Silicon mirrors for UV-optical
space telescopes
Instrument Technology
Center Code 550
David Content, NASA GSFC
•
Abstract
– Several forms of silicon lightweight mirrors are in development.
As compared to SiC, silicon is much easier to polish with nearly
equal mechanical & thermal properties allowing much more
aggressive lightweighting than glass or metal materials. Recent
developments are discussed
– Other candidate materials (gr/Ep composite, Ni, Al) are
mentioned; other talks cover glass and SiC
Outline
Instrument Technology
Center Code 550
– Materials & fabrication discussion
• What is the ideal mirror fabrication solution for large precision
telescopes?
• Brief discussion of alternatives beyond SiC and glass
• Why silicon?
– Foam-core Si [slides thanks to B. Goodman @ Schafer]
– Single crystal Si
• McCarter Engineering
• GSFC in-house work (V. Bly)
What is the ideal solution for lightweight
precision UV/optical mirrors?
Instrument Technology
Center Code 550
•
Current attempts (e.g. JWST etc.) to make large aperture lightweight
mirrors have
– Been moving towards technical success in demonstrating
lightweighting (e.g. real values of areal density ≤ 25 kg/m2
– Have failed in their promise of dramatic cuts in the cost metric
$M/m2 (e.g. JWST promised <1, now ~ 4)
– In general, costs are heavy in lightweighting and in polishing
•
Ideal solution would involve replication
– Separates polishing and lightweighting (to some extent)
– Re-use of mandrel saves cost
– However no replicated solution is yet nearly precise enough
What is the ideal solution for lightweight
precision UV/optical mirrors?
Instrument Technology
Center Code 550
•
Next step back is rapidly formed substrates, e.g. casting, foam body
work, or new technologies (spray-on substrate)
– Castable or foamcore mirror blanks still require polishing but save
on lightweighting costs
– I don’t think this will be affordable for ~10m or up (space, filled
aperture) telescopes
– But this is where the state of the art is today
•
Back to discussion of materials for polishable UV telescopes:
Instrument Technology
Center Code 550
Materials comparison for candidate
lightweight mirrors
Figures of merit
Basic properties
Property
Table 1: lightweight materials properties
desired Al
ULE
units
value 6061 Si
3
density, ρ
kg/m
Young's Modulus, E
GPa
high
CTE, α
1.E-6/K
Thermal conductivity, k
W/m.K
Thermal Diffusivity, D
1.E-6m /s
small
2
Be I-70 SiC [CVD
Zerodur H
e- Ni
α]
gr/Ep
2700
2330
2210
2530
1850
3210
8908
1780
68
131
67
91
287
465
200
93
low
22.5
2.6
0.03
0.05
11.3
2.4
11
0.02
high
167
148
1.31
1.64
216
198
7
35
high
69
94.3
0.78
0.77
57.2
84.2
Ability to be diamond turned
high
high
high
low
low
low
low
high
low
Difficulty of superpolishing
low
medium
low
low
low
high
high
low
high
Self Deflection, ρ/E
low
39.7
17.8
33.0
27.8
6.4
6.9
44.5
19.1
low
0.135
0.018
0.023
0.030
0.052
0.012
1.57
0.001
low
0.326
0.028
0.038
0.065
0.198
0.029
Steady state Thermal, α/k
um/W
Transient thermal, α/D
sec/m .K
Cost of finished optic
2
$
$$
$$
$$
$$$
$$$
$$
$$
Most critical for lightweight UV/optical mirrors: self deflection, polishability, cost
Omissions –C/SiC – see following talk (R. Keski-Kuha)
Instrument Technology
Center Code 550
Same data – graph format
materials comparison (low is better)
steady state thermal, alpha/k .
Å Increasing thermal stability
10.0000
e- Ni
1.0000
Al 6061
0.1000
Be I-70 H
SiC [CVD α]
0.0100
0.0010
(Ideal material here)
x
Zerodur ULE
Si
gr/Ep
0.0001
0
10
20
30
40
Self Deflection, rho/E
Å Increasing stiffness per unit weight
50
Al, Be, gr/Ep, Ni, replicated glass, etc
Instrument Technology
Center Code 550
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Al – recently developed bare Al polishing process (GSFC) to ~10Å
microroughness, with BSDF (angular scattering) similar to glass
– Works on standard alloys, e.g. 6061
– However, large CTE and relatively low modulus make this unlikely
for large apertures
– Probably still quite cost-effective for some applications, as can be
used with (cheap) Al structures
• Readily diamond turnable and polishable (to ~1m panels)
– Is amenable to foam-core mirror blank construction also
•
Be – various attempt to polish have not succeeded to the point where
bare Be is usable; Ni-plated Be can be polished, but bi-metallic stress
may cause thermal problems for some applications
– Not proven for UV-optical
– Very expensive and long lead time (longest of any material)
Al, Be, gr/Ep, Ni, replicated glass
Instrument Technology
Center Code 550
•
Gr/Ep
– Successful on large apertures (e.g. MAP microwave reflectors)
– Print-through of fibers is a major issue for UV-optical applications
•
Ni
– Relatively easy to polish, but very heavy (8908 kg/m3)
– Relatively low modulus and thermal conductivity
– Must be used as a very thin electroplate, with bimetallic issues
•
Replicated or formed glass
– Being developed for both soft and hard x-ray grazing incidence
telescopes for Constellation-X – Dr. W. Zhang (GSFC) et al.
