Testing light concentrators prototypes for the Cherenkov Telescope Array
Testing light concentrators prototypes for the Cherenkov Telescope Array François Hénault, Pierre-Olivier Petrucci, Laurent Jocou, Brahim Arezki, Yves Magnard Institut de Planétologie et d’Astrophysique de Grenoble Université Grenoble-Alpes, Centre National de la Recherche Scientifique B.P. 53, 38041 Grenoble – France Bruno Khélifi, Pascal Manigot Laboratoire Leprince-Ringuet, Ecole Polytechnique, 91128 Palaiseau – France Jean-François Olive, Pierre Jean Institut de Recherche en Astrophysique et Planétologie, 31028 Toulouse– France Michael Punch Université Paris 7 Denis Diderot, 75205 Paris – France
for the CTA Consortium Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Plan of presentation • • • •
The Cherenkov Telescope Array (CTA) Principle of Cherenkov telescopes Light Concentrator requirements Prototypes definition – Winston cones – Nonimaging lens
• Test bench description – Design – Error analysis – Measurement procedure
• Experimental results • Conclusion
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA)
MST prototype
• More than 100 collecting telescopes in South and North Hemispheres (Chile and Canary Islands) – Including ~ 40 Medium-size telescopes (MST) of 12 m diameter Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Principle of Cherenkov telescopes • To collect very faint UV pulses at ground level, generated by high-energy cosmic Gamma-rays interacting with atmosphere • Focal plane equipped with ~1800 photomultipliers (PM) • Each PM equipped with a light concentrator (LC) having two main functions: Stray-light
• To maximize concentration efficiency (fill dead spaces between PMs) • To reject stray-light originating from terrestrial environment
Y
UV photons Y’ LC Detectors αT
Cherenkov Telescope
X’ X
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
Focal plane
Z
Support structure
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Light Concentrator requirements • Most critical requirements: Spectral range and Optical transmission REQUIREMENTS Spectral range Cut-off angle αC
MST telescope half-angle αT (nominal) Optical transmission for all angles 0 ≤ α ≤ αT and all polarization states of light Entrance aperture y’ Shape error Photomultiplier Tube (PMT)
VALUES From 300 to 600 nm Depending on the optical design for CPC αC = 28.5 ± 0.5 deg. αC = 26 ± 0.5 deg. for nonimaging lens αT = 21.2 deg. T ≥ 80 % on the full spectral range (goal 85%) Hexagonal of width 49 mm flat to flat ≤ 0.1 mm Hamamatsu R12992-100 series
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Prototypes definition: Winston cones
50 mm
Photo-cathode
• Made of three petals of molded plastic • Coated with high-reflective layers • Will be protected from harmful desert environment by a large common Plexiglas window
54 mm
Optical model
Mechanical model
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
Assembled prototype San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Prototypes definition: Nonimaging lenses
• •
End stop
50 mm
T
31.6 mm
Photo-cathode
T
End stop
Photo-cathode
50 mm
T
•
Two different types: plano-convex and aspheric Made of FK5 glass (good transmission in near-UV range) Anti-reflection coated on both faces Also act as protective windows
T
•
32.5 mm
Plano-convex lens
Aspheric lens
Exit stop
Mechanical model Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
Prototype San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Test bench design Remote Light Sources Block Off-axis parabola
X XR
Fiber head
Rotation stage
Baffle
OF
YR
15 deg.
