New polishing technology for large mirrors and lenses

D. D. Walker, A. T.H. Beaucamp, D. Brooks, A. King, R. Morton, G. McCavana

Research output: Chapter in Book/Report/Conference proceedingConference contribution

3 Citations (Scopus)

Abstract

The Precessions™ process, and the 200mm capacity 'All' machine which embodies it, are outlined in the accompanying paper 'New method to control form and texture on industrially-sized lenses'. In this paper, we summarise the development of the process to address larger optical components. The first machines constructed for this application are of 600mm capacity. A conceptual design of a 1-2m class machine of bridge-configuration has also been produced. Unlike the All machine, the work-piece on these larger machines lies in a horizontal plane i.e. with the rotation axis of the work-piece spindle vertical. This configuration is preferred because it allows the use of a hydrostatic support-system for the work-piece; important for large optics in general, and particularly so for light-weight optics. The machine can also provide a clear vertical path above the work-piece for optical testing, or for access by lifting gear. It is interesting to consider how the Precessions™ process can be scaled. As an example, consider first that the type of surface-form remains the same; it might, for example, be an f/2 parabola. Consider then that the work-piece diameter, the bonnet-diameter, the polishing spot-sizes used, and the tool-path on the surface, are all stretched by a factor of two. The area of the polishing spot and that of the work-piece have both increased by a factor of 4, but the number of convolutions of the spiral tool-path remains the same. Since the spot instantaneously addresses the same fraction of the overall work-piece surface-area, the terms all cancels in relation to cycle-time. However, for the same tool-rpm, the larger bonnet delivers a proportionally higher surface-speed, because the polishing action is further from the axis of rotation. Therefore, the volumetric removal rate (proportional to contact-area times surface-speed) tends to vary as the cube rather than the square of the scale-factor. As a result, the larger bonnet working over the larger work-piece with the same polishing pressure, will remove twice the depth of material in the same time. The above scaling could, in principle, reduce cycle times on larger parts. In practice, we have found that excessive removal-rates lead to impractically short dwell-times, especially near the centre of a spiral tool-path. Such scaling also demands increased traverse-speeds and accelerations from the machine, and this rapidly becomes a limitation. Moreover, as the process converges on final form, smaller depths of material need to be removed. In practice, we have found it necessary to moderate the process by diluting the slurry and by moving from a polyurethane tool-surface to a material that delivers a lower-removal rate, such as Multitex. We are also developing bonnets that are compatible with operation at a reduced polishing pressure, in order to give additional control for fine-removal. The current status of the process development is as follows. A form error of ∼80nm peak-to-valley has been achieved aspherising a 100mm diameter part. This work revealed some subtle but significant limitations in the optimisation code, mostly concerned with convergence and residual ripples. Recent work has focussed on this aspect, and the revised code is now ready for polishing trials. A 150mm diameter part is currently in-process, and work is about to commence on a 300mm fused silica mirror for a space application.

LanguageEnglish
Title of host publicationOptifab 2003
Subtitle of host publicationTechnical Digest
EditorsHarvey M. Pollicove, Walter C. Czajkowski, Toshihide Dohi, Hans Lauth
PublisherSPIE
Number of pages1
Volume10314
ISBN (Electronic)9780819451040
DOIs
Publication statusPublished - 19 May 2003
Externally publishedYes
EventOptifab 2003: Technical Digest - Rochester, United States
Duration: 19 May 200322 May 2003

Conference

ConferenceOptifab 2003: Technical Digest
CountryUnited States
CityRochester
Period19/05/0322/05/03

Fingerprint

Polishing
polishing
Lens
Lenses
Mirror
lenses
mirrors
Tool Path
Optics
axes of rotation
Vertical
Optical testing
Scaling
Optical Testing
precession
Dwell Time
Fused Silica
Parabola
Configuration
Polyurethanes

Cite this

Walker, D. D., Beaucamp, A. T. H., Brooks, D., King, A., Morton, R., & McCavana, G. (2003). New polishing technology for large mirrors and lenses. In H. M. Pollicove, W. C. Czajkowski, T. Dohi, & H. Lauth (Eds.), Optifab 2003: Technical Digest (Vol. 10314). [103140C] SPIE. https://doi.org/10.1117/12.2284001
Walker, D. D. ; Beaucamp, A. T.H. ; Brooks, D. ; King, A. ; Morton, R. ; McCavana, G. / New polishing technology for large mirrors and lenses. Optifab 2003: Technical Digest. editor / Harvey M. Pollicove ; Walter C. Czajkowski ; Toshihide Dohi ; Hans Lauth. Vol. 10314 SPIE, 2003.
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Walker, DD, Beaucamp, ATH, Brooks, D, King, A, Morton, R & McCavana, G 2003, New polishing technology for large mirrors and lenses. in HM Pollicove, WC Czajkowski, T Dohi & H Lauth (eds), Optifab 2003: Technical Digest. vol. 10314, 103140C, SPIE, Optifab 2003: Technical Digest, Rochester, United States, 19/05/03. https://doi.org/10.1117/12.2284001

New polishing technology for large mirrors and lenses. / Walker, D. D.; Beaucamp, A. T.H.; Brooks, D.; King, A.; Morton, R.; McCavana, G.

Optifab 2003: Technical Digest. ed. / Harvey M. Pollicove; Walter C. Czajkowski; Toshihide Dohi; Hans Lauth. Vol. 10314 SPIE, 2003. 103140C.

