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.
The manufacture of the next generation of large astronomical telescopes, the extremely large telescopes (ELT), requires the rapid manufacture of greater than 500 1.44m hexagonal segments for the primary mirror of each telescope. Both leading projects, the Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT), have set highly demanding technical requirements for each fabricated segment. These technical requirements, when combined with the anticipated construction schedule for each telescope, suggest that more than one optical fabricator will be involved in the delivery of the primary mirror segments in order to meet the project schedule. For one supplier, the technical specification is challenging and requires highly consistent control of metrology in close coordination with the polishing technologies used in order to optimize production rates. For production using multiple suppliers, however the supply chain is structured, consistent control of metrology along the supply chain will be required. This requires a broader pattern of independent verification than is the case of a single supplier. This paper outlines the metrology requirements for a single supplier throughout all stages of the fabrication process. We identify and outline those areas where metrology accuracy and duration have a significant impact on production efficiency. We use the challenging ESO E-ELT technical specification as an example of our treatment, including actual process data. We further develop this model for the case of a supply chain consisting of multiple suppliers. Here, we emphasize the need to control metrology throughout the supply chain in order to optimize net production efficiency.
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.
Severely aspheric lenses have surfaces which depart from spherical by millimeters rather than microns. Optical testing of these lenses can be achieved using aspheric null mirrors. Although such mirrors can give full null compensation at diffraction limited precision most severely aspheric lenses are made by molding for applications which allow many fringes of form error. Diamond turned aspheric null mirrors verified by Form Taysurf methods give a test lens plus mirror systems which can be tested using either interference or a less sensitive autocollimation or encircled energy method more economical and more appropriate to the quality required.