Bala K. Balasubramanian
Jet Propulsion Laboratory
California Institute of Technology
© 2016 California Institute of Technology. Government sponsorship acknowledged.
HabEx Technology Meeting3 August 2016
Jet Propulsion Laboratory
Mirror Coatings for large aperture UV to IR Telescopes
Jet Propulsion Laboratory
California Institute of Technology
Jet Propulsion LaboratoryCalifornia Institute of TechnologyRequirements and Challenges
• UV Optical IR telescope optics covering FUV to NIR
• High Reflectance including the far UV down to 90nm
• Large area, meter class optics
• High Uniformity
• Polarization
• Stability in the environment, robust protection
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyHigh Reflectivity Metals
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Reflectance of 200nm thick film on glass for four metals
Theoretical calculations based on optical constants from Palik
0
10
20
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50
60
70
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100
200 300 400 500 600 700 800 900 1000 1100 1200
Wavelength (nm)
Refl
ecta
nce (
%)
Ag
Al
Au
Cu
Ag
Al
Au
Cu
Aluminum is the obvious choice
Jet Propulsion LaboratoryCalifornia Institute of TechnologyTechnical Challenges
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Performance impact due to:
1. Chemical (contamination, oxidation, stoichiometry)AbsorptionInstability/durability
2. MicrostructuralScatteringWater vapor adsorption
3. Uniformity over large area
4. Polarization sensitivity
Jet Propulsion LaboratoryCalifornia Institute of TechnologyUnprotected Aluminum Mirror
Unprotected Al
Unprotected Al with ~ 2 nm of oxide
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyUnprotected Aluminum
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On the Vacuum-Ultraviolet Reflectance of Evaporated Aluminum before and during Oxidation*R. P. MADDEN, L. R. CANFIELD, AND G. HASS; JOSA Vol:53 No:5, May 1963
Jet Propulsion LaboratoryCalifornia Institute of TechnologyUnprotected Aluminum
• Reduction of aluminum reflectance following air exposure has power law dependence on time– Power law exponent also has at least exponential dependence on wavelength
Oxidation induced reflectance reduction in the near UV of an Al mirror sample; Models predictions match a progressive increase of oxide formation.
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyProtective layers
Properties of Typical Deposited Thin Films for UV Optical Applications
Material Band Energy (eV) ~λ Cut Off (nm)
lithium fluoride (LiF) 12 – 13 95
aluminum fluoride (AlF3) 11 – 12 105
magnesium fluoride
(MgF2)
10 – 11 115
calcium fluoride (CaF2) 9 – 10 125
lanthanum fluoride (LaF3) 8 – 9 140
silicon oxide (SiO2) 7 – 8 160
aluminum oxide (Al2O3) 6 – 7 190
• Aluminum has the highest reflectance in the ultraviolet, but reflectance below 200 nm is strongly suppressed by the presence of any surface oxide
• Protective coatings can be applied to pristine Al surfaces to prevent oxidation and even enhance reflectivity due to interference effects
• Currently developing and optimizing ALD processes at JPL for the three best candidate protective materials
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyBackground
• Standard coatings fall well below the natural reflectance of aluminum
– A thin, dense, absorption free protective coating could greatly improveperformance from 90-120 nm
• FUV has a significant number of spectral lines that are of great interest to astronomers
– Stellar and galaxy evolution; protoplanetary disks and exoplanet atmospheres
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Ref: Keski-Kuha et al., ASP Conference Series, vol. 164, (1999)
Jet Propulsion LaboratoryCalifornia Institute of TechnologyA Protected Aluminum Mirror
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Conventional Deposition
Jet Propulsion LaboratoryCalifornia Institute of TechnologyAl+LiF+AlF3 mirror aging performance
Measured reflectance of a bi-layer protected Al mirror sample measured 6, 8, 10 and 14 months after fabrication showing excellent stability. FUV to NIR spectral range.
Conventional Thermal Evaporation
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyAl+LiF+AlF3 mirror aging performance
Measured reflectance of a tri-layer Al mirror sample measured 6, 8, 10, 14 and 23 months after fabrication showing excellent stability. Expanded view of the FUV spectral range.
