For the (CNC) polishing of aspheres, generally a compliant, sub-aperture tool is applied, which may cause mid- spatial frequency errors on the surface of the workpiece. The tolerance on surface figure is commonly given in peak-to-valley (PV) or root-mean-square (RMS). Even if a surface is fabricated within specified tolerances according to one of the mentioned metrics, the optical performance may be inadequate for the desired application. For the specification of the tolerance on mid-spatial frequency errors, several other characteristics have been proposed, e.g. power spectral density (PSD) or surface slope error. This paper presents an investigation into the mid-spatial frequency form error of mass-produced aspheres, discusses the results and draws relevant conclusions.
Many processes applied in Computer Controlled Optical Surfacing (CCOS)1–4 utilise a conformal polishing tool (e.g. the Precessions™ process) which is well suited to correct surface errors that are larger than or similar in size as the contact area between tool and surface, applying a different dwell time as needed. However, due to the conformal nature of the tool it has no significant effect on surface errors with smaller dimensions: the peaks and valleys of the error will be polished equally.
To remove these smaller dimension errors with a conformal tool is impractical. The preferred approach therefore is to apply a sufficiently rigid tool of sufficient size so that it preferentially removes material from the peaks and not from the valleys. A larger tool is capable of smoothing surfaces where the peaks are further apart and also benefits from a larger removal rate, reducing the total process time.
To achieve a uniform contact the form of the tool should be the inverse of the local form of the surface. This is trivial for spherical or plano surfaces, but present problems when the surface is aspherical. The mismatch between a rigid tool and the workpiece increases with the size of the tool for a given asphere, which leads to an upper limit of the tool size that can be used.
The work reported in this article presents a numerical analysis of the mismatch of rigid tools applied on E-ELT prototype segments. It can be readily applied to aspheric or free-form surfaces for which an analytical approach is difficult or impossible and furthermore it provides a detailed analysis of the form of this mismatch, including spatial frequency content. Additionally, an analysis and experimental work is presented to determine the applicability of sub-aperture rigid tools for the polishing of E-ELT segments.
We report on the first-ever demonstration of grinding and polishing full-size, off-axis aspheric, mirror segments as
prototypes for an extremely large telescope, processed entirely in the final hexagonal shape. We first describe the overall
strategy for controlling form and mid spatial frequencies, at levels in the vicinity of <10nm RMS surface. This relies first
on direct CNC grinding of the base-form of these 1.4m segments, using the Cranfield BoX™ machine. The segments are
then mounted on a custom designed (Optic Glyndwr Optoelectronic Engineering Group) three segment hydraulic
support, and CNC polished on a Zeeko IRP 1600 machine using a variety of custom tooling. We overview the fullaperture
and sub-aperture metrology techniques used to close the process-loop and certify quality, all of which operate
with the segment in-situ on the IRP1600. We then focus on the pristine edge-definition achieved by the combination of
tool-lift and smoothing operations; results never previously demonstrated on full-size pre-cut hexagonal segments.
Finally, the paper discusses the feasibility of scaling the process to deliver 931 segments in seven years, as required for
the E-ELT project.
This paper addresses two challenges in establishing a new process chain for polishing hexagonal segments for
extremely large telescopes:- i) control of edge and corner profiles in small-tool polishing of hexagons, and ii)
achieving the required smoothness of the bulk aspheric form. We briefly describe the performance of a CNC-grinding
process used to create the off-axis asphere, which established the input-quality for subsequent processing. We then
summarize processes for smoothing ground mid-spatials and pre- and corrective polishing using Zeeko CNC
machines. The impact of two cases is considered; i) all processing stages are performed after the segment is cut
hexagonal, and ii) final rectification of a hexagon after cutting from an aspherised roundel, as an alternative to ionfiguring.
We then report on experimental results on witness samples demonstrating edges and corners close to the EELT
segment specification, and results on a full-aperture spherical segment showing excellent surface smoothness.
