Segmented mirror polishing experiment

Robert A. Jones

An experiment was conducted to demonstrate the capability of polishing segmented mirrors. A mirror seg-ment was figured with the computer controlled polisher using special software and tooling. This processrapidly reduced surface figure errors and produced a good figure to the segment's edges.

1. Introduction

The construction of very large optical telescopes canbe prohibitively costly due to the massive weight ofconventional primary mirrors. One method of reducingthe cost of such telescopes is the use of a segmentedprimary mirror. Such a mirror has several advantages:lower blank cost, ease in handling and transporting, lessrisk due to breakage, simpler coating equipment, anda more lightweight compact telescope.1

The fabrication of the individual segments posescertain problems. The elements will be off-axis seg-ments of the desired composite aspheric surface.Therefore, the fabrication processes cannot rely onsurface figure symmetry. Also, the segments are non-circularly shaped pieces which must have a good figureright to their edges. Sizable edge errors are unaccept-able for segments since the edges would be in apertureof the composite mirror produced by the segments.

A brief experiment was conducted to demonstrate thecapability of computer controlled polishing a segmentedmirror. An available 0.47-m diam polished sphericalmirror was used. The mirror was cut along the straightedges and sandblasted outside the curved edges asshown in Fig. 1. The resulting mirror was a 60° seg-ment of a 0.91-m mirror with a 30% obscuration. Themirror had a radius of curvature of 0.62 in resulting inthe segment mirror being f/0.66.

The mirror was tested using the phase measuringinterferometer (PMI). The use of this equipmenteliminated the need to construct a new test setup.Since the PMI was set up to test a spherical surface and

The author is with Perkin-Elmer Corporation, Norwalk, Con-necticut 06856.

Received 18 September 1981.0003-6935/82/030561-04$1.00/0.© 1982 Optical Society of America.

could not be disturbed, the segment was figured as asphere. However, the mirror was fabricated in a man-ner suitable for producing an off-axis segment of anaspheric mirror.

The mirror was polished with the computer-con-trolled polisher (CCP) using special software and tool-ing. The final surface figure was to be the best effortsince time was limited and the initial surface errors wereinitially unknown. The goal for the CCP effort was toreduce the surface error as rapidly as possible whilemaintaining a good figure out to the segment edges.

11. Path Software

The required software for computer-controlled pol-ishing of mirror segments was written. The computeralgorithm accepted PMI test data and generated thecontrol tapes for the CCP. These control tapes desig-nated the path of the CCP tool over the surface of the.mirror and the time alloted to the various portions ofthis path. The polishing path used for the segmentmirror consisted of concentric arcs with connectingsteps. These arcs were centered on the center of thecomposite mirror, which would be formed by six suchsegments. These arcs represented paths of equal mirrorsurface height for an aspheric segment. The connectingsteps between the arcs were parallel to the straight edgesof the segment.

The arc raster path started with the step from the endof the inner arc to the end of the second arc, movedalong the second arc to its other end, stepped to thethird arc, and moved along that arc. The path contin-ued this outward motion until completing the outer arcas shown by the path in Fig. 2 with the large arrowheads.Next the path moved to the end of the next inward arcand moved along that arc. The path continued thisinward motion until completing the inner arc as shownby the path in Fig. 2 with the small arrowheads for thesteps.

The steps were interlaced such that the resulting pathhas passed twice over each inside arc while passing onceover the outside pattern consisting of the innermost arc,

1 February 1982 / Vol. 21, No. 3 / APPLIED OPTICS 561

Fig. 1. *Mirror in metrology cell. An available 0.47-m diam mirrorwas used to form a 600 segment of a 0.91-m diam mirror.

Fig. 2. Polishing path for CCP tool. Polishing path consisted of anarc raster-type path.

the outermost arc, and the two connecting edge lines.The specific characteristics of the path could be ad-justed by input parameters to the computer program.These path descriptors included the radii of the inner-most and outermost arcs, the number of arcs, and thedistance from each arc end to the edge of the seg-ment.

