Design of light concentrators for Cherenkov telescope observatories
Design of light concentrators for Cherenkov telescope observatories François Hénault, Pierre-Olivier Petrucci, Laurent Jocou Institut de Planétologie et d’Astrophysique de Grenoble F-38041 Grenoble – France Bruno Khélifi, Pascal Manigot, Stéphane Hormigos Laboratoire Leprince-Ringuet, F-91128 Palaiseau – France Jürgen Knödlseder, Jean-François Olive Institut de Recherche en Astrophysique et Planétologie F-31028 Toulouse– France Michael Punch Université Paris 7 Denis Diderot, F-75205 Paris – France Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Principle of Cherenkov telescopes • To collect very faint UV pulses at ground level, generated by high-energy, cosmic γ–rays • Focal plane equipped with hundreds of photomultipliers (PM) • Each PM equipped with a light concentrator (LC) having two main functions: Stray-light Y
• To maximize concentration efficiency (fill dead spaces between PMs) • To reject stray-light from terrestrial environment
UV photons Y’ LC Detectors αC
Cherenkov Telescope X
X’ Focal plane
Z
Support structure
Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
The Cherenkov Telescope Array (CTA)
• More than 50 collecting telescopes in South and North Hemispheres – ~ 10 Large-size telescopes, ∅ = 24 m, 40 Medium-size telescopes, ∅ = 12 m, 10 Small-size telescopes, ∅ = 6 m Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
MST light Concentrator requirements • Most critical requirements: Spectral range and Cut-off angles (linked to telescope aperture) REQUIREMENTS
VALUES
Spectral range
From 300 to 600 nm ≥ 75 % on full spectral range (goal 80%)
Optical transmission Cut-off angle αC
26 deg. ≤ αC ≤ 28 deg.
Rejection rate outside of cut-off angle αC
≤ 5 % on full spectral range
Crosstalk between two adjacent LCs
≤ 3 % on full spectral range Hexagonal, ∅ = 50 mm flat to flat
LC entrance aperture Curvature radius of photo-detector (PM)
20 mm
Diameter of photo-sensitive area (PM)
34 mm
Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Reflective solution: Winston cones (CPC) • Already implemented on most existing Cherenkov observatories
• “Truncated” Winston cone, length = 54 mm (instead of 66 mm) to save volume and mass • Safety margin ≥ 1 mm between cone and PM to prevent electrostatic discharges Y’
50 mm
Photo-cathode
Z X’
54 mm
Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Dioptric solution: Methodology • All computations carried out with Zemax commercial optical software in sequential mode • Only FoV edge is optimized Follows the Edge-ray Principle • Provides quick, first-guess Nonimaging lens designs
Aperture stop ∅ 32 mm
Do not optimize FoV center
Plano-convex lens Conf. 8834 – Nonimaging Optics X
Only optimize FoV edge San Diego, 08-25-13
Aspherical lens 6
Design of light concentrators for Cherenkov telescope observatories
Dioptric solution: Plano-convex lens • Plano-convex lens of hexagonal section, curved face on telescope side, flat face on PM side • Selected material: FK5 glass or equivalent – Good transmission on full Cherenkov spectral range – Moldable (cost reduction for mass production)
50 mm
Photo-cathode
• Anti-reflective coated on both faces
Y’
X’
Z 31.6 mm Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Dioptric solution: Aspheric lens • Starting from the plano-convex design, the flat exit face is slightly aspherized • Both designs include a ∅ 32 mm aperture stop, located 1 mm from PM entrance face (as for reflective design)
50 mm
Photo-cathode
Y’
Z
X’ 32.5 mm Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Achievable performance • Most important performance criterions are: 1
• Absolute transmission (e.g. at FoV center) Rejection curve
• Slope of rejection curve at cutoff angle αC
0.8
0.6
0.4 Hexagonal CPC, cut-off = 28° Plano-convex lens, cut-off = 26°
0.2
Aspheric lens, cut-off = 26° 0 0
5
10
15
20
25
30
35
40
45
Field angle (deg.)
Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Cone Vs. lens trade-off Criterions
HEXAGONAL CONE
CONCENTRATING LENS
Typical cost (per unit) Weight (per unit) Size of entrance area Need for protective window Cut-off angle for straylight rejection Maximal slope of rejection curve Typical absolute transmission Alignment tolerance (wrt PM) Spot size on cathode Dead spaces in focal plane Encumbrace (along optical axis)
15 Euros 15 grams Not critical Yes 28 deg. 12% / deg. 0.8 Moderate 24 mm ≤ 1 mm 54 mm
20-30 Euros 40-50 grams Should be ≤ 50 mm No (lens acting as window) 26 deg. 3% / deg. 0.96 (with AR coatings) Moderate 32 mm ≤ 1 mm ≈ 32 mm
• Protective window needed in sandy or dusty environment – Reflective design: requires thin Plexiglas window at cone entrance – Dioptric design: no, additional component, lens naturally acting as window Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Prototyping activities: Winston cones • Made of three petals of molded plastic • Coated with highreflective layers
CAD view
Assembled cone
• Can accommodate a thin protective entrance window Cone petals Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
Prototyping activities: Nonimaging lens Y’ • Mounted into a baffle with hexagonal section – Supports the lens – Prevents crosstalk between adjacent lenses – Ends on circular aperture stop near PM entrance face
Baffle
Aperture stop
zS Z zPM
DS
PM cathode
Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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Design of light concentrators for Cherenkov telescope observatories
First experimental results
• Preliminary results obtained on CPC prototype (rough data, being not calibrated) • “Bump” near FoV edge
Measured signal (V)
• Dedicated test bench developed at IRAP (Toulouse, France) to measure the rejection curves of prototyped concentrators
FoV angle (deg.)
Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
13
Design of light concentrators for Cherenkov telescope observatories
Conclusion • We presented two designs of light concentrators for Cherenkov observatories: – Classical Winston cones (reflective design) – Nonimaging lenses (dioptric design) • Following the Edge-ray Principle • Optimized using standard optical software
• Both types of concentrators were prototyped, preliminary test results already obtained for cones • Extensive test campaign to start in September 2013 • Do you want to help us for design or manufacturing issues ? Please e-mail to:
[email protected] Conf. 8834 – Nonimaging Optics X
San Diego, 08-25-13
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