We describe progress on a novel process-chain being used to produce eight 1.4m hexagonal segments as prototypes for
the European Extremely Large Telescope - a Master Spherical Segment as a reference, and seven aspheric segments. A
new pilot plant integrates a bespoke full-aperture test-tower designed and built by OpTIC Glyndwr, with a Zeeko 1.6m
polishing machine. The process chain starts with aspherising hexagonal segments on the Cranfield BoX™ grinder,
followed by smoothing, corrective-polishing and edge-rectification using the Zeeko CNC platform. The paper describes
the technology and progress, and anticipates how the process-chain is expected to evolve through the seven segments to
increase both process-speed and surface-quality.
The footprint of the Fluid Jet Polishing process is determined by the shape of the nozzle as well as by the orientation of the slurry beam with respect to the local surface normal. Besides, no tool wear occurs and the footprint remains constant during the manufacturing process allowing shape corrections in a deterministic way. To that aim, FJP has been implemented on a CNC machine and applied for both shaping of previously polished aspheres and polishing of fine ground a-spheres. In this paper, results will be presented showing the application of FJP as a sub-aperture shape correction method. Besides, experimental data will be reported demonstrating FJP's capability of polishing previously fine ground surfaces. The wear rate depends on the sharpness of the abrasives and their kinetic energy. It can thus be adjusted by various parameters, among others the applied pressure, slurry concentration and abrasive sizes. In this paper, an additional process parameter is identified allowing the application of the same polishing compound for wear rates ranging from nanometers to micrometers. This large wear range is achieved by mixing a well controlled amount of gas into the slurry flow allowing the abrasives to travel at higher speeds.
This article presents the recent achievements with Jules Verne, a sub-aperture polishing technique closely related to Fluid Jet Polishing . Whereas FJP typically applies a nozzle stand-off distance of millimeters to centimeters, JV uses a stand-off distance down to 50 μm. The objective is to generate a non-directional fluid flow parallel to the surface, which is specifically suited to reduce the surface roughness [2, 3]. Different characteristic Jules Verne nozzle geometries have been designed and numerically simulated using Computational Fluid Dynamics (CFD). To verify these simulations, the flow of fluid and particles of these nozzles has been visualized in a measurement setup developed specifically for this purpose. A simplified JV nozzle geometry is positioned in a measurement setup and the gap between tool and surface has been observed by an ICCD camera. In order to be able to visualize the motion of the abrasives, the particles have been coated with fluorescence. Furthermore, these nozzles have been manufactured and tested in a practical environment using a modified polishing machine. The results of these laboratory and practical tests are presented and discussed, demonstrating that the CFD simulations are in good agreement with the experiments. It was possible to qualitatively predict the material removal on the processed glass surface, due to the implementation of appropriate erosion models [4, 5] in the CFD software.
Through single scratch experiments on glass, the transition from ductile to brittle mode grinding is analysed and the ductile regime dependent on load and coolant type is determined. The influence of different coolants on the grinding mode is discussed and certain manufacturing "tricks" used in the optical shop are explained. The results are verified by loose abrasive grinding experiments.
High detection performance is required for an operational system for the detection of landmines. Humanitarian de-mining scenarios, combined with inherent difficulties of detecting landmines on an operational (vibration, motion, atmosphere) as well as a scenario level (clutter, soil type, terrain), result in high levels of false alarms for most sensors. To distinguish a landmine from background clutter one or more discriminating object features have to be found. The research described here focuses on finding and evaluating one or more features to distinguish disk-shaped landmines from background clutter in infrared images. These images were taken under controlled conditions, with homogenous soil types. Two methods are considered to acquire shape-based features in the infrared imagery. The first method uses a variation of the Hough transformation to find circular shaped objects. The second method uses the tophat filter with a disk-shaped structuring element. Furthermore, Mahalanobis and Fisher based classifiers are used to combine these features.