Edges in CNC polishing: from mirror-segments towards semiconductors, paper 1: edges on processing...

Edges in CNC polishing: from mirror-segments

towards semiconductors, paper 1: edges on

processing the global surface

David Walker,1,2,3,*

Guoyu Yu,1,2

Hongyu Li,1,4

Wilhelmus Messelink,1,3

Rob Evans,2 and

Anthony Beaucamp3,5

1Department of Physics and Astronomy, University College, Gower St, London, WC1E 6BT, UK

2OpTIC-Glyndŵr, Glyndŵr University, St Asaph Business Park, Fford William Morgan, St Asaph,, North Wales,

LL17 0JD, UK 3Zeeko Ltd, 4 Vulcan Court, Vulcan Way, Coalville, Leicestershire, LE67 3FW, UK

4Research Center for Space Optical Engineering, Harbin Institute of Technology, Harbin, 150001, China 5Dept. of Mechanical Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan

*[email protected]

Abstract: Segment-edges for extremely large telescopes are critical for

observations requiring high contrast and SNR, e.g. detecting exo-planets. In

parallel, industrial requirements for edge-control are emerging in several

applications. This paper reports on a new approach, where edges are

controlled throughout polishing of the entire surface of a part, which has

been pre-machined to its final external dimensions. The method deploys

compliant bonnets delivering influence functions of variable diameter,

complemented by small pitch tools sized to accommodate aspheric mis-fit.

We describe results on witness hexagons in preparation for full size

prototype segments for the European Extremely Large Telescope, and

comment on wider applications of the technology.

©2012 Optical Society of America

OCIS codes: (220.0220) Optical design and fabrication; (220.5450) Polishing; (230.4040)

Mirrors; (110.6770) Telescopes.

References and links

1. P. Wehinger, “Steward Observatory Mirror Lab,” (accessed March. 2012) http://mirrorlab.as.arizona.edu/.

2. J. Nelson and T. S. Mast, “Construction of the Keck Observatory,” Proc. SPIE 1236, 47–55 (1990).

3. P. Alvarez, J. M. Rodríguez Espinosa, and F. Sánchez, “The Gran Telescopio Canarias (GTC) project,” New

Astron. Rev. 42(6-8), 553–556 (1998).

4. H. M. Martin, J. H. Burge, B. Cuerden, W. B. Davison, J. S. Kingsley, W. C. Kittrell, R. D. Lutz, S. M. Miller, C.

Zhao, and T. Zobrist, “Progress in manufacturing the first 8.4 m off-axis segment for the Giant Magellan

Telescope,” Proc. SPIE 7018, 70180C (2008).

5. J. Lubliner and J. E. Nelson, “Stressed Mirror Polishing. 1: A technique for producing nonaxisymmetric mirrors,”

Appl. Opt. 19(14), 2332–2340 (1980).

6. J. E. Nelson and G. M. Smith, “W. M. Keck Observatory,” Bull. Astron. Soc. 22, 310 (1990).

7. J. W. Pepi, “Test and theoretical comparisons for bending and springing of the Keck segmented 10 m telescope,”

Opt. Eng. 29(11), 1366–1372 (1990).

8. R. Gilmozzi and J. Spyromilio, “The European Extremely Large Telescope (E-ELT),” The Messenger 127, 11–

19 (2007).

9. L. Christensen, “ESO moves one step closer to the first Extremely Large Telescope,” (accessed Sept. 2011)

http://www.eso.org/public/announcements/ann11034/.

10. E. Swat, “ESO prototype segment specification,” E-SPE-ESO-300–0150 Issue 4, 29th July, (2009).

11. X. Tonnellier, P. Morantz, P. Shore, and P. Comley, “Precision grinding for rapid fabrication of segments for

extremely large telescopes using the Cranfield BoX,” Proc. SPIE 7739, 773905, 773905-8 (2010).