OP Off-axis parabola
Aperture stop
LC
OR
PMT
α
ZR Z
Optical Bench Enclosure
Fiber head
Diffusers
LC and PM support
Fiber head
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
Rotation stage
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Test bench error analysis • Typical repeatability error of 0.34 % (worst case 1.3 %) for rejection curves and relative transmission measurement • Typical absolute error of 1.6 % (worst case 2.5 %) for spectral transmission curves
Error Source
Type
Beam non-uniformity
Bias
1.23
1.23
Light source and PM intensity
Drift
0.02
0.02
Light source and PM intensity
Random
0.01
0.06
PM voltage adjustment
Random
0.02
0.05
LC positioing error (XYZ)
Random
0.12
0.34
LC positioing error (roll angle)
Random
0.09
0.16
LC shape deformation
Random
0.28
0.66
Repeatability error (%)
0.34
1.29
Absolute error (%)
1.57
2.52
8.04
RMS Error (%) Max. Error (%)
#1
#2
#3
#4
2 ∆ R(α ) in %
Current values (x 100)
3
8.03
8.02
1 0 -1 -2
8.01
Light source / PMT response stability
-3
0
1
2
3
Time (hours)
4
5
-45 Light concentrator shape deformation
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
-30
-15
0
15
30
α (deg)
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
1
0.8
0.8
0.6 0.4 0.2
1
-45
-30 α-
4
Spectral transmission curves
420 nm
440 nm
480 nm
517 nm
-15
0
15
α (deg)
0.4 0.2
2
390 nm
420 nm
440 nm
480 nm
517 nm
0
30 α+
10
20
30
40
α (deg)
C
0.92
1.6
0.9
1.4 1.2 1 0.8 0.6 0.4
3
325 nm
0
αT
1.8
Efficiency E(α )
3
390 nm
0
C
Radiallly integrated curves
325 nm
0.6
0.2
325 nm
390 nm
420 nm
440 nm
480 nm
517 nm
0 0
10
20
30
40
α (deg)
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
Transmission
2
Symmetrized rejection curves
1
Rejection RS(α )
1
Calibrated rejection curves
Rejection R(α )
Measurement procedure
0.88 0.86 0.84 T at 0 deg. 0.82
4
T (integrated) 0.8 325
390
420
440
480
517
Wavelength (nm)
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Experimental results: Winston cones Spectral transmission curves
92 90 Transmission (%)
• Two different series: standard or enhanced reflective coatings • Results are well above specification: from 85 to 90 % for enhanced coating series
88 86 84 82
Symmetrized rejection curves
1
boost6
boost7
boost8
alu-prot5
alu-prot6
alu-prot7
alu-prot8
80 325
392
420
440
480
517
Filter (nm)
0.8
Standard vs. enhanced coating
92
0.6
90 Transmission (%)
Rejection RS(α )
boost5
0.4 0.2
325 nm
390 nm
420 nm
440 nm
480 nm
517 nm
88 86 84 boost series 82
0
alu-prot series
0
10
20
30
40
80
α (deg)
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
325
392
420
440
480
517
Filter (nm)
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Nonimaging lenses vs. cones • Raw rejection curves show different aspects Lenses show higher response 1
Winston cones 1
0.8 Rejection R(α )
0.8 Rejection R(α )
Lenses
0.6 0.4 0.2
325 nm
390 nm
420 nm
440 nm
480 nm
517 nm
Dissymmetry due to lens/baffle parallelism error
0.6 0.4 0.2
325 nm
390 nm
420 nm
440 nm
480 nm
517 nm
0
0 -45
-30 α-
C
-15
0 α (deg)
15
Cut-off angle αC
• Cut-off angles
-45
30 + αC
Winston cones Nonimaging lens
-30
-15
0
15
30
Spurious reflections α (deg) above Cut-off Average (deg.)
Maximal Standard deviation (%) deviation (%)
27.9 (φ = 0°) 29.4 (φ = 30°) 26.5
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
Requirement (deg.)
0.7
0.3
28.5
1.8
1.4
25.5
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Nonimaging lenses vs. cones • Final comparison between nonimaging lenses, Winston cones, and cones + Plexiglas window – Lenses are more efficient than cones alone (+5-11 %) and cones + window (+11-19 %) – depending on wavelength 100
Window transmission loss (%)
Transmission (%)
95 90 85
Cone
80
hex. asp. lens 75 70 325
12
Nonimaging lens
Cone + Window
boost7
Window transmission losses
8
alu-prot5
4
boost7 boost8 Mean
boost7 + window 0
392
420
440
480
517
325
Filter (nm)
Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
392
420
440
480
517
Filter (nm)
San Diego, 08-06-17
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Testing light concentrators prototypes for the Cherenkov Telescope Array
Conclusion • Two different types of light concentrators have been designed for the Cherenkov Telescope Array (CTA) – Classical Winston cone – Nonimaging lens (Following Edge-ray Principle)
• Both types of concentrators have been prototyped, a test bench was developed in our laboratory • Extensive test campaign led to the following conclusions: – Pure performance is in favor of nonimaging lenses. But they present some drawbacks: • Stray reflections above cut-off angle • Heavier mass • Higher cost
• Thus Winston cones were selected as baseline for CTA Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIV
San Diego, 08-06-17
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