Conventional Thermal Evaporation
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyStability of Al with Thin AlF3 Layers by ALD
– ALD AlF3 coatings have a measured long-term stability, and can also extend the short wavelength cutoff when compared to traditional methods
– Layers as thin as 3 nm have been demonstrated to be effective in suppressing the oxidation of aluminum
Stability of Al mirror (sample K series) coated with thin AlF3 layer by ALD
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyAluminum Evaporation Rate Dependence
• Even in UHV conditions (base pressure ~ 2 x 10-9 Torr), the reflectivity dependence on evaporation rate is significant– Impact on the saturated value of reflectance as well as the
rate of degradation08/03/2016 Balasubramanian, JPL/Caltech 14
Jet Propulsion LaboratoryCalifornia Institute of TechnologyFUV performance of ALD AlF3/Al mirror samples
Model fits (dotted lines) of measured (symbols) FUV reflectance of unprotected (sample K1) and AlF3 protected samples (K2 to K5).
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyAl+LiF+AlF3 mirror, Al+AlF3(ALD) mirror
• Conventional Thermal Evaporation (Z11, Z15)• ALD of AlF3 on e-beam Al (K5 and Q11)
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• FUV reflectance of o tri-layer mirror samples produced by conventional thermal evaporation o bi-layer mirror samples produced by e-beam and ALD
• Optimization of layer thicknesses necessary to improve performance
Ly-a 121.6nmLy-b 102.6nmLy Limit 91.2nmBalmer-g 108.5nm
Jet Propulsion LaboratoryCalifornia Institute of TechnologyProtected Al mirrors from JPL and GSFC
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GSFC Data Courtesy: Manuel Quijada
0
10
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40
50
60
70
80
90
100
90 100 110 120 130 140 150 160 170 180 190
Ref
lect
ance
(%
)
Wavelength (nm)
Protected Al mirrors from JPL and GSFC produced with different processes
Al +MgF2 (GSFC)
Al+LiF (GSFC)
JPL065-11513Al+LiF+AlF3
JPL Z15FS(MgF2/LiF/Al)
Al2O3 (3A) + ALD AlF3(3nm) + Al
Al2O3 (0A) + ALD AlF3(2nm) + Al
Jet Propulsion LaboratoryCalifornia Institute of TechnologyOptimization
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The calculated reflectance at 121.6 and 102.6 nm as a function of coating thickness for films of MgF2, AlF3, and LiF on ideal Al.
J. Hennessy, et al., JATIS 2(4), 041206 (2016)
Measured FUV reflectance (symbols) and the corresponding calculated optical model (dashed lines) of ALD AlF3 protective coatings of various thickness deposited on evaporated Al thin films.
Throughput after 3 reflections: with 60% R from each optic at 100nm, throughput will be 0.6^3 = 0.22
Jet Propulsion LaboratoryCalifornia Institute of TechnologySummary
• Protected Aluminum mirrors with ~75% reflectance at 110nm with long term stability have been produced
• These mirrors currently show ~55% reflectance at 100nm
• Protective fluoride layers coated with Atomic Layer Deposition indicate potentially better performance (>60% at 100nm) and stability
• References
• Balasubramanian, et al., Proceedings of SPIE vol. 9602-19 (2015)
• Hennessy, J., April D. Jewell, Frank Greer, Michael C. Lee, and Shouleh Nikzad. "Atomic layer deposition of magnesium fluoride via bis (ethylcyclopentadienyl) magnesium and anhydrous hydrogen fluoride." Jl. of Vacuum Science & Technology A 33, no. 1 (2015): 01A125.
• Hennessy, J., A. D. Jewell, K. Balasubramanian, and S. Nikzad, “Ultraviolet optical properties of aluminum fluoride thin films deposited by atomic layer deposition,” (submitted) Jl.of Vacuum Science & Technology A, JVST A 34, 01A120 (2016).