12. P. Comley, P. Morantz, P. Shore, and X. Tonnellier, “Grinding metre scale mirror segments for the E-ELT

ground based telescope,” CIRP Annals, Manufacturing Technology 60(1), 379–382 (2011).

13. X. Tonnellier, P. Shore, P. Morantz, A. Baldwin, D. Walker, G. Yu, and R. Evans, “Sub surface damage issues

for effective fabrication of large optics,” Proc. SPIE 7018, 70180F, 70180F-10 (2008).

14. C. R. Dunn and D. D. Walker, “Pseudo-random tool paths for CNC sub-aperture polishing and other

applications,” Opt. Express 16(23), 18942–18949 (2008).

#170198 - $15.00 USD Received 7 Jun 2012; revised 3 Aug 2012; accepted 3 Aug 2012; published 14 Aug 2012(C) 2012 OSA 27 August 2012 / Vol. 20, No. 18 / OPTICS EXPRESS 19787

15. D. W. Kim, W. H. Park, S. W. Kim, and J. H. Burge, “Parametric modeling of edge effects for polishing tool

influence functions,” Opt. Express 17(7), 5656–5665 (2009).

16. F. W. Preston, “The theory and design of plate glass polishing machines,” J. Soc. Glass Technol. 11, 214–256

(1927).

17. G. Yu, H. Li, and D. D. Walker, “Removal of mid spatial-frequency features in mirror segments,” J. Eur. Opt.

Soc. Rap. Pub. 6, 11044 (2011).

18. C. Song, D. D. Walker, and G. Yu, “Misfit of rigid tools and interferometer sub-apertures on off-axis aspheric

mirror segments,” Opt. Eng. 50(7), 073401 (2011).

19. D. D. Walker, D. Brooks, A. King, R. Freeman, R. Morton, G. McCavana, and S. W. Kim, “The ‘Precessions’

tooling for polishing and figuring flat, spherical and aspheric surfaces,” Opt. Express 11(8), 958–964 (2003).

20. H. Li, G. Yu, D. D. Walker, and R. Evans, “Modeling and measurement of polishing tool influence functions for

edge control,” J. Eur. Opt. Soc. Rap. Pub. 6, 11048 (2011).

21. D. D. Walker, A. Beaucamp, C. Dunn, R. Evans, R. Freeman, R. Morton, X. Wei, and G. Yu, “Edge-control and

surface-smoothness in sub-aperture polishing of mirror segments,” Proc. SPIE 7018, 67–76 (2008).

22. H. Jing, C. King, and D. D. Walker, “Measurement of influence function using swing arm profilometer and laser

tracker,” Opt. Express 18(5), 5271–5281 (2010).

1. Introduction

With the 8.4m mirrors produced by the Steward Observatory Mirror Lab monolithic primary

mirrors for optical/IR astronomical telescopes have probably reached their ultimate size [1].

This is not limited by manufacturing technology, but practical considerations due to the costs

of large production equipment and coating plants, and the shipping of mirrors through a road

network to a remote mountain site. Therefore, optical/IR telescopes from 10m aperture and

upwards have segmented primary mirrors. The two Keck telescopes [2], and Gran Telescopio

Canarias (‘Grantecan’) [3], epitomize the tessellated hexagonal segment approach; the

forthcoming Giant Magellan Telescope (GMT) [4], the use of circular segments.

The now-considered “classical” method to manufacture hexagonal segments is based on

stressed-mirror lapping and polishing of the blank to a true spherical form whilst in its circular

state. The applied stresses are then relaxed, after which the mirror approximates to the

required off-axis aspheric form [5]. The part is then cut to the final hexagonal shape, but this

causes some warping [6] due to changed boundary-conditions in the stress-field. The mirror is

finished by ion-figuring in a vacuum chamber. The method was used successfully for the

thirty-six segments for each of the Keck telescopes, and a comparison of test and theoretical

predictions of mirror stressing has been reported [7].

