On machine Measurement for Precision Corrective polishing of Aspheres and Freeform Surfaces

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Nathaniel Lawson
5 years ago
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1 On machine Measurement for Precision Corrective polishing of Aspheres and Freeform Surfaces David Walker, Christopher King University College London Zeeko Ltd & Zeeko Research Ltd Based at the OpTIC Technium, North Wales 1

2 Sub title An update on an on going product development Acknowledgements Funding from PPARC (now STFC) PIPSS scheme Collaboration between UCL, Zeeko Ltd and the OpTIC Technium 2

3 Areas of interest include Optical lenses and mirrors Head up displays Defence and space optics Telescope and instrumentation optics Hip and knee joint implants Turbine blades Moulds and dies Mandrels for X ray mirrors But, we ll address larger components for now! 3

4 The polishing technology Range of CNC polishing machines Zeeko Classic inflated bonnet tools Fluid jet polishing Polish rough surfaces Rectify measured form errors Finish Measure tool removal function ( influence function ) Numerical optimisation of process Measure surface form-error Run machine Iterate 4

5 Aspheres & free form surfaces Measurement dynamic range Need for null ( corrective ) optics The Hubble Space telescope problem! For large parts Risk of damage in handling Substantial down time measuring off machine, moving the component re adjusting at test setup and back on machine thermal stabilization 5

6 6

7 A case study European Extremely Large Telescope project 42m optical telescope with 960 hexagonal segments Each segment: 1.4m off axis asphere Total requirement of 1,148 segments (inc. spares) Contract at OpTIC Technium to produce seven prototype segments using Zeeko technology Particular issue of measuring segment edges Role of on machine metrology 7

8 On machine metrology Demountable interferometer module to fit tool chuck of the larger Zeeko machines Overlapping sub aperture measurements Sub apertures cover pupil optimally Sub apertures are stitched in software Residuals in overlap regions => estimate of uncertainty of stitched results 8

9 Advantages Increased dynamic range for on and off axis aspheres Close up test reduces effects of air turbulence Increased spatial resolution (more pixels) Handle large parts Handle steeply convex parts Possible application to convex/concave parts (e.g. saddles) 9

10 Key issues Size envelope of module Environmental conditions Vibration: ambient, machine servos Heat load from machine and process Effectiveness of stitching software 10

11 Test rig for software development Stepper motor rolling element slide ways Old Zygo PTI interferometer with Vidicon Vibration isolated bench 11

general case, stitching will not work well without

12 Stitching Individual phase maps require correction for geometric distortion Distortion in optical system In the general case, stitching will not work well without distortion correction, especially on strongly curved surfaces. 12

13 Effect of distortion 13

14 Stitching Stitching then proceeds by optimization of the residuals in the overlap areas. The stitching error is estimated from residual noise, merit function and analysing data in the sub aperture overlap areas. 14

15 Example stitching a convex part 15

16 First results 1. Full aperture measurement 16

17 2. Raw sub aperture measurements 17

18 3. Comparison 18

19 4. Difference full-aperture & stitched PV=92 nm RMS 11 nm Much of the residual error is systematic. Calibration is predicted to leave a few nm residual. Expect residuals ~ X10 improvement after calibration 19

20 Stitching a part with complex features (140mm diameter) 20

21 Stitched result 21

22 Module design: vibration and thermal Collaboration with 4D Technologies Corp. 4D are developing a compact version of their simultaneous phase shift interferometers Short exposures freeze vibration Interferometer tests in machine environment confirm suitability for metrology Polishing environment is flooded with polishing slurry at constant temperature when in use. 22

23 Module concept in polishing machine X,Y,Z,A,B motions 23

24 Next stages Validate stitching on aspheres Evaluate prototype 4D compact interferometer Integrate optics and interferometer into on machine module Formal consideration of uncertainty of measurement Delivery of first commercial system 24

25 Thank you! 25

Aspheres and freeforms

Aspheres and freeforms Historically, the disadvantages of poor manufacturability and metrology determined the choice of using classical optics for optomechanical instrumentation. Worldwide however, a lot