– Incredibly low areal densities (<1 kg/m2)
– Significant figure precision advancement needed for UV/optical
applicability – currently at few arcsec slope error level
– Achieved microroughness already suitable for uv/optical
What is SLMSTM?
•
•
Instrument Technology
Center Code 550
SLMSTM have a silicon foam core (85-95% porosity) enclosed by a
continuous CVD polycrystalline silicon shell (like an M&M)
CVD Silicon can be deposited to 2 inch thickness at 1 meter diameter
Silicon
Foam
Continuous pores
65-100 pores per inch
Polycrystalline Silicon Closeout
(0.01-0.05” typical)
Foam core can be CNC machined
to virtually any shape to ± 0.002 inch
Foam vs Web Structures For
Lightweighting
Roles/Requirements
Support against polishing
pressures
Foam
stiffness/high modes
1-g sag (proportional to
Webs
Fully distributed load paths
Concentrated load paths to print-
under mirror surface, easier
through as lines, difficult metrology
metrology mount
Dynamics/stability/
Instrument Technology
Center Code 550
Higher stiffness, first mode
frequency
Pockets = 10 microns
mount
More mass for same stiffness, first
modes
Pockets = 100,000 microns
pocket width)
Micrometeoroid protection
Natural bumper material and
Little or none
ripstop
Reliability/ Redundancy
Many alternate load paths graceful crush fail
Structural failure effect greater catastrophic fail
Instrument Technology
Center Code 550
SLMSTM Manufacturing Process
•
Current capability is φ 30 cm horizontal (30 cm x 45 cm for flats in
vertical)
•
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•
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60 cm facility on-line September 2003
Mirror manufacturing time 7-12 weeks for flats and spheres
Polishing times for aspheres add 3-6 months
Metal or dielectric coatings are readily applied
Silicon Foam
4 – 6 weeks
Polycrystalline Silicon Close-out
2 – 4 weeks
HEL Coatings for Operation at
HF, DF and 1.315 µm
Polished CVD Silicon
1 – 2 weeks
Instrument Technology
Center Code 550
SLMSTM Have High Structural Efficiency
Mirror Comparison
Weight Areal Density
kg
5 inch diameter x 0.5 inch Single Crystal Silicon
5 inch diameter x 0.5 inch thick SLMS
5 inch diameter x 0.69 inch Single Crystal Silicon
•
•
0.382
0.125
0.515
kg/m
30
9.88
40.7
2
st
1 Mode
hertz
5508.0
7625.0
7625.0
SLMSTM is 1/3 weight of same size Single Crystal Silicon mirror, and has
~30% higher first frequency
Equivalent stiffness Single Crystal Silicon Mirror is 4X heavier than a
SLMSTM and ~30% thicker
1st Mode with Tangent Mount
is 5047 Hz
1st Mode with Kinematic Mount
is 1801 Hz
Instrument Technology
Center Code 550
SLMSTM Technology Maturity
10 cm
Single Segment
Manufacture
NASA,
AFRL
Phase I
25 cm
IR& D
LASIT
25 cm
Active
50 cm
System
House
NASA
Phase II
Multi-Segment
Manufacture
Stiffness
= Demonstrated
AFRL
Phase II
By
Analysis
ISO 9000
Process
By
Analysis
System
House
NASA
Phase II
System
House
System
House
System
House
IR&D
LASIT,
et.al.
System
House
NASA
Phase II
Polishability
NASA
Phase I
VLA Coating
AFRL
System
House
High Power
Test
AFRL
System
House
1m+
AFRL
Phase III
Relay
Mirrors
= 1-2 year funded
= In Discussion or
Proposed
1.5 m Approach - 56cm Hexes
- 3 Center Hexes + Wedges
Instrument Technology
Center Code 550
Single crystal Silicon lightweight
mirrors – McCarter Engineering
•
SCSi is unique material – all advantages of Si
previously discussed, but crystal form – no
internal stress
– Relatively high materials cost
•
McCarter approach is to machine pieces of
sandwich assembly and frit bond to assemble stiff
mirror blank; post-assembly polishing (TRW)
– Shown to be cryostable (0.1λ rms ∆ RT-LN2)
– Amenable to all Si chemical processing from
lithography
• MRF (magnetorheological finishing) highly
effective in figuring & polishing SCSi
•
12.5cm sphere delivered to GSFC
– 19kg/m2, 34nm rms figure error, 6Å roughness
•
Overall – requires expensive materials, assembly,
and post polishing
Instrument Technology
Center Code 550
Single crystal Silicon lightweight
mirrors – V. Bly approach
•
•
•
•
•
Same material – bulk SCSi – as McCarter
New features:
– 1. lightweight AFTER polishing
• Lack of stress means no warpage
as would happen with other
materials
– 2. rapid lightweighting process
(proprietary)
This separates polishing from
lightweighting. Often the polishing is the
harshest environment the lightweighted
blank sees (before launch). This allows
higher lightweighting
10cm flat – 9 kg/m2, 16nm rms figure
Now working on spheres to 30cm for Earth
Science instrument testbed
Instrument Technology
Center Code 550
Summary
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Instrument Technology
Center Code 550
Si is a near-equivalent to SiC, but is more readily polishable
Multiple mirror fabrication paths exist
These include edge-bonding to tile larger apertures than can be made
using available Si crystal boules or Si foam blanks
All types are polishable to precision specifications
– Several have been demonstrated to handle high laser power,
which is a similar requirement to superpolishing for UV
applications
Thanks to V. Bly, B. Goodman, D. McCarter, W. Zhang for material included here