The design of the European Extremely Large telescope (E-ELT), with its original 42m

aperture [8], deployed 984 x 1.4m across-corners segments, with a total requirement of 1,148

segments including one complement of spares (based on 6-fold symmetry). In June 2011 [9],

ESO announced a de-scope to 39.3m aperture on cost-grounds. The corresponding segment

numbers are now 798 in the telescope and 931 with spares.

The 36 segments for each of Keck 1&2, and Grantecan, are akin to repeated prototype-

scale manufacture. In contrast, the numbers for the E-ELT demand a new approach,

approaching mass-production. Key pointers are minimizing manual interventions (e.g. for

attachment and removal of the stressing fixtures), mitigating risk associated with cutting

hexagonal when the part has significant added value, and the use of ion figuring which is slow

and requires evacuation and further handling. The emphasis moves from minimizing times for

individual process-steps, to risk and total cycle time including all ancillary operations.

For these reasons, we report on a new approach which has been developed in the context

of E-ELT segment fabrication. All surface processing is performed on the blank in its final

hexagonal shape. Minimum manual intervention is needed, no stressing, and processing

entirely in air. Edge-control presents the most difficult challenge, combined with speed of

processing. We have considered sacrificial wasters around the perimeter of a segment to

mitigate edge-roll in polishing. However, we have rejected this option because (i) adhesive-

bonds can potentially distort the surface, and (ii) due to the extra handling and risk with

waster attachment and detachment, which we contend is not suitable for a mass-production

process.


The current paper reports on process development results on witness parts from the

perspective of edge and corner control, supported by 3D interferometer measurements on

complete hexagons, and embracing multi-stage processes for increased speed. The methods

developed are scalable applicable to larger and smaller sizes, and to other market sectors. In

particular, we have received commercial enquiries for edge-control on a variety of

components already cut to their final external dimensions. One example concerns thermal

imaging semiconductors that have been diced to final size, and the polishing is required to

unify the thickness of a deposited layer. Another case concerns the Zerodur datum straight

edges that form part of the interferometric metrology in wafer stepper machines. Other edge-

critical applications arise in image or pupil slicing optics, and in optics where loss of captured

energy is critical.

2. Interpretation of ESO’s specification for full-size prototype mirror segments

The segment specification provided by ESO [10], defines the ‘useful area’ of the segment as

the bulk surface excluding a peripheral zone 10mm wide (requirement) and 6mm wide (goal),

as shown in Fig. 1. The maximum edge-misfigure in the peripheral zone is specified to be

<400nm PV wavefront, and the average of the six edges <200nm PV wavefront. This

translates to <200 and <100nm surface misfigure, respectively. This specification has

introduced two ambiguities which we have identified and resolved as follows:-

• The PV metric is extremely sensitive to one or more anomalous pixels in the

interferometer data. We have adopted the PVq (95%) metric, which is more

representative of the functional requirement of the segments.

• The datum with respect to which the edge-misfigure is to be measured is ambiguous, but

has been resolved through the following metrology protocol:-

1. The part is measured on the interferometer.

2. A 0.5mm wide band around the periphery is removed from the data, to represent the

margin for final beveling after all other operations are complete.

3. Tip/tilt, de-focus and astigmatism are removed from the resulting data set.

4. The Useful Area is defined as the surface excluding the 10mm wide peripheral zone.

5. The Useful Area is cut out of the data set and analyzed to provide the RMS.

6. The remaining 10mm wide hexagonal ring is divided into six individual trapezoidal

edge-segments, each of which is analyzed separately to provide the PVq (95%) edge

misfigure numbers.

Fig. 1. Analysis of interferometry data for edge mis-figure.


