Integrated On-Machine Metrology Systems
Christopher W. King
UCL, Optic Technium, St Asaph, N. Wales, UK
Abstract – Producing On-Machine Metrology systems can present many benefits for the production of opticalcomponents - particularly where complex and delicate parts are concerned. To fulfil the requirements for onmachine metrology a suite of tools would be necessary to characterize manufactured parts at all spatial scalesof interest. This extended abstract details a development to address the on-machine metrology problem,
particularly in the optics industry, but applicable to other sectors of precision engineering.

Introduction

ELT Segment Example

Great advances have been made in recent years in the stateof the art in ultra-precision polishing machines, so that
deterministic computer controlled material removal ispossible on almost any geometry, provided that adequate
metrology exists. Our motivation for this project is to open
new ground for the production of ultra-precision surfaces
be enabling on-machine metrology. Such metrology is by
no means new, but for the traditional optical shop, onmachine metrology is new becoming possible thanks to theapplication of science and computer control and dataacquisition.

We take the example of a typical segment for an Extremely
Large Telescope (ELT)[1]. The segments are usuallyaspheric, entirely off-axis and have a very long base radiusof curvature (30 - 90 m). The low order terms (say, 0.5 m-1- 200 m-1) could be measured accurately by interferometry,
but this would not give an assessment of the radius of theoptic leaving a gap in the PSD description of the surface.
Furthermore, there may be a small gap between the form
interferometry and white light interferometry at certain
spatial frequencies. A sample plot showing approximate
coverage in spatial frequency terms is shown in Figure 1.

Typically optical shops use phase-shifting interferometry
for optics form measurement. To this we can add whitelight interferometry for texture measurement, and perhaps
profilometry for lower accuracy form work. These tools
normally reside in a metrology room away from thepolishing work. However, transferring parts between
polishing machine and the metrology instrument canbecome a risky process where large parts are concerned.
We can also add to this the inconvenience of re-aligning
the part with the polishing machine's coordinate frame.
We have been working on a series of projects to allow
metrology to take place on the part processing machines.
The work includes form interferometry, texture
measurement and other ancillary measurements to verify,
for example, radius of curvature.
Optical Surface Characterization
In this work, we have concentrated on form and texture
measurements for optics. Form terms are measured byphase-shifting interferometry at 633 nm, or profiling-type
instruments to calculate low-order terms. Scanning whitelight interference microscopes provide texture information
either directly on the surface under test, or on a replica of asample area of the surface. For ultra-precision work, forexample, X-ray mandrels and mirrors for large segmented
telescopes, the specifications are very high. This meansthat nowadays it is becoming a requirement to provide acomplete coverage in frequency for the power spectral
density of a surface with no gaps.

Figure 1. Approximate diagram of instrument coverage forcommon instruments used in optics characterization.
I=interferometry, SI=stitching interferometry, WLI=white lightinterferometry, SPM=scanning probe microscopy.

For ELT-like segments, the characterization could involve
profilometry for form and radius measurement,interferometry as the main form metrology, and then whitelight interference microscopy to measure the surface finish.
Typically, the tests for ELT segments will be very
elaborate in order to meet the demanding specifications forform error, aperture coverage and surface finish.
Collaborative work at the Optic Technium in N. Wales isusing all the tools mentioned in Figure 1 except for SPM
for ELT segments, with the advantage that the profiler data
can cross-check the form data from the interferometer.
On Machine Measurements
The desire to measure parts on-machine imposes someconstraints on the types of equipment that can be used. In
general, the polishing machine will be located on a lab orworkshop floor with no isolation from vibrations from thePage 1 of 4

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underlying buildings. Furthermore, many machines will
have some active components generating unwanted
vibrations, even when not processing parts. The enabling
technology for full-aperture 3D form measurements onmachine has been vibration insensitive phase shifting
interferometry, often allowing measurements to be made inquite harsh environments. In recent years it has become
possible to buy different types of these interferometers(Twyman-Green and Fizeau for example) from different
manufacturers. A similar argument applies to measurementof surface finish. Typically, smaller parts are placed on themicroscope table and direct measurements are made. This
is not possible with larger optics where surface replicationis commonly used. However, even vibration insensitive
white light interferometry is becoming available. Finemechanical adjustment of such instruments is therefore
important and will be addressed later.

able to be fitted with different CGH elements for a wide
range of optical surface prescriptions
Optical Interferometer and Test Tower
To ensure the maximum flexibility a 6" Fizeau
interferometer was chosen such that 4" or 6" transmission
elements could be used. The system is a 4D FizCam 3000
unit that provides the vibration insensitivity necessary forthe application.
The test tower comprises a stiff steel structure with anupper platform and access ladder enabling the user toadjust the interferometer transmission lens and CGH
element together. Computer and alignment displays arelocated at the upper platform to allow this to be performed
by a single person. In previous work we have used anisolated gantry adjacent to the test tower to control theinterferometer, but in this work, once the transmission
sphere and any CGH are set, all other adjustments arecarried out from the ground level.
5-Axis Stage
The accurate positioning of the interferometer is achievedby a motorized 5-axis stage. The optical axis of theinterferometer is generally coincident with the rotary axisof the polishing machine, and the stage is able to move in x,y, z, tilt-x and tilt-y. With experience gained in building amanual prototype stage we designed and built a CNC 5axis stage to position the interferometer accurately with thespecifications shown in Table 1.
Table 1. Specifications of the 5-axis CNC stage.

Figure 2. Zeeko IRP 1000 machine, 5-axis motorized stage and6" Fizeau interferometer.

