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Stephen D. FantoneOptical Activities in Industry is re-ported by Stephen D. Fantone, OptikosCorporation, 286 Cardinal MedeirosAvenue, Cambridge, MA 02141. Ste-phen welcomes letters, news and com-ments for this column which should besent to him at the above address.

Vacuum activated polishing laps produce smoothaspheric surfaces

Wiktor Rupp and Vadim Plotsker

A novel approach to computer controlled fabrication of aspheric optical surfaces utilizes semi-flexiblevacuum activated polishing tools to produce high quality, smooth surface geometry on off-axis mirrorcomponents.

A fundamental requirement for producing smoothoptical surfaces is good conformity of the lapping toolto the polished surface. In the case of sphericalsurfaces this condition is inherently fulfilled. How-ever, in the case of aspheric surfaces the condition forproper tool fit is the major technical problem.

Conventionally, polishing of aspheric surfaces isperformed by using combination of flexible, large sizetools for assuring proper surface smoothness and

Wiktor Rupp is with Itek Optical Systems, 10 Maguire Road,Lexington MA 02173.

Vadim Plotsker is with Insight Technology Inc., 300 Bedford St.,Manchester, NH 13101.

Received 6 June 19920003-6935/93/071048-03$05.00/0.( 1993 Optical Society of America.

small size tools for local surface geometry correction.Although, such an approach can produce satisfactoryresults, it relies entirely on the skill of the opticianand is very time consuming. This fabrication ap-proach becomes extremely difficult if an off-axisaspheric surface has to be fabricated. To overcomethe cost and schedule uncertainties of the conven-tional approach computer controlled surfacing meth-ods have been recently developed. They can beclassified into two groups. One is based on relativelylarge tools (about 10% of the blank area) wherefiguring capability is provided by real time control oftool geometry. This approach has beem termed"Stress Lap Polishing".' The second approach isbased on relatively small tools (about 1% of blankarea) which operate with constant tool pressuredistribution in the tool/work-piece interface. The

1048 APPLIED OPTICS / Vol. 32, No. 7 / 1 March 1993

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figuring capability for this approach consists of locallyvarying the tool dwell time. Since the tool travelpattern and dwell time are computer controlled thisapproach has been termed "Computer ControlledOptical Surfacing" (CCOS).2 The first approach re-quires that the blank surface is in strain free condition.For this reason this approach is used on passivemirror blanks which are rigid enough not to printthrough the support pattern or light-weighting ribpattern. In order to address fabrication of light-weighted mirrors or thin mirrors for active systems asmall tool approach has been developed. Its figuringaction depends only on tool kinematics and does notrequire a strain free condition of the blank for thefiguring function. To prevent print-through effectsfrom rib pattern for light-weighted blanks, the nor-mal tool loading force has been replaced with suctionforce created in the tool workpiece interface by partialvacuum. In the original approach the constant pres-sure distribution in the tool/glass interface was as-sured by proper tool guidance along isoastigmaticpattern of the aspheric surface. In this case the toolsurface would conform to the blank surface by wearin the case of the grinding process, or by pitch flow inthe case of the polishing process. To satisfy thisrequirement the tool has to be guided along a circularpattern centered on the vertex point of the asphericsurface.

This approach does not present any problems in thecase of round blanks. In the case of rectangularblanks the concentric isoastigmatic tool travel pat-tern can not be realized without requiring that thetool walks out of the blank surface at the middle zoneof the upper edge of the blank as shown on fig 1. Toovercome this problem the tool travel pattern havebeen changed to straight lines parallel to the blanksedges. This approach, however, forces the tool totravel across the lines of the isoastigmatic pattern,exposing it to surface misfit and thus violating therequirements for equal pressure distribution in thetool/glass interface. To eliminate this effect, thetool body has been provided with a degree of flexibilityso that under the forces created by vacuum in thetool/glass interface the tool would conform to the

glass surface. This paper describes the first applica-tion of this approach in the fabrication of a rectangu-lar of-axis mirror component.

The dimensions of the mirror component were 28in x 15 in. It represented a segment of a F/0.85elliptical parent with a conic constant of -0.69. Thelargest semi-flexible tool was diamond shaped, havingan area of about 12 sq. in. and representing about2.5% of the blank area. In extreme positions thistool would see as much as 12 microns mismatch to theblank surface. Typically, applied vacuum in theinterface was 20 in.Hg. which corresponds to about a4 psi polishing pressure. The tool has been run inrectangular pattern as marked on fig 1. The dis-tance between the scanning lines was set to 0.2 in.Final figuring was done with a small, star shaped toolhaving a surface area of about 2 sqr in. The testingof the surface geometry was performed at the centerof curvature using a null lens in conjunction with anunequal path interferometer. Figure 2 representswave-front geometry of the finished mirror. It metthe surface geometry accuracy requirement whichwas specified as 0.007 microns surface RMS over anyaperture of 6 in. in diameter. There was no overallsurface geometry requirements since the mirror wasused in a scanning mode.

The described above fabrication process demon-strated that excellent surface smoothness can beachieved in computer controlled surfacing. Although,by no means did the process indicate that it reachedits performance limit. There are two CCOS processelements which address the surface smoothness:The natural smoothing tendency of a pitch polishinglap and the figuring process. The surface irregular-ity which corresponds to a spatial period equal to 25%or less of the tool size is totally removed by smoothingand it is partially removed if it corresponds to aspacial period of about 50% of the tool size. Conse-quently the surface irregularity corresponding to aspacial period larger than 25% of the tool size has tobe addressed by figuring. This requires an input ofdata points density of spacial period of about 25% of

tool travel patterns

isoastigmaticpattern * parent vertex

Fig. 1. In the mirror fabrication, the semiflexible surfacing tooltravels across the isoastigmatic pattern.

Fig. 2. Interferogram of an off-axis mirror segment of a F/0.85elliptical parent.

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tool size. If this requirement is met any surfaceirregularity not removed by tool smoothing tendencycan be addressed in figuring process. In the de-scribed fabrication case the size of the smallestpolishing tool corresponded to one inch. The chosendistance for the data points defining the surfacegeometry was 0.20 in.

The process applied in the fabrication of this mirrordemonstrated also that tool fit, which usually getsmore critical as the f/number of the parent increases,is no longer a critical element in computer controlledoptical surfacing as long it is enforced by a combina-tion of some tool flexibility and vacuum suction in thetool/glass interface. Since, the figuring process isapplied step by step to local elements of the mirrorsurface, there is no upper limit on the size of themirror blank as long proper equipment for handlingthis size is available. There is however a limitation

on the lover size of the fabricated blank. It isdictated be the smallest size of a lapping tool whichcan be practically applied to the process. In order toresolve the asphericity of commonly used asphericsurfaces the smallest lapping tool has to have an areaof less than 1% of the blank size. The smallest toolused so far at Itek was about 1 square in. Conse-quently, the process is applicable to any, practicallyuseful F/numbers and any blank sizes over about 10in. as long as proper size equipment is available.

References1. J. R. P. Angel et al., "Progress toward making lightweight 8m

mirrors of short focal length," Advanced Technology OpticalTelescopes IV, Lawrence D. Barr, ed. SPIE, 1990, 636-640.

2. R. A. Jones, W. J. Rupp, "Rapid optical fabrication withcomputer controlled optical surfacing," Optical Engeneering 30(12), 1992, p 1962-1968.

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