3. Outline of the new segment process chain

Our E-ELT process-chain starts with a blank pre-machined hexagonal to final external

dimensions (other than a small overage on thickness), and with a rear cavity for the lateral

support system. Next, the off-axis asphere is ground using the Cranfield University BoXTM

ultra-precision grinding machine, which was optimized for low-slope optics. The grinding

preserves pristine edges, and delivers a surface with <1 µm RMS and 6 µm p-v form error,

when mounted on a precision diamond-turned grinding fixture [11, 12]. Sub-surface damage

is ~6 µm on Zerodur; a little more on ULE [13]. Our experience is that the grinding process

leaves residual mid spatial frequency errors, which are not removed by bonnet polishing, even

after long runs. This is because the very property of the bonnet that allows it to adapt to the

spatial frequency content in the asphere, also allows it to adapt to the mid spatials.

After BoXTM

grinding, we apply half of the specified 1mm final bevel, and the segment

proceeds through polish/metrology cycles. The final 0.5mm bevel is applied at the end. This

provides a contingency for rolling of the extreme edges in polishing, as they are subsequently

removed.

For the work reported in this first paper, we utilize 200mm and 400mm across corners

hexagonal witness parts, pre-machined to a 3m concave spherical radius, and lapped with C9

aluminum oxide on a matching convex cast iron tool.

The standard tooling for the Zeeko machines comprises compressible bonnets, covered

with standard polishing cloths, and which naturally adapt to the local asphere. The bonnet is

rotated about its axis by the H-axis spindle. The rotation axis is precessed with respect to the

local normal to the surface being polished, in different pre-determined directions, delivering a

near-Gaussian integrated influence function. The tool is moved across the surface of the part

in a pre-determined tool-path and dwell-time control is used to rectify measured form errors.

The standard tool-path is a regular raster. However, other tool-paths can confer advantages,

such as the pseudo-random, zero-crossing tool-path [14], which can help randomize a surface.

Standard bonnet polishing can be supplemented by other specialized tooling, and we

describe two examples of a family of tools mounted on bonnets, as shown in Fig. 2: (i) a pitch

button used with cerium oxide, which plays a key role in edge-rectification, and (ii) a

‘grolishing’ smoothing tool comprising a brass button used with C9 aluminum oxide abrasive.

4. The challenge of edges – rotating rigid tools

4. 1 Analysis of rigid tools edge effect

We first consider a pivoted, rotating, rigid tool, much smaller than the segment, performing a

predetermined regular tool-path at uniform traverse-speed, in the presence of an abrasive

slurry. Three main effects arise when the surface of the tool overlaps the edge of the segment:

a. For the tool-path to be fully executed and give uniform removal, all areas of the

segment should experience the same treatment. To achieve this, the boundary of the

tool-path should allow the tool to leave the part completely at every point along the

edge. However, were even the tool’s center to reach the edge of the part, the tool

would tip. The envelope of the tool-path must therefore be constrained, the tool-path

is consequently incomplete, and the missing removal tends to turn the edge-zone up.

b. The area of the tool in contact with the edge decreases as the tool overhangs. For a

given tool-mass, or constant applied force, the pressure exerted by the tool increases.

This tends to roll the edge-zone down.

c. The bow-wave of slurry, when the local speed-vector due to the tool-rotation attacks

the edge of the part, can turn the very extreme edge down.


The first two are geometric. The third disturbs the physical process of removal. A fourth

factor applies to a compliant bonnet when the polishing spot encroaching an edge:

d. The bonnet material can additionally deform and mold around the edge of the part,

and cause a highly localized increase in applied pressure, creating a sharp down-turn.

The effects (a) and (b) above are amenable to numerical modeling and calculation

respectively, parametric results being presented in Figs. 3, 4, 5, and 6. Effects (c) and (d) are

mitigated by applying the final half of the bevel after all other processing is complete.

Fig. 2. An example of a pitch

tool (left) and a brass

grolishing tool (right) mounted

on a bonnet.

Fig. 3. A rigid tool

overhanging a segment corner

and edge (right), along its

tool-path trajectory.

Considering the incomplete tool-path Case (a) above, the efficiency of removal at the

extreme edge of the part is given below, and plotted in Fig. 4.