• J. Hennessy, Kunjithapatham Balasubramanian, Christopher S. Moore, April D. Jewell, Shouleh Nikzad, Kevin France, Manuel Quijada, “Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition,” J. Astron. Telesc. Instrum. Syst. 2(4), 041206 (2016), doi: 10.1117/1. JATIS.2.4.041206
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyAcknowledgements
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John Hennessy, Shouleh Nikzad, Nasrat Raouf, Stuart Shaklan, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
Paul Scowen, Arizona State University, Tempe, AZ 85287
Manuel Quijada, Javier Del Hoyo, NASA – GSFC (FUV Reflectance)
David Sheikh, and Josh Saadia, Zecoat Corporation, Torrance, CA
The work is performed at Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA Cosmic Origins Program
Thank You
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Backups
Jet Propulsion LaboratoryCalifornia Institute of TechnologySignificant FUV Spectral Lines
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Wavelength (nm) Species Significance68.1, 69.4 Na IX Coronal Gas (> 106 K) Diagnostic (density,
ionization state, etc.)77.0 Ne VIII Warm-Hot Gas (5105 - 106 K) Diagnostic (density,
ionization state, etc.)91.2 H, Lyman Limit Ionization Energy of Atomic Hydrogen
97.7 C III Gas Electron Density Diagnostic
99.1, 175.0 N III Gas Temperature Diagnostic102.6 H, Ly- Lyman Series H Recombination Line
103.2, 103.8 O VI Recombination Line Doublet108.5, 164.0 He II Balmer- line for He117.5 C III Gas Electron Density Diagnostic
120.6 Si III Optically thin emission line of Silicon
121.6 H, Ly- Lyman Series H Recombination Line
123.8, 124.3 N V Gas Emission Diagnostic130.4 O I Geocoronal Triplet Emission Line
133.5 C II Absorption Line for ionized Carbon
139.4, 140.3 Si IV Emission Line of Silicon140.7 O IV] Gas Density sensitive doublet148.8 N IV] Gas Diagnostic Line – sensitive in particular to
electron collision strengths154.8, 155.1 C IV Gas density-sensitive doublet
Courtesy: Paul Scowen
Jet Propulsion LaboratoryCalifornia Institute of TechnologyKepler
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Reference:
D.A.Sheikh, et al., Proc SPIE vol.7010 (2008)
Jet Propulsion LaboratoryCalifornia Institute of TechnologyKepler
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Reference:
D.A.Sheikh, et al., Proc SPIE vol.7010 (2008)
Reflectance is ~ 92% for l > 400 nm.
Radial uniformity is better than 30 nm pvover 1.6 m dia.
Technology development is required for achieving better performance.
Jet Propulsion LaboratoryCalifornia Institute of TechnologyDeposition Chambers
1.2m thermal / ebeam evaporation chamber (Zecoat Corp) with a moving source
Beneq ALD reactor (JPL)Oxford ALD reactor (JPL)
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyLarge ALD chambers
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Commercial solutions for large area atomic layer deposition include (left) systems for
high performance optical coatings [MLD Technologies, mldtech.com], (middle)
deposition on meter-class substrates for photovoltaic applications [Putkonen 2009],
and (right) large area roll-to-roll ALD reactor [Beneq, beneq.com]
Jet Propulsion LaboratoryCalifornia Institute of TechnologyVUV enhanced Al mirrors
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Al+LiF Mirror FUV Performance (GSFC)
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0.4
0.5
0.6
0.7
0.8
0.9
1.0
100 110 120 130 140 150
Ref
lect
an
ce
Wavelength (nm)
Hot Deposition
Fit
FUSE
28
Recipe: Al (43nm, ambient)+LiF(8nm, ambient)+LiF(16.4nm, 250°C)
Rave(100-150nm): 59% (FUSE) 75% (Hot)
Manuel Quijada, GSFCSep 2014
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0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
100 105 110 115 120 125 130 135 140 145 150
Ref
lect
an
ce
Wavelength (nm)
Al (50nm)+LiF(24.4nm)Al (50nm)+LiF(15nm)+MgF (5nm)Al (50nm)+LiF(18nm)FUSE
Al+LiF Mirror FUV Performance Cont..
29
Manuel Quijada, GSFCSep 2014
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Jet Propulsion LaboratoryCalifornia Institute of Technology
Polarization
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Jet Propulsion LaboratoryCalifornia Institute of TechnologyOblique Incidence on Mirrors
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• p and s Reflectances and Phase differences are different and vary with angle of incidence (AOI)
• Consequence phase and amplitude maps of orthogonal polarizations are quite different across the aperture
• Leakage terms from cross polarizations also have different amplitudes and phases
• DM can not correct wavefront error for both x and y polarizations simultaneously inadequate wavefront correction for coronagraphy
• Potential solutions optimized coatings through design and manufacture
• How well we can do this?