Research output: Chapter in Book/Report/Conference proceedingConference contribution

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N2 - The Precessions™ process, and the 200mm capacity 'All' machine which embodies it, are outlined in the accompanying paper 'New method to control form and texture on industrially-sized lenses'. In this paper, we summarise the development of the process to address larger optical components. The first machines constructed for this application are of 600mm capacity. A conceptual design of a 1-2m class machine of bridge-configuration has also been produced. Unlike the All machine, the work-piece on these larger machines lies in a horizontal plane i.e. with the rotation axis of the work-piece spindle vertical. This configuration is preferred because it allows the use of a hydrostatic support-system for the work-piece; important for large optics in general, and particularly so for light-weight optics. The machine can also provide a clear vertical path above the work-piece for optical testing, or for access by lifting gear. It is interesting to consider how the Precessions™ process can be scaled. As an example, consider first that the type of surface-form remains the same; it might, for example, be an f/2 parabola. Consider then that the work-piece diameter, the bonnet-diameter, the polishing spot-sizes used, and the tool-path on the surface, are all stretched by a factor of two. The area of the polishing spot and that of the work-piece have both increased by a factor of 4, but the number of convolutions of the spiral tool-path remains the same. Since the spot instantaneously addresses the same fraction of the overall work-piece surface-area, the terms all cancels in relation to cycle-time. However, for the same tool-rpm, the larger bonnet delivers a proportionally higher surface-speed, because the polishing action is further from the axis of rotation. Therefore, the volumetric removal rate (proportional to contact-area times surface-speed) tends to vary as the cube rather than the square of the scale-factor. As a result, the larger bonnet working over the larger work-piece with the same polishing pressure, will remove twice the depth of material in the same time. The above scaling could, in principle, reduce cycle times on larger parts. In practice, we have found that excessive removal-rates lead to impractically short dwell-times, especially near the centre of a spiral tool-path. Such scaling also demands increased traverse-speeds and accelerations from the machine, and this rapidly becomes a limitation. Moreover, as the process converges on final form, smaller depths of material need to be removed. In practice, we have found it necessary to moderate the process by diluting the slurry and by moving from a polyurethane tool-surface to a material that delivers a lower-removal rate, such as Multitex. We are also developing bonnets that are compatible with operation at a reduced polishing pressure, in order to give additional control for fine-removal. The current status of the process development is as follows. A form error of ∼80nm peak-to-valley has been achieved aspherising a 100mm diameter part. This work revealed some subtle but significant limitations in the optimisation code, mostly concerned with convergence and residual ripples. Recent work has focussed on this aspect, and the revised code is now ready for polishing trials. A 150mm diameter part is currently in-process, and work is about to commence on a 300mm fused silica mirror for a space application.

AB - The Precessions™ process, and the 200mm capacity 'All' machine which embodies it, are outlined in the accompanying paper 'New method to control form and texture on industrially-sized lenses'. In this paper, we summarise the development of the process to address larger optical components. The first machines constructed for this application are of 600mm capacity. A conceptual design of a 1-2m class machine of bridge-configuration has also been produced. Unlike the All machine, the work-piece on these larger machines lies in a horizontal plane i.e. with the rotation axis of the work-piece spindle vertical. This configuration is preferred because it allows the use of a hydrostatic support-system for the work-piece; important for large optics in general, and particularly so for light-weight optics. The machine can also provide a clear vertical path above the work-piece for optical testing, or for access by lifting gear. It is interesting to consider how the Precessions™ process can be scaled. As an example, consider first that the type of surface-form remains the same; it might, for example, be an f/2 parabola. Consider then that the work-piece diameter, the bonnet-diameter, the polishing spot-sizes used, and the tool-path on the surface, are all stretched by a factor of two. The area of the polishing spot and that of the work-piece have both increased by a factor of 4, but the number of convolutions of the spiral tool-path remains the same. Since the spot instantaneously addresses the same fraction of the overall work-piece surface-area, the terms all cancels in relation to cycle-time. However, for the same tool-rpm, the larger bonnet delivers a proportionally higher surface-speed, because the polishing action is further from the axis of rotation. Therefore, the volumetric removal rate (proportional to contact-area times surface-speed) tends to vary as the cube rather than the square of the scale-factor. As a result, the larger bonnet working over the larger work-piece with the same polishing pressure, will remove twice the depth of material in the same time. The above scaling could, in principle, reduce cycle times on larger parts. In practice, we have found that excessive removal-rates lead to impractically short dwell-times, especially near the centre of a spiral tool-path. Such scaling also demands increased traverse-speeds and accelerations from the machine, and this rapidly becomes a limitation. Moreover, as the process converges on final form, smaller depths of material need to be removed. In practice, we have found it necessary to moderate the process by diluting the slurry and by moving from a polyurethane tool-surface to a material that delivers a lower-removal rate, such as Multitex. We are also developing bonnets that are compatible with operation at a reduced polishing pressure, in order to give additional control for fine-removal. The current status of the process development is as follows. A form error of ∼80nm peak-to-valley has been achieved aspherising a 100mm diameter part. This work revealed some subtle but significant limitations in the optimisation code, mostly concerned with convergence and residual ripples. Recent work has focussed on this aspect, and the revised code is now ready for polishing trials. A 150mm diameter part is currently in-process, and work is about to commence on a 300mm fused silica mirror for a space application.

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Walker DD, Beaucamp ATH, Brooks D, King A, Morton R, McCavana G. New polishing technology for large mirrors and lenses. In Pollicove HM, Czajkowski WC, Dohi T, Lauth H, editors, Optifab 2003: Technical Digest. Vol. 10314. SPIE. 2003. 103140C https://doi.org/10.1117/12.2284001