Ill. Equipment

Figure 3 is a photograph of the CCP, which can beused for working pieces up to 2 in in diameter. Thecomputer controls the two positional drives, the twotilts, and the height. The CCP computer is suppliedwith data on path step. As the tool assembly movesover the workpiece the computer constantly obtainsprecise information from encoders on the actual toolcoordinate values. For each path step the computerdetermines the velocities required to get from its presentmeasured position to its next desired position in thealloted time. These velocity values are supplied via adigital interface to velocity servo systems which supplythe machine motions. The use of controlled tilt mo-tions keeps the polishing head normal to the mirrorsurface as it follows the prescribed path over the mirror.These velocities also determine the time spent polishingthe various regions of the mirror, which provides thecontrolled material removal.2 4

IV. CCP Tool

The CCP polishing tools consist of pads much smallerthan the workpiece size. These pads are maintainedunder constant pressure by use of an air plenum whichcontains the ends of the rods on which the pads aremounted. The pressure may be higher than for con-ventional fabrication since workpiece distortion pro-duces only minor effects when using small pads. Thepads are free to rotate or tilt slightly to best fit theworkpiece surface. Two rotational motions are sup-plied to produce an epicyclic tool motion. This con-stant epicyclic motions is much faster than the varyingpositional motion and is the main cause of material re-moval. By varying the size, location, and number of thegrinding or polishing pads various tool configurationscan be used.

The desire to obtain a good surface figure out to theedge of the segment was the dominant factor in the se-lection of the CCP tool for polishing the mirror segment.If the tool was moved up to the edge without over-hanging the edge during the polishing, the edge wouldhave been left high since the pad is over that region forless time. Any attempt to spend more time near theedge during such a polishing run could have resulted inremoving too much material inside this edge region.The method used to take down the edge was the use ofa pad overhanging the edge. As a pad overhung theedge the pressure distribution under the pad changed,with the resultant pressure greatest at the edge.(Specific pad distribution curves have been obtainedby computer modeling.)

Previous fabrication efforts used separate edge runswith an overhanging pad to take down the up edge leftfrom prior figuring runs. On this effort a decision wasmade to combine the figuring and edge runs into a singlerun. Therefore, a single pad was used which could dothe normal figuring (and smoothing) over the main areabut could also overhang the edge for edge control.

Fig. 3. Photograph of the CCP. Polisher is the third-generationcomputer-controlled polisher designed and built by Perkin-Elmer.

562 APPLIED OPTICS / Vol. 21, No. 3 / 1 February 1982

B. ISOMETRIC FIGURE PLOT

X,2F

Fig. 4. Computer plots showing the surface-figure error of the mirrorprior to CCP polishing. Fringe spacing is X/20, while the contour linesare X/40 in the pseudointerferogram. ( is 0 .6 3 3 /Am). Isometric plotshows that the initial surface is higher than desired along most of the

cut edges.

and roll error on the parent piece remains. The high-upedges made figuring more difficult since good figure isrequired up to those edges, and the CCP normally tendsto leave edges up.

The CCP process was used in an iterative manner toproduce the desired workpiece surface. Figure 5 is aflow diagram for each polishing cycle. First, the-surfacewas tested with a phase-measuring interferometer toprovide digital surface-error data. Next, the measuredworkpiece surface errors were compared with the errorspredicted prior to the last polishing operation. Theparameters for the next CCP run were chosen, and acontrol tape was computer generated. Based on theprior surface errors, the control tape data, and the CCPparameters, the expected figure progress for the nextCCP cycle was predicted. The control tape and pa-rameters were used with the CCP to polish the work-piece. After the prescribed polishing time the work-piece was tested again and the cycle repeated.

The CCP was used for five iterative cycles, with PMItesting after each cycle. During this polishing, con-sisting of <4 h, the surface figure was improved from0.042 X to 0.012 X rms error. This figuring progress is

A. PSUEDO INTERFEROGRAM B. ISOMETRIC FIGURE PLOT

Fig. 5. Flow diagram for a CCP polishing cycle.