Edge to edge:

1 11 1( cos( ) ) sin(2 cos( ) ))

2 2edge

AS ar R d R ar R d R

A

π

π

− −= = ⋅ ⋅ − ⋅ − − ⋅ (1)

Corner to corner

0 0 (1 3 / 2)corner

S d= ≤ ≤ − (2)

1 2 1 2

2 2

1 3 1 3(2 sin ( 2 ( )) sin(2 sin ( 2

2 2 2 2

1 3 1( )) ( 2 ( )) (1 3 / 2)

2 4 2

cornerS ar R Rd d R d ar R Rd d

R d Rd d R d R d R

π

− −= ⋅ ⋅ − − − − ⋅ −

− − + − − − − ⋅ ≤ ≤

(3)

1 1

2 2 2 2 1

1 5 5( cos( ) sin( cos( ) ))

2 6 6

3 3( ( ( ) ( ) ) ( ) 2

4 3

cornerS ar d R R ar d R R

d R R d R R R d R

π π

π

π

− −

−

= ⋅ − − ⋅ − − − ⋅

+ ⋅ − + − − ⋅ ≤ ≤

(4)

where, R is the radius of the spot size, d is the overhang distance as per Fig. 3, A1 is the area of

the overhang, and A is the area of the full spot. As per section 4.1a, zero tool overhang

corresponds to the extreme edge of the circular polishing spot just touching the extreme edge

of the part, giving zero removal at this precise location. 100% removal would occur were the

polishing spot permitted to leave the part entirely (prohibited in practice by tipping of the

tool).


Fig. 4. The polishing efficiency at the precise edge for different edge-overhang due to

incomplete tool path.

Fig. 5. Profiles of pressure

distribution at corner for

different edge-overhangs.

Fig. 6. Pressure distribution in

the contact area between tool

and work- piece for an

overhang of 35mm.

Consider a rigid tool pressed onto a part with a constant force at the center of the tool. The

pressure distribution over the contact area must change if the tool hangs over the edge, due to

the reduced contact-area. Assuming that the tool does not tip, the sums of the moments must

be equal to zero. Due to the lost contact on one side of the tool this results in a pressure

distribution skewed towards the edge of the part, which leads to edge-roll. This effect has

been simulated for a circular rigid tool near the corner of a hexagonal part for different values

of tool-overhang. Starting with a uniform pressure distribution over the contact area, an

iterative optimization has been performed finding the least squares of three equations: the total

force and the sums of the two moments.

The profiles in the radial direction of the resulting pressure distributions for different

overhang distances of the tool are shown in Fig. 5. These results are local minima and not

necessary global minima since the problem is under-defined. The results are in agreement

with the linear pressure distribution model used by D.W. Kim et al. [15] on a single edge, with

the addition that if the tool overhangs two edges (near a corner) the lines of equal pressure are

no longer parallel to an edge but in a direction in-between that of the two edges, as can be

seen from the pressure distribution in Fig. 6.

The opposing effects of Figs. 4 and 5 are amenable to optimization, available variables

being:

a. Z-offsets, and hence the range of spring-forces exerted by the tool

b. tool-path traverse-speeds (and so, the effective local dwell-times)


c. H-axis speeds (tool rotation speeds)

d. Tool overhang at the edge

In terms of modeling volumetric removal rate, the first three can be continuously varied as

a function of tool-overhang, but are effectively equivalent through Preston’s Law [16].

4.2 Experimental results for edge-control in the grolishing process

We have referred above to the mid spatial content in BoX-ground surfaces. Bonnet polishing

proves to be extremely slow in removing these features. A pitch button and cerium oxide

slurry is effective, and grolishing with a brass button and C9 abrasive is faster (albeit leaving a

grey surface) [17,18]. We have also confirmed that these process-steps (i) do not introduce

significant new mid spatials on a cylindrical form representative of the segment asphericity,

and (ii) can be ported onto flat parts (representing an R = 84m base-radius segment).

