Eng, Ron

NASA Technical Reports Server

Cryogenic optical testing of space telescope mirrors
Ron Eng
Optics and Imaging Branch
NASA Marshall Space Flight Center
Instrumentation for Astronomical Space Science (IASS) workshop 2018 
https://ntrs.nasa.gov/search.jsp?R=20180007891 2019-08-31T17:14:00+00:00Z
NASA Marshall Space Flight Center
Oct 18-19, 2018 IASS workshop 2018 2
Marshall Space Flight Center
Space Transportation, Propulsion Systems, Space Systems, and Science
Huntsville, Alabama
Oct 18-19, 2018 IASS workshop 2018 3
Space Transportation, Propulsion Systems
Oct 18-19, 2018 IASS workshop 2018 4
Space Systems and Science
Oct 18-19, 2018 IASS workshop 2018 5
X-ray cryogenic facility (XRCF)
Large test chamber:
• 7.3 x 22.9 m (O.D. x L) horizontal 
cylinder
• 6 x 18.3 m (I.D. x L)  test volume
• 4.25 x 9.4 m (I.D. x L) Helium shroud
• < 22.5 m ROC without modification 
• Up to 30 m ROC with modifications
Cryo shroud enclosure: 320º to 20º K
Refrigeration system: 2 gaseous helium 
refrigerators; each capable of ~1 kW at 20K.
Vacuum systems: 10-8 Torr
X-ray source: 527 m guide tube
History
Testing grazing-incidence x-ray telescopes (Chandra, Solar X-ray Imager, Solar B) since 1992.  
Cryogenic optical interferometric testing of normal incidence, visible & IR optics (NMSD, 
AMSD, JWST, AMTD) since 1999.
Oct 18-19, 2018 IASS workshop 2018 6
Chandra X-Ray Observatory
Oct 18-19, 2018 IASS workshop 2018 7
JWST PMSA test configuration
7.3 x 22.9 m vacuum chamber
6 x 18.3 m test volume
5 m dia. gaseous helium-cooled shroud
16 m ROC; ~4 m dia. (JWST 6 PMSA) 
2 closed-loop helium cryogenic refrigeration systems <20 deg. K (2 KW capacity)
Existing structure prevents testing mirrors with ROC < 3.5 meters
A pressure tight enclosure (PTE) configuration to test mirror with short ROC < 3.5 meter
Oct 18-19, 2018 IASS workshop 2018 8
XRCF class 2K clean room
Oct 18-19, 2018 IASS workshop 2018 9
James Webb Space Telescope (JWST)
NASA, ESA, and CSA
2021 launch
0.6 – 30 microns (visible to mid IR)
4 scientific instruments
6.5m primary mirror
L2 orbit, 1,500,000 km
Science objectives: first light, formation of galaxies, birth of stars and planets, and 
origin of life
Technical challenges: deployable segmented telescope and structure, lightweight yet 
stable optics at 40 degrees Kelvin operational temperature. 
Oct 18-19, 2018 IASS workshop 2018 10
James Webb Space Telescope (JWST)
Oct 18-19, 2018 IASS workshop 2018 11
JWST orbit
•~1,500,000 km from earth vs ~650 km for Hubble
•30 to 60 deg. K operational temperature
Oct 18-19, 2018 IASS workshop 2018 12
HST & JWST primary mirror comparison
2.4 m dia.
3.6 m2
6.5 m (18 mirror segments)
25 m2
Oct 18-19, 2018 IASS workshop 2018 13
JWST primary mirror segment at XRCF
Oct 18-19, 2018 IASS workshop 2018 14
JWST mirror optical test instrument
Oct 18-19, 2018 IASS workshop 2018 15
Backplane stability test article (BSTA)
Design (ATK):
Graphite Epoxy composite 
M55J/T300 laminate
consist of tubes (1mm thick) 
and gussets (2.5mm thick) 
Dimensions 
2.8m x 2.5m   (9.5 ft x 8.3 ft)
Mass 
53 kg
Strength
2g, 1.25 S.F. yield
Thermal cycling
320 to 25 deg. K
Oct 18-19, 2018 IASS workshop 2018 16
BSTA delivered to MSFC for cryo testing
Oct 18-19, 2018 IASS workshop 2018 17