Fig. 7. Computer plots showing the surface-figure error of the mirrorafter CCP polishing. These plots are at the same scale as Fig. 4 and

show the edge correction.

I-- i/100

0 I . , , I . I

0 50 100 1 50 200 250POLISHING TIME - MINUTES

Fig. 6. Plot of CCP figuring progress for the mirror segment. Pol-ishing consisted of <4 and produced a mirror with an rms error of only

0.012X (0.0076 Mm).

V. Figuring Progress

After preparation the mirror segment was tested withthe PMI and found to have a surface error of 0.0425-4rms, where X is 0.633 um. The surface is shown in Fig.4. The first figure shows pseudofringes with A/20spacing between fringes and contour lines at A/40 in-tervals. The second figure is an isometric plot of thesurface after removal of tilt and power radius. Themirror is seen to have high-up edges along the cut,presumably the result of the cutting process or the un-dercutting along those edges. Also the residual of hole

Fig. 8. Testplate interferogram of the edge of the mirror-fringespacing is /2. The testplate measurement verifies that the surface

figure is good to the edge of the segment.

1 February 1982 / Vol. 21, No. 3 / APPLIED OPTICS 563

0.04

? 0.03

° 0.02

a 0.01

. . . . . . . . . . .

A. PSUEDO INTERFEROGRAM

25

/50

.

,

.

shown in Fig. 6. More important than the rms errornumber was the edge figuring progress. Figure 7 indi-cates the actual surface figure in a similar manner as Fig.4. As shown in these plots the high-up edges have beentaken down without introducing any figure roughnessand without the need for separate edge polishing runs.As shown in the testplate interferogram of Fig. 8, wherefringe spacing is X/2, the mirror has a good figure to theedges.

VI. Conclusions

An experiment was conducted to develop and dem-onstrate the capability of figuring segmented mirrorswith computer-controlled polishing. The requiredsoftware for polishing mirror segments was generated.The polishing path consisted of concentric arcs withconnecting steps. An available spherical mirror was cutto form a 60° segment of a 91-cm (36-in.) diam mirror.The initial surface figure was 0.042X rms error, with

DEUTSCHE GESELLSCHAFTFUR ANGEWANDTE OPTIK

much of this error due to the surface being up along thetwo cut edges. In just five cycles the CCP was able tocorrect the up edges and improve the surface figure to0.012X rms error. The capability of the CCP to polishsegmented mirrors has been demonstrated. In fact, theCCP approach is ideally suited to the fabrication ofsegments. Since the process achieves controlled re-moval by regulating tool velocities, there is no need forany surface symmetry.

This experiment was supported by Perkin-ElmerIR&D funding. I wish to acknowledge the contribu-tions of G. Marks and J. Pecunia to portions of the CCPeffort.

References1. J. Nelson, Proc. Soc. Photo-Opt. Instrum. Eng. 172, 31 (1979).2. R. A. Jones, Appl. Opt. 19, 2072 (1980).3. R. A. Jones, Appl. Opt. 16, 218 (1977).4. R. A. Jones, Opt. Eng. 18, 390 (1979).

SCHWEIZERISCHE GESELLSCHAFTFUR OPTIK UND ELEKTRONEN-MIKROSKOPIE

The Deutsche Gesellschaft fr angewandte Optik (DGaO - GermanSociety for Applied Optics) and the Schweizerische GesellschaftfUr Optik und Elektronenmikroskopie (SGOEM - Swiss Society forOptics and Electron Microscopy) announce a joint conference on

- Optical Surfaces- Thin Films- Principles for Imaging.

The conference will be performed June 1-5, 1982 at the ConventionCentre Lucerne/Switzerland.

Further topics will be

- Lens Design- Pattern Recognition- Image Processing- Holography.

Informations and programmes are available from the Secretary ofDGaO:

H.-J. PreuBc/o Ernst Leitz Wetzlar GmbHPostfach 2020

D-6330 Wetzlar

564 APPLIED OPTICS / Vol. 21, No. 3 / 1 February 1982