The brass button was profiled with one surface concave to match the 80mm convex radius

of curvature of a standard R80 bonnet. The other surface was profiled to match the part. The

working area of the Zeeko machine was contained with polythene sheet to prevent machine

contamination by the C9 abrasive. Grolishing experiments were performed to characterize

edge-profiles as a function of tool-overhang at the ends of the tool-path, as shown in Fig. 7.

The precise edge-geometry is then selected to give optimum performance of subsequent

polishing operations. Pitch button polishing exhibits similar edge-features.

The tool was 50mm diameter, rotated at 150rpm, and the tool-path traversed over the

surface of the hexagonal part was a regular raster of spacing 5mm with tool-lift at the ends.

The traverse speed along the raster was 500mm/minute. In each case, a set of three

measurements represents separate profilometer scans across different pairs of edges or

corners. These have been separated vertically on the figures for clarity of presentation, and

this separation does not reflect a difference in removal-rate or depth.

Fig. 7. Form Talysurf profilometry of edge and corned profiles, related to tool overhangs. The

set of three plots corresponds to the three edge scans across corners or flat.


5. The challenge of edges—bonnet polishing

5.1 The philosophy of practical edge control

The Zeeko Precessions process in its standard form utilizes compliant bonnets, covered with

polishing cloth, spun about their axis, precessed about the local normal to the surface, and

operated in the presence of re-circulated cerium oxide slurry. High removal rates suitable for

segment fabrication result from the high surface-speed and applied pressure [19].

A compliant bonnet pressed against a surface acts differently from a rigid tool,

specifically:-

a. Changing the Z-offset modifies the size of the polishing spot (influence function)

b. The precess angle changes the direction of the local speed vector

c. The influence function is pseudo-Gaussian, rather than M-shaped with a central zero

d. Bonnet deformation around the extreme edge of the segment, as mentioned above

The ability to reduce the spot-size along the tool-path, by decreasing the Z-offset (lifting

the bonnet) has proved particularly useful for edge control [20, 21]. Once the leading edge of

the full-size spot encounters the edge of the part, the spot-size is controlled, by optimizing the

trajectory of the bonnet. By this means, the spot’s leading edge can remain registered with the

segment-edge, avoiding any overhang. This is shown schematically in Fig. 8, where a raster

tool-path proceeds towards the segment-edge (dashed). The raster then turns around in air (not

shown).

Fig. 8. Polishing spot encountering an edge with tool-lift enabled.

The reduced removal in the edge-zone can be compensated by increasing dwell-times

(reducing traverse speed), or increasing H-axis speed. Even so, the removal at the precise edge

remains exactly zero. This can be mitigated by allowing the spots to overlap the edge slightly,

within a range constrained by the onset of the bonnet molding around the extreme edge and

turning it down. Note that the edge-overhang can be varied independently for each spot-size.

Based on the above, the overall strategy we have developed is as follows. The largest

bonnets/spots available are first used for fast pre-polishing. The tool-lift parameters are

defined to give an edge-zone meeting the following criteria:-

• never dipping below the extrapolated bulk-form

• of minimum height (microns), but with a contingency for any process-variability

• of width sufficient to give benign slopes, always within the measurement-range of the

full-aperture interferometric test


The last criterion has required a careful analysis of the interferometric configuration for

measuring a full-size segment, including the lateral sampling of the interferometer used in that

application. The test of witness parts is configured to approximate this condition.

The form-corrective process then proceeds through smaller bonnets delivering a range of

smaller spots, which gives capability progressively to control the edge profiles.

During measurement, fiducial shadow-masks are temporarily attached to the optical

surface to identify the true edge (start of bevel), and are imaged through the interferometer.

By this means, loss of data due to an unexpected sharp down-turn can be correctly identified.

On witness parts, this is further confirmed by scanning the part with an Extended Range Form

Talysurf stylus profilometer. To provide a datum for absolute measurement of removal-depth

throughout processing, witness parts are marked with a linear scratch at the approximate

center. The scratch-depths are measured across the scratches with the Form Talysurf.

