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Mid-spatial frequency (MSF) errors challenge freeform manufacture, not in the least due to tool-misfit. This can compromise the performance of functional surfaces and is difficult to remove by post-processing. Our previously reported work on an effective process-chain for aluminum polishing demonstrated the ability to remove MSFs by hard-tool grolishing. In this paper, we describe MSF removal on an aluminum mirror, deformed to a saddle-like freeform shape, using power spectral density (PSD) as a diagnostic. CNC Precessions bonnet polishing was optimized to minimize output MSFs, then a non-Newtonian (n-N) tool was used to attenuate the residual MSFs that were present. Our approach was distinct from the approach pioneered by University of Arizona, in that we adopted small-tool polishing on the saddle-like part, with removal rate restored by rotating the n-N tool. In order to define the optimum window of rotation speeds, the dynamic behavior of the n-N material was explored by modelling and experiments. The tool was deployed on an industrial robot, and we describe a novel 'hyper-crossing' tool-path with wide sweeping paths, which is the logical opposite of the unicursal zero-crossing paths we have previously reported. This new path has proved ideally suited to robots, given their high velocity/acceleration capabilities. Detailed results are presented from the PSD viewpoint.

In the context of industry 4.0, building data clouds in manufacturing technologies is the fundamental step to approach intelligent automatic process. This new technology has proved to efficiently and effectively monitor five important responses (removal rate, texture, surface accuracy, edge-profiles and mid-spatial frequencies) with more than 95% repeatability in real time. This faster and inexpensive process can serve for the nowadays high-quality segmented telescopes. We illustrate the underlying problem by reference to a case-study - the challenge of an average manufacturing rate of 11.4 segments per month for the 39.3m optical/infrared European Extremely Large Telescope (E-ELT) project.

Development of major optical projects demands great amounts of freeform optics, which requires mass-production technology. The atmospheric pressure plasma processing (APPP) based on chemical etching shows the advantage of high efficiency in form generation. However, the surface texture gets deteriorated with roughness to 60 nm Ra. Bonnet polishing can achieve ultra-smooth surface texture using flexible inflated tool. In this study, a combined processing chain consisting of APPP and bonnet polishing was proposed, in which APPP plays a role in form generation and bonnet polishing plays a role in rapid surface smoothing. Firstly, the form generation using APPP and the surface finishing using bonnet polishing were analyzed. Then the form-preserving ability of bonnet polishing under various conditions, including polishing parameters and surface form characteristics, was investigated experimentally. Consequently, the influence of bonnet polishing on surface form change was analyzed. Two sinusoidal freeform surfaces with 5 mm and 16 mm spatial period were generated using APPP and then smoothed using bonnet polishing. The experimental results show that the polishing induced form change is smaller than 20 nm PV with roughness improved to 2 nm Ra, which verifies the feasibility of the combined processing chain.

Edge mis-figure is regarded as one of the most difficult technical issues for manufacturing the segments of extremely large telescopes, which can dominate key aspects of performance. A novel edge-control technique has been developed, based on 'Precessions' polishing technique and for which accurate and stable edge tool influence functions (TIFs) are crucial. In the first paper in this series [D. Walker Opt. Express 20, 19787-19798 (2012)], multiple parameters were experimentally optimized using an extended set of experiments. The first purpose of this new work is to 'short circuit' this procedure through modeling. This also gives the prospect of optimizing local (as distinct from global) polishing for edge mis-figure, now under separate development. This paper presents a model that can predict edge TIFs based on surface-speed profiles and pressure distributions over the polishing spot at the edge of the part, the latter calculated by finite element analysis and verified by direct force measurement. This paper also presents a hybrid-measurement method for edge TIFs to verify the simulation results. Experimental and simulation results show good agreement.

The removal of mid-spatial-frequency errors is a challenging issue in most subaperture polishing technologies. A novel "grolishing" technology has been developed to deal with grinding errors of spatial wavelengths from 1 to 50 mm with the help of power spectral density analysis and filter theory. This grolishing process was implemented on Zeeko's IRP polishing machine, on which all the subsequent polishing was performed. This has greatly reduced the process time. Although different abrasive have been used, the process is self-contained. The process parameters have been optimized to leave an edge upstand of peak-to-valley of 1 μm over a width of 40 mm.

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.