• If polarization effects can be mitigated, can we remove the polarizers thus simplifying design?
Jet Propulsion LaboratoryCalifornia Institute of TechnologyOptimizing Variables
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1. Apply one optimum coating to all surfaces, preferably single
layer for simplicity
2. Compensate polarization effect from one surface by different
coatings on another surface
2. Apply tapered coatings over the aperture to mitigate angle of
incidence effects
3. Add a correction optic, such as a diffractive element
Different Approaches
Jet Propulsion LaboratoryCalifornia Institute of TechnologyPupil Fields
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TelescopePolarizing Beam Splitter
Deformable Mirror
Occulting Mask and Lyot Stop Final Image
Deformable Mirror
Occulting Mask and Lyot Stop Final Image
x Pol
y Pol
xy yy yxxxi i ii
xx xy yy yxA e a e A e a e
Total FieldY polarization main term
Y polarization cross term
( )
1i xxxy xy
xx
ae
A
X polarization field after DM correction
Leakage or residual due to cross term
Balasubramanian, et al, Proc SPIE 5905, 2005
Jet Propulsion LaboratoryCalifornia Institute of TechnologyPolarization
Wavelength = 600nm
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Rp/Rs from unprotected Al and Ag
Optical constants from Essential Macleod standard materials
0.99
0.992
0.994
0.996
0.998
1
0 5 10 15
Angle of Inc. (deg)
Rp
/Rs
Rp/Rs (Ag)
Rp/Rs (Al)
Wavelength 600nm
Delta Phase (p-s) from unprotected Al and Ag
Optical constants from Essential Macleod standard materials
-3
-2.5
-2
-1.5
-1
-0.5
0
0 5 10 15
Angle of Inc (deg)
Delt
a P
hase (
P-S
) d
eg
Delta Phase Ag
Delta Phase Al
Wavelength 600nm
Jet Propulsion LaboratoryCalifornia Institute of TechnologyUnprotected Aluminum
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For R 91%,
Six surfaces will yield a
throughput of
(0.91)^6 = 0.568
Four surfaces will yield
(0.91)^4 = 0.685
For R = 86%
Four surfaces yield
(0.86)^4 = 0.547
At AOI = 12 deg
Rp at 800nm = 86.2%
Rs at 800nm = 86.8%
DR at 800nm = 0.6%
DR at 800nm after 4 surfaces =
1.6%
Reflectance (%) vs Wavelength (nm)
Unprotected Aluminum
Phase difference (deg) bet p and s on reflection
vs Wavelength (nm)
94
84
400 1000
178
182
AOI 0 deg
AOI 12 deg
All angles 0 to 12 deg
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Single layer SiO2 protection
Rp, Rs for AOI 2 to 14 deg
124nm SiO2 on silver
95
95.5
96
96.5
97
97.5
98
98.5
99
99.5
100
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
ecta
nce (
%)
0 deg
2 deg - p
2 deg - s
4 deg - p
4 deg - s
6 deg - p
6 deg - s
8 deg - p
8 deg - s
10 deg - p
10 deg - s
12 deg - p
12 deg - s
14 deg - p
14 deg - s
Delta phase vs wavelength for AOI 2 to 14 deg
124nm SiO2 on silver
177
178
179
180
181
182
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
Delt
a P
hase (
p-s
) (d
eg
)
2deg
4deg
6deg
8deg
10deg
12deg
14deg
s (blue curves) and p (red curves) Reflectances vs Wavelength
8nm Si3N4 / 124nm SiO2 / 15nm Si3N4 / Ag / Substrate
95
95.5
96
96.5
97
97.5
98
98.5
99
99.5
100
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
ecta
nce (
%)
0 deg
2 deg - p
2 deg - s
4 deg - p
4 deg - s
6 deg - p
6 deg - s
8 deg - p
8 deg - s
10 deg - p
10 deg - s
12 deg - p
12 deg - s
14 deg - p
14 deg - s
Delta phase vs wavelength for AOI 2 to 14 deg
8nm Si3N4 / 124nm SiO2 / 15nm Si3N4/ 300nm Ag / Substrate
177
178
179
180
181
182
