Modern Optical Metrology Techniques used in James Webb Space TelescopeSegment Optics and Space Instruments

Modern Optical Metrology Techniques used in James Webb Space TelescopeSegment Optics and Space Instruments

1 Department of Electrical and Telecommunications Engineering, New York City College of Technology2Instrument Systems and Technology, Optics Division, NASA Goddard Space Flight Center

Agossa Segla1, Edwin Olaya1, Viviana Vladutescu1

Phillip Coulter2, Theo Hadjimichael2, Raymond Ohl2

AbstractAbstract

Theoretical backgroundTheoretical background

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 non-contact 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.

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.

Two-beam interference fringes

)cos(**2 212121 IIIII

2

21

minima 2

1cos2

maxima1cos2

md

md

interfering beams phase difference

Fig. 1. Schematic of MIC-1 Interferometer

Fig. 2. Destructive and constructive interference requirements

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

Theoretical background cont’dTheoretical background cont’d

Methods and instruments Methods and instruments

1

2

3

4

5

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

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 SA3. In SA connect to the LR 4. Measure TB positions 5. Scan spherical gold mirror to determine size and edges6. Change LR position and repeat Step 57. Fit the scanned points from the different positions into the same XYZ coordinate system8. Fit the mirror scanned

Results Results

Square flat mirror

Circular flat mirror

Concave mirror

MIC 1Interferometer S/N 913

MIC 1Interferometer S/N 915

MIC- 1 InterferogramsSerial #915

Concave Mirror

Flat Mirror

1. High Tilt 2. Less Tilt with better alignment

LUPI CALIBRATION

Fig. 6. Interferogram comparison from Interferometer s S/N 913 and S/N915 (qualitative comparison presented here)

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.

Procedure for Mirror scan

Fig. 5. Tip-tilting the spherical mirror to get a higher return power

TB#1

TB#2TB#3

Reference points

Laser radar

Spherical mirror

Laser beam

Fig. 4. Set up for LR measurement

S=10x

Since we are using the ratio of Δ/S absolute values are not necessary

Δ=1.5x

nmx

xnm

SHeightErrorSurface

46.4710

5.1*

2

82.632

2

Surface error calculationCircular flat mirror errors measured with interferometer SN 913

with interferometer SN 915

%55.2100*2

2051.2

051.22.76

345.156

54.2"*3

345.156#

345.1562

69.312

2

Error

mm

mm

cm

mm

D

fF

mmR

f

Fig. 7. 1 Scanned mirror, 2 SA instruments scanning the mirror

R = radius of curvature of the mirrorD = diameter of the mirrorF = focal length of the mirrorF# = f number

MIRROR SCAN

Conclusion Conclusion 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 AcknowledgmentsReferences and Acknowledgments James H. Burge, “Use of a commercial laser tracker for optical alignment”, Proc. of SPIE , vol.

6676, doi: 10.1117/12.736705, 2007Metris 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.

12

cT

R *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.

T = time, c=speed of light