized for the milky PMT coating

Significant additional improvements to come
OLD TRICK OF ASTRONOMERS TO INCREASE QE
SMALL PRISM
PHOTOCATHODE
A TOY MODEL
MOST LIGHT TRAPPED
INSIDE HIGH n MATEREIAL
FRACTION OF LIGHT SCATTERED OUT
GRADED DIFUSSE
SCATTER LAYER
WITH MICROSCOPIC
VOIDS
PHOTOCATHODE