Modern Optical Metrology Techniques used in James Webb Space Telescope
Segment Optics and Space Instruments
Agossa Segla1, Edwin Olaya1, Viviana Vladutescu1
Phillip Coulter2, Theo Hadjimichael2, Raymond Ohl2
1 Department
of Electrical and Telecommunications Engineering, New York City College of Technology
2Instrument Systems and Technology, Optics Division, NASA Goddard Space Flight Center
Abstract
Deep space studies have been of high interest for wide range of space
investigations. James Webb Space Telescope (JWST), the follower of Hubble,
will be launched in 2018 with the scope of exploring the universe and
improving our understanding of processes such as the assembly and evolution
of galaxies, star birth, first light, reionization, and the formation of planets
orbiting other stars as well as objects in our solar system. The JWST’s
telescope collects light from distant stars and galaxies, has sensors that convert
that light into digital images and spectra. The presented work involves noncontact measurements of mechanical and optical system components on JWST.
In general, the calibration of the uncalibrated laser unequal path interferometer
(LUPI) was performed successfully and also the LR measured the surface of
the gold concave mirror with a high accuracy.
Theoretical background
I  I 1  I 2  2 I 1 * I 2 * cos(  1   2 )
1

*c
T = time, c=speed of light
2
 Laser Radar performs non-contact measurements to points and surfaces or
scans features. It can rapidly sample metal, plastic and composite as-built
surfaces and compare them directly to a CAD model. It uses laser beams to scan
and process the signal echoed from targets, to create a virtual picture of the area.
with interferometer SN 913
with interferometer SN 915
Flat Mirror
S=10x
5
Surface
4
2
Fig. 3. 1 MIC-1 Interferometer (LUPI) with alignment aperture screen (5)
Procedure for LUPI calibration
•Adjust the reference mirror (using tip-tilt adjustment screws) to super-impose the return
disc of light on the alignment aperture (1).
•Adjust Horizontal, vertical, and focus lead screws for coincidence of diverger focal
point of the optics under test (2,3,4).
•Adjust lead screws while observing return circular disc of light on alignment
aperture/screen (3, 4).
•Block reference beam. Focus projection lens for best edge sharpness of optics under test.
•While observing fringe pattern, slide contrast tuning assembly (C. T. A. ) in either
direction until a “strong “pattern is seen.
•Compare measurement result to an independent, known value
 47 . 46 nm
Concave
mirror
10 x
f 
1
R
F#
Fig. 7. 1 Scanned mirror,
2 SA instruments scanning the mirror

312 . 69
2
2
 156 . 345 mm
2
f
D
Error
MIC 1
Interferometer
S/N 915


156 . 345 mm

156 . 345 mm
3"* 2 . 54 cm
2 . 051  2
 2 . 051
76 . 2 mm
* 100  2 . 55 %
2
R = radius of curvature of the mirror
D = diameter of the mirror
F = focal length of the mirror
F# = f number
Spherical mirror
Conclusion
TB#3
TB#2
TB#1
maxima
 The Laser Radar system operates using a sensor to direct a focused
invisible infrared laser beam to a point and coherently process the reflected
light. As the laser light travels to and from the target, it also travels through
a reference path of calibrated optical fiber in an environmentally controlled
module. The two paths are combined to determine the absolute range to the
point. The laser Radar makes precise measurement in a millisecond. The
distance measurement is then combined with the positions of the two
precision encoders to determine a point on a surface in space.
1 .5 x
    
 

 
 2  S 
MIC 1
Interferometer
S/N 913
Laser beam
Fig. 1. Schematic of MIC-1 Interferometer
*
Height
Circular
flat mirror
 MIRROR SCAN
Laser radar
Where:
θ is the angle of the beam with the normal to the mirror surface,
d is the distance between the reference and measured or movable mirror,
m is the fringe order
λ is the wavelength
Error
632 . 82 nm
 2 
  
2
  
minima
Square flat
mirror
Fig. 6. Interferogram comparison from Interferometer s S/N 913 and
S/N915 (qualitative comparison presented here)
Since we are using the ratio of Δ/S absolute values are not necessary
Procedure for Mirror scan
1

2 d cos    m   
2

Fig. 2. Destructive and constructive interference requirements
2. Less Tilt with better
alignment
1. High Tilt
phase difference
2 d cos    m  1 
Concave
Mirror
Δ=1.5x
Methods and instruments
2
Serial #915
Surface error calculation
Circular flat mirror errors measured
3
Two-beam interference fringes

R 
T
MIC- 1 Interferograms
 LUPI CALIBRATION
1
 Laser interferometers offer the ultimate in high accuracy, repeatable and
traceable measurement, using externally mounted optical components. An
interferometer which uses a laser as a light source; because of the
monochromaticity and high intrinsic brilliance of laser light, can operate with
path differences in the interfering beams of hundreds of meters, in contrast to a
maximum of about 20 centimeters (8 inches) for classical interferometers. For
this project we used the MIC-1 Laser Interferometer, it is a Twyman-Green
interferometer.
interfering beams
Results
Theoretical background cont’d
Reference points
Fig. 4. Set up for LR measurement
Fig. 5. Tip-tilting the spherical
mirror to get a higher return power
1. Set up the optics and place the Tooling Balls (TB) in the proximity of the mirror (the actual
tooling balls were 0.5 inch in size and were placed on the stand of the mirror so we could change
the position of the mirror instead of changing the position of the LR points to a shape (spherical)
2. Turn on the LR and Add instrument Metris Laser RADAR into SA
3. In SA connect to the LR
4. Measure TB positions
5. Scan spherical gold mirror to determine size and edges
6. Change LR position and repeat Step 5
7. Fit the scanned points from the different positions into the same XYZ coordinate system
8. Fit the mirror scanned
The results showed in the first project showed a surface error height uncertainty of 47.46 nm
which is the accuracy of our calibration of the LUPI. In the second project, F# of 2.051 was
deducted from the radius obtained from SA after fitting the measured gold spherical mirror
with the SA's CAD model. This measurement gave an acceptable error of 0.5% when
compared to the actual F# of 2 of the mirror. The calibration of the LUPI was performed
successfully and the LR measured the surface of the gold concave mirror with a high accuracy.
References and Acknowledgments
 James
H. Burge, “Use of a commercial laser tracker for optical alignment”, Proc. of SPIE , vol.
6676, doi: 10.1117/12.736705, 2007
Metris Laser Radar (http:www.nikonmetrology.com)
Buccini Instrument Company, “ MIC-1 Laser Interferometer Users Guide for S/N# 915”
James C. Wyant, “Introduction to Interferometric Optical Testing,”
(www.optics.arizona.edu/jcwyant)
http://www.kinematics.com
 I would like to gratefully acknowledge the enthusiastic supervision of Dr. Raymond Ohl, Phillip
Coulter, and Theo Hadjimichael during this work. I would like to show my greatest appreciation to
Prof. Viviana Vladutescu. I can't say thank you enough for her tremendous support and help. My
thanks and appreciations also go to NASA- CIPAIR in developing this project.