JWST mirror backplane test instrument
Simultaneous speckle pattern 
interferometer (4D)
Spectra-Physics Quanta-Ray PRO 
Nd:YAG 290-10, 1000mJ pulsed 
laser @532nm, 9ns pulse, 10Hz 
repetition rate
1000 x 1000 pixels, 9 microns pixel 
resolution
vibration insensitive with patented 
pixelated phase mask technology 
where a set of 4 pixels has 
discrete phase shift over entire 
array 
fast camera shutter speed
designed to measure large diffuse 
objects for change
5nm rms measurement 
repeatability
Oct 18-19, 2018 IASS workshop 2018 18
Simultaneous phase shifting interferometer
Micro-polarizer array camera 
sensor
Spatial phase shifting overcomes 
previous single frame or temporal 
phase shifting interferometer 
technique
Overcomes vibration and air 
turbulence in long optical path test 
setup found in astronomical 
telescope metrology in vacuum 
test chamber
Oct 18-19, 2018 IASS workshop 2018 19
Small chamber for mirror characterization
Oct 18-19, 2018 IASS workshop 2018 20
Cryo test of 12 SiC mirrors (~150 mm dia. each)
Oct 18-19, 2018 IASS workshop 2018 21
Test configuration for < 800 mm dia. mirror
1.5 - 2.5 m ROC
Oct 18-19, 2018 IASS workshop 2018 22
JWST PMSA test configuration
Existing structure prevents testing mirrors with ROC < 3.5 meters
A pressure tight enclosure (PTE) configuration to test mirror with short ROC < 3.5 meter
Oct 18-19, 2018 IASS workshop 2018 23
Test configuration for < 3.5 m radius of curvature mirror 
Oct 18-19, 2018 IASS workshop 2018 24
Test configuration for < 3.5 m radius of curvature mirror 
Oct 18-19, 2018 IASS workshop 2018 25
Test envelop for large and small chambers
Chamber Max Diameter Max radius of curvature
Large 4.25 m 22.5 m
Small 0.8 m 2.5 m
Oct 18-19, 2018 IASS workshop 2018 26
Optical test equipment in pressure tight enclosure (PTE)
Oct 18-19, 2018 IASS workshop 2018 27
Thermal optical test surface figure error
- =
Predicted SFE uses:
• as-built CTE distribution
• as-built shape from X-ray CT
• includes prying (due to aluminum frame) and all possible forces 
reacting between mount and bond pad
Residual SFE could be CTE inhomogeneity
Gravity sag (predicted vs measured)
Oct 18-19, 2018 IASS workshop 2018 28
Predicted 
580 nm rms
Measured
582.5 nm rms
Thermal gradient test
Oct 18-19, 2018 IASS workshop 2018 29
Future test plans
Oct 18-19, 2018 IASS workshop 2018 30
Current test facility modifications 
• Predictive thermal control
• Passive thermal
• Active thermal control
• Low CTE glass-ceramic mirrors
• Low CTE ceramic mirrors
• Low CTE metal mirrors
• Additive manufactured mirrors
Oct 18-19, 2018 IASS workshop 2018 31
Acknowledgments
Phil Stahl: PI
Michael Effinger: program manager
Mark Baker, Bill Hogue, Jeffrey Kegley, Richard Siler, John Tucker, Ernest Wright: XRCF 
thermal-vac test support team
Thomas Brooks: thermal-mechanical analysis
Brent Knight, Frank Tsai: modal analysis
Alex McCool, Russel Parks: modal test
Ron Beshears, Dave Myers: X-ray computed tomography
Darrell Gaddy: thermal IR video
Brian Odom: MSFC historical photos
Oct 18-19, 2018 IASS workshop 2018 32
Thank you
Dr. Jeong-Yeol Han (KASI)
Ron Eng
ron.eng@nasa.gov
https://optics.nasa.gov


Cryogenic Optical Testing of Space Telescope Mirrors

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