Other factors are also important like thermal time constants.
For easy measurements on-machine, either good thermal
control is required, or parts with short thermal time
constants, or materials with very low linear expansion
coefficients. For the ELT example, the parts are generally
low expansion glasses and testing can be carried out atnear 20 °C.
An Integrated Development
We have developed an integrated solution for polishingand measuring moderately large concave optics up to 1 min diameter. The system is based around a Zeeko IRP 1000
polishing machine and a 5-axis motorized stage housing a6" Fizeau interferometer. A CAD model of the set-up isshown in Figure 2. The work was based around a prototype
test tower placed above an IRP 1200 machine at the UCL
lab at Optic Technium in N. Wales[2]. A new setup was
designed able cope with a wider range of base radii ofcurvature from approximately 1.1 - 2.4 m, and also to beAxis
XYZ
A (tilt-y)
B (tilt-x)

Range± 100 mm
± 100 mm-1440 mm± 1 deg± 1 deg

Resolution
1 um
1 um
1 um
0.135 arc sec
0.135 arc sec

The z-axis travel allows measurement of surfaces withradii of curvature between 1.1 and 2.4 m. Figure 3 shows aCAD model of the stage and interferometer.
Control of the stage is via a handheld controller, with adigital readout of the axis positions, and the system isdesigned primarily for testing rotationally symmetricspheres and aspheres, but can be arranged to measure othergeometries depending on the exact surface specification.
The stage is also equipped with a CGH holder mounted ona sliding rail system with fine focus to allow different
CGH elements to be used with different transmission
optics as shown in Figure 4.

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Figure 3. CAD model of the 5-axis stage system with the Fizeauinterferometer. The long vertical structure houses the z-axis
leadscrew and servomotor.

Figure 5. The complete system of polishing machine, test tower
and interferometer.

Figure 6. Left: Part received polished spherical to 5 PV and 1
RMS. Right, 1st aspherization run showing targeted 3 departurefrom best-fit sphere.

Figure 4. CGH holder, CGH and interferometer optics.

Complete System
The full system is shown in Figure 5 where theinterferometer has been moved down to a testing position
inside the enclosure of the IRP 1000 polishing machine.
Initial Work: a half-metre class asphere
Initial work was to polish an f/3.7 convex hyperboloid of
450 mm diameter with just under 30 um of asphericdeparture. We took delivery of a Zerodur part polished tobest-fit radius.

Using the on-machine metrology with just an f/3.3transmission sphere we improved the form toapproximately 1 wave spherical before starting to polishin the asphere. After assessing that the removal rates werepredictable, we targeted to push the form out to the asphere
over several polishing runs in order that it would beimageable using the f/3.3 and CGH. Figure 7 showsphasemaps of the part measured by the system using theCGH until 85% of the surface by clear aperture was in
specification (80 nm PV and 16 nm RMS). We willcontinue processing the part edges until we are out to100% of the clear aperture at 450 mm dia.

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along a diameter whilst measurements were made with thelaser-tracker. After acquisition and SMR ball radiuscompensation we determined the base radius by aspherefitting.
Completing the measurement
As a final on-machine measurement we measured thesurface roughness using an on-machine white lightinterferometer. The Zeeko STA1 is an instantaneous whitelight interferometer from 4D technology designed tooperate on a machine platform, and especially a Zeeko
IRP-type machine. The prototype unit with a x10 objective
measured the surface of the asphere on-machine andestablished a surface roughness of 1 nm RMS, Figure 9.

Figure 7. Convergence of the surface. Left-to-right, top-tobottom. Part after polishing run 23, PV=23. After run 25,
PV=4.7. After run 29, PV = 2.2. After run 34. PVq = 79 nm,
RMS = 14 nm. Final plot is shaded to highlight slopes.

The polishing and measurement results show that thesystem can be used to converge aspheric surface forms atthe /8 level. Previous results on repeatability show thatthe system should be adequate for at least /10[3].
Radius Term
The CGH test of the f/3.7 part can only give the correct
radius of curvature if the distance between the asphere andthe CGH is well controlled to a few 10s of um. In order tocross check the radius measurement, a laser tracker wasused[4] along with a Sphere Mounted Retro-reflector(SMR) that was mounted in custom tooling into thepolishing machine's tool chuck, Figure 8.

Figure 9. Left, texture measurement, RMS is approx. 1 nm. Right,
STA1 and mirror during measurement. Note, some fringe print
through is visible in this plot which is an average of 64
measurements.

Currently the instrument is controlled and mounted
manually, but we will extend the functionality of thesystem by including automatic probing and focusing
routines such that a grid of sampling points can be chosenon the surface and the points measured automatically.
Conclusion
We have described a new integrated polishing andmetrology system designed to process aspheric surfaces up
to 1 m in diameter and how we have used this system todetermine base-radius of curvature, form error and surface
texture for an aspheric part. We will continue develop thetechnology and to process the mirror to polish out to fullclear aperture and will report on the work in greater detailin the future.
References

Figure 8. Measuring the base-radius using a laser tracker andSMR. The IRP machine is used to position the SMR on points
along a diameter gently, and without applying any force to thesurface under test. Only the mass of the SMR and the central
shaft is resting on the mirror.

The tool-holder was designed to support the weight of theSMR, but to allow slack when the SMR was placed incontact with the optical surface to avoid damage. The IRP
machine was used to position the SMR at several points[1] Spiromillio, J. et al., “Progress on the European Extremely
Large Telescope” ESO Messenger No. 133., Sept. 2008.
[2] Walker, D., Yu, G., “ELT Segment Process DevelopmentReport”, OpTIC Technium ESO Consortium Report, April
2009.
[3] King, C., et al., “On-Machine Metrology for Aspheric
Optics Testing” Proceedings of ASPE Summer TopicalMeeting, June, 2010.
[4] Zobrist, T. “Application of Laser Tracker Technology for
Measuring Optical Surfaces” PhD Thesis, University ofArizona, 2009.

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