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
Delt
a P
hase (
p-s
) (d
eg
)
2 deg
4 deg
6 deg
8 deg
10 deg
12 deg
14 deg
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Complementary coatings
s (blue curves) and p (red curves) Reflectances vs Wavelength
24nm Si3N4 / 44nm SiO2 / 29nm Si3N4 / 42nm Al2O3/ 300nm Ag / Substr
95
95.5
96
96.5
97
97.5
98
98.5
99
99.5
100
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
ecta
nce (
%)
0 deg
2 deg - p
2 deg - s
4 deg - p
4 deg - s
6 deg - p
6 deg - s
8 deg - p
8 deg - s
10 deg - p
10 deg - s
12 deg - p
12 deg - s
14 deg - p
14 deg - s
s (blue curves) and p (red curves) Reflectances vs Wavelength
19nm Si3N4 / 39nm SiO2 / 15nm Si3N4/ 300nm Ag / Substrate
95
95.5
96
96.5
97
97.5
98
98.5
99
99.5
100
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
ecta
nce (
%)
0 deg
2 deg - p
2 deg - s
4 deg - p
4 deg - s
6 deg - p
6 deg - s
8 deg - p
8 deg - s
10 deg - p
10 deg - s
12 deg - p
12 deg - s
14 deg - p
14 deg - s
Delta phase vs wavelength for AOI 2 to 14 deg
19nm Si3N4 / 39nm SiO2 / 15nm Si3N4/ 300nm Ag / Substrate
177
178
179
180
181
182
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
Delt
a P
hase (
p-s
) (d
eg
)
2 deg
4 deg
6 deg
8 deg
10 deg
12 deg
14 deg
Delta phase vs wavelength for AOI 2 to 14 deg
24nm Si3N4 / 44nm SiO2 / 29nm Si3N4/ 42nm Al2O3/ 300nm Ag / Subst
177
178
179
180
181
182
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Refl
Delt
a P
hase (
p-s
) (d
eg
)
2 deg
4 deg
6 deg
8 deg
10 deg
12 deg
14 deg
Jet Propulsion LaboratoryCalifornia Institute of TechnologyA Case Study for TPF coronagraph telescope architecture
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Deep UV to NIR space telescopes and exoplanet coronagraphs: a trade study on throughput, polarization, mirror coating options and requirementsKunjithapatham Balasubramanian, Stuart Shaklan, Amir Give’on, Eric Cady and Luis MarchenJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena, CA 91109Techniques and Instrumentation for Detection of Exoplanets V, edited by Stuart Shaklan,Proc. of SPIE Vol. 8151 (2011)
Figure 1. Telescope and instrument layout
Jet Propulsion LaboratoryCalifornia Institute of TechnologyA Case Study cont’d……
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Deep UV to NIR space telescopes and exoplanet coronagraphs: a trade study on throughput, polarization, mirror coating options and requirementsKunjithapatham Balasubramanian, Stuart Shaklan, Amir Give’on, Eric Cady and Luis MarchenJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena, CA 91109Techniques and Instrumentation for Detection of Exoplanets V, edited by Stuart Shaklan,Proc. of SPIE Vol. 8151 (2011)
Figure 13. Case C. LiF/MgF2 protected Al mirrors per tables 3 and 4 in the reference below
Jet Propulsion LaboratoryCalifornia Institute of TechnologyA Case Study cont’d……
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Deep UV to NIR space telescopes and exoplanet coronagraphs: a trade study on throughput, polarization, mirror coating options and requirementsKunjithapatham Balasubramanian, Stuart Shaklan, Amir Give’on, Eric Cady and Luis MarchenJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena, CA 91109Techniques and Instrumentation for Detection of Exoplanets V, edited by Stuart Shaklan,Proc. of SPIE Vol. 8151 (2011)
Figure 14. Case D. AlF3/LaF3 protected Al mirrors per tables 3 and 4 in the reference below