5.2 Modeling of edge profiles with bonnet polishing

To model the tool’s performance accurately in the edge zone of the part, a series of influence

functions (IFs) have been generated specifically in that area. These IFs augment the ones

taken in the bulk area to give a better representation of the polishing process over the whole

area (including the edges) of the part. Simulation software, capable of predicting the edge

profile has been developed in MATLAB. The empirical Ifs, both on the part and overlapping

the edge, are interpolated, scaled according to the dwell-times, and summed for each pixel

over the part.

The aim of the modeling was to achieve the targeted material removal, whilst keeping the

slopes of the error in the edge-zone sufficiently small so that they can be measured using an

interferometer. At each position in the edge zone, the slope of the error has been kept under

the limit that the interferometer can resolve by adjusting the tool offsets and the local dwell

time. This model has been verified by an experiment on a 200mm across corners, hexagonal

Zerodur part, polished with an R160mm bonnet tool and 45 mm spot size. A comparison is

shown in Fig. 9, after correction for base-radius and volumetric removal rate. It can be seen

that the modeling and experimental results show reasonable agreement, providing a useful

tool for further work.

Fig. 9. Preliminary modeling and experimental results of tool lift.

6. Edge control in global polishing: experimental results

We report below on experiments processing 200 and 400mm across corners witness parts with

R = 3m concave. The flexibility of the edge processing means that the detailed edge-definition

can be tuned as required. For example, the process can be constrained so that the profile never

dips below the extrapolated bulk form anywhere around the part as per Figs. 10 and 11


(effective for a possible local edge-rectification process-step). Alternatively, edge-misfigure

can be balanced about the neutral position as per Fig. 12. Phase maps and numerical results

are presented after allowance for the final 0.5mm of edge beveling.

6.1 200mm hexagonal part, Zerodur

Measurements of a 200mm across-corners hexagonal Zerodur witness part, R = 3m concave,

are shown in Figs. 10, 11, and 12. This part was prepared by abrasive-lapping, and it was then

bonnet pre-polished on an IRP1200 Zeeko machine. The tool was a 160mm radius-of-

curvature (‘R160’) bonnet, precessed at 15 degrees, and with Z-offset to deliver a 45mm full

spot-size. Measurements were conducted using a 4D Technologies 6000 simultaneous phase

interferometer. The part of Fig. 10 was then form-corrected using an R80 bonnet and 20mm

spot size, in order to narrow and reduce the peripheral up-stand. Results are in Fig. 11.

Fig. 10. Fringes and phase map from turned-up edges after pre-polishing with R160 bonnet and

60mm spot. The surface figure of the entire surface including edge-zone, but excluding the

0.5mm allowance for the bevel is 1084nm RMS.

Fig. 11. Fringes and phase map after correction with R80 bonnet and 20mm spot. The surface

figure of the entire surface including edge-zone, but excluding the 0.5mm allowance for the

bevel is 67nm RMS.

Finally, the entire part was treated with a 100mm diameter pitch tool, furnished with

grooves in the traditional manner, and mounted on a metal carrier, on the Zeeko machine. The

tool-path was a regular raster with 4mm spacing, the rotation-speed was 260rpm and the

traverse speed along the raster was 3000 mm/min. The operation of the tool was separately

qualified on a cylindrical asphere representative of the segment, in order to ensure that it did

not introduce any surface defects due to aspheric misfit. Results are shown in Fig. 12.


Fig. 12. Fringes (left), phase map (centre), and PVq (95%) edges (right), after rectification with

a pitch tool.

