Aluminum (pure or alloy) mirrors attract increasing interest, having Young’s Modulus and density similar to glasses. Advantage of high diffusivity offsets disadvantage of high thermal expansion coefficient and means that the mirror reaches thermal equilibrium rapidly. High ductility supports extreme light-weighting and complex machining, including fluid-cooling channels in high-energy applications, and integral interface components. Aluminum mirrors are also tolerant to vibrations and shock loads. The material is amenable to single point diamond turning (SPDT) and does not require optical coating. However, SPDT tends to produce mid-spatial frequency artefacts, which are difficult to remove, especially for aspheres and free-forms. These introduce diffraction effects and compromise stray light performance. In our previous research, we have demonstrated the potential of industrial robots to automate manual interventions with CNC polishing machines, and to provide surface-processing capabilities in their own right. We have also presented research concerning the mismatch between rigid and semi-rigid tools (including non-Newtonian tools), and aspheric surfaces. In this paper, we report on polishing of spherical and aspheric aluminum mirrors using an industrial robot. This includes tool-design, tool-path generation, texture control and removal of the mid-spatial frequency artefacts. We have investigated removal-rates and textures achieved, using different specialized slurries, polishing pads and special tool-paths. An effective process has been established, achieving Sa of 5nm on a 400mm square witness sample and a 490mm elliptical off-axis parabolic mirror.
After the formal acceptance of our fabrication of E-ELT segments, we aim to further accelerate the mass production by introducing an intermediate grolishing procedure using industrial robots, reducing the total process time by this much faster and parallel link. In this paper, we have presented research outputs on tool design, tool path generation, study of mismatch between rigid, semi-rigid tool and aspheric surface. It is indicated that the generation of mid-spatial frequency is proportional to the grit size and misfit between work piece and tool surfaces. Using a Non-Newtonian material tool with a spindle speed of 30 rpm has successfully reduce the mid-spatial error. The optimization of process parameters involve the study the combination effects of the above factors. These optimized parameters will result in a lookup table for reference of given input surface quality. Future work may include the higher spindle speed for grolishing with non- Newtonian tool looking for potential applications regarding to form correction, higher removal rate and edge control.
This paper builds on previous reported work describing the marriage of robots and CNC polishing machines, both for the pre-processing of parts, and to automate operations hitherto manually conducted on the CNC platforms. This paper reviews strategies for metrology, then takes the work a stage forward by reporting the use of a robot to automate the exchange of a part between CNC machine and metrology station, the probing of the part, and the capture of interferometer data. This constitutes an important step towards realization of an automated manufacturing cell.
Following formal acceptance by ESO of three 1.4m hexagonal off-axis prototype mirror segments, one circular segment, and certification of our optical test facility, we turn our attention to the challenge of segment mass-production. In this paper, we focus on the role of industrial robots, highlighting complementarity with Zeeko CNC polishing machines, and presenting results using robots to provide intermediate processing between CNC grinding and polishing. We also describe the marriage of robots and Zeeko machines to automate currently manual operations; steps towards our ultimate vision of fully autonomous manufacturing cells, with impact throughout the optical manufacturing community and beyond.
Zernike polynomials have emerged as the preferred method of characterizing as-fabricated optical surfaces. From here,
over time, they have come to be used as a sparsely sampled representation of the state of alignment of assembled optical
systems both during and at the conclusion of the alignment process. We show here that it is possible to develop the field
dependence that analytically interconnects the coefficients of the Zernike polynomial (which has to-date been
characterized only by its aperture dependence) as a more complete representation of an aligned rotationally symmetric
optical system and in a paper to follow a misaligned optical system. This significant expansion to this valuable
polynomial provides an important new tool for characterizing high performance optical systems throughout the optical design, fabrication, assembly, and interim and acceptance test process.
In this paper we address two interrelated issues important to primary mirror segments for extremely large telescopes - edge-control, and the detailed topography over the segment surface. Both affect the intensity and distribution of stray
light and infrared emissivity. CNC polishing processes typically deploy spiral or raster tool-paths, tending to leave
repetitive features. We compare and contrast two novel families of pseudo-random tool-paths for Precessions CNC
polishing. We then show how CNC control of the three-dimensional tool-path can optimize edge-profiles. Finally, we
demonstrate fluid-jet polishing used to clean up residual edge defects.
The requirements of space and defence optical systems and ground-based astronomy (especially extremely large telescopes) are providing optical fabricators with new challenges. These challenges particularly concern process speed, determinism and automation, and tighter tolerances on surface form and texture. Moreover, there is a growing demand for complex off-axis and 'freeform' surfaces and for larger components of the ~1m scale.
With this in view, we first report on form-correction on a smaller analogue of the IRP1200: an IRP400 in service in industry. We then report on the design, commissioning and preliminary process-development results from the first of the scaled-up 1.2m capacity CNC polishing machine from Zeeko, Ltd. This machine delivers the 'Classic' bonnet-based process, together with two new processes: fluid-jet polishing and the hybrid soft-grinding/polishing process called 'Zeeko-Grolish.' We indicate how this trio of processes running on the same machine platform with unified software can provide an unprecedented dynamic range in both volumetric removal rate and removal spot-size. This leads into a discussion of how these processes may be brought to bear on optimal control of texture and form. Preliminary performance of the 1.2m machine is illustrated with results on both axially-symmetric and more complex removal regimes. The paper concludes with an overview of the relevance of the technology to efficient production of instrumentation-optics, space optics and segmented telescope mirrors.
The recent upsurge in the demand for off-axis and complex "freeform" optical surfaces is driving the development of novel processes for their fabrication. This paper focuses on recent developments of the Precessions CNC polishing process for freeform surfaces, including off-axis as a special case. First, the surface-prescription and metrology-data, and their relation to the data-input for the polishing machines, are considered. The relevance of consistent coordinate frames is emphasised. An outline of how the process can 'polish' a ground freeform part (improve the texture), and then 'figure' the part (reduce the form errors) is given. Specific experimental case-studies are then presented, illustrating the versatility of the process on different materials and forms. Recent work is included in which the process-speed has been moderated in order to remove tens of nanometres of stock material, rather then the more usual hundreds of nanometres to tens of microns as in the standard Precessions process. The relevance of this to improving the ultimate surface-precision that should be achievable by this method is described. As a final illustration, the potential of the process to the rapid fabrication of the hundreds to thousands of 1-2 metre class mirror segments required for extremely large telescopes is considered.