6.2 400mm hexagonal part, borosilicate glass

The edge control process has also been demonstrated on a 400mm hexagonal, borosilicate

glass part. In this process, an R200 bonnet was deployed, delivering a 55mm spot size. This

was used in pre-polish to achieve a higher removal rate than the previous R160 bonnet. An

R80 bonnet and 20mm spot size were then used to correct form. Finally, the entire surface

was polished by a pitch tool on the Zeeko machine. The final results are shown in Fig. 13,

where 23nm RMS was achieved for the entire surface including the edge-zone (excluding the

0.5mm margin), and edge results PVq (95%) were measured according to the protocol defined

in Section 2.

Fig. 13. Final results on the 400mm across corners part: fringes (left), phase map (center) and

PVq (95%) edges (right).

7. Conclusion and further work

In this paper we have discussed the context of edges on extremely large telescope segments,

and briefly reviewed the E-ELT edge specification. We have also drawn attention to

increasing enquiries we have received for edge-control in other industrial applications.

The paper has reviewed the physical mechanisms causing edge-misfigure in loose-

abrasive processing, and presented a preliminary modeling method which can be used to

optimize processes for specific applications. We have gone on to consider three basic

manufacturing processes in the context of edge-properties – hard tooling for (i) grolishing and

(ii) pitch-polishing (both methods to mitigate mid-spatial frequency defects from prior CNC-

grinding), and (iii) compliant bonnets for pre- and corrective polishing.

We have outlined a novel process-chain for E-ELT segments and other components that

are sensitive to edge mis-figure. All surface processing steps can be conducted after the part

has been pre-machined to the final external dimensions. In support, we have conducted

numerous trials using 200mm and 400mm witness parts, where the blank was pre-machined to

the final external dimensions, and the surface profiled leaving a grey surface. A sample of

results has been presented.


We have shown how pre-polishing the global surface with R160 and R200 compliant

bonnets can be configured to produce surfaces with gently-sloping peripheral up-stands and

no downturned edges, amenable to full-aperture interferometric measurement. Further results

have demonstrated how these surfaces can be corrective-polished using R80 bonnets, leaving

residual edge features that can be controlled by traditional pitch techniques on the Zeeko

machine. We have drawn attention to the flexibility of the methods in regard either to tuning

for optimum edges where residual misfigure is turned both up and down, or for tuning where

all residuals are turned-up. Overall, results are close to the ESO specification.

Future work will follow two parallel tracks. First, we plan to deploy a larger bonnet

(nominally R400) before the R200, delivering 100-110mm spot-sizes, and leaving a broader

turned-up edge-zone, again amenable both to interferometry and to controlled reduction using

the R200 process. The objective is to accelerate the pre-polish phase for the entire surface. To

support the practical use of such tooling, we should have the capability to measure the tool

influence functions on a flat witness part. This demands a sizeable land of material around the

influence function to provide a suitable datum for measurement. With this in view, we have

developed the use of swing arm profilometry for this purpose, which we have reported [22].

Second, we have started developing a new local edge rectification method as a final

process step, which is constrained to operate within the edge-zone of the part. This has two

roles. The first is to address the natural tendency of hard tools to leave corners that are slightly

higher than the edges. The second is to provide a method to correct residual edge misfigure.

In summary, the work reported has demonstrated for the first time a methodology for

manufacturing segments for extremely large telescopes, where all the optical work is

performed with the blank machined to its final dimensions. This methodology clearly has

potential in the other applications to which we have referred, and which we shall pursue in

due course.

Acknowledgments

We gratefully acknowledge financial support under an RCUK Basic Technology Translation

Grant, ‘Ultra Precision Surfaces: A New Paradigm’, through an R&D project funded under

the EPSRC Integrated Knowledge Centre in Ultra Precision and Structured Surfaces, and

through an STFC IPS grant. Particular thanks are also due to Glyndŵr University, and to the

Vice Chancellor Prof. Mike Scott, in regard to their substantial commitment and financial

support of the segment project. We also wish to thank Zeeko Ltd for assistance in many ways,

not least developing the code for edge-control in polishing. H. Li’s PhD studentship has been

sponsored by UCL and OpTIC, and W. Messelink through a UCL Impact studentship.


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