Grinding metre scale mirror segments for the E-ELT ground based telescope

Comley, Paul; Morantz, Paul; Shore, Paul; Tonnellier, Xavier

doi:10.1016/j.cirp.2011.03.120

Cited by 68 publications

(37 citation statements)

References 7 publications

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“…The grinding leaves clean edges with no measurable misfigure. Grinding an off-axis aspheric full-size segment, the form-error is <1 μm RMS and 6 μm p-v form-error (measured by coordinate measuring machine, with the segment still mounted on its precision diamond-turned, hard grinding fixture) [11,12]. Sub-surface damage is ~ 6 μm on Zerdor; a little more on ULE [13].…”

Section: Overview Of the New Manufacturing Approachmentioning

confidence: 99%

Edge-control and surface-smoothness in sub-aperture polishing of mirror segments

et al. 2012

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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 E-ELT segment specification, and results on a full-aperture spherical segment showing excellent surface smoothness.

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Section: Overview Of the New Manufacturing Approachmentioning

confidence: 99%

Edge-control and surface-smoothness in sub-aperture polishing of mirror segments

et al. 2012

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“…This is then mounted into the Cranfield University BoX TM ("Big Optics") CNC grinding machine [6]. The off-axis asphere is then ground into the hexagonal part.…”

Section: Grinding the Base-aspherementioning

confidence: 99%

Advanced Abrasive Processes for Manufacturing Prototype Mirror Segments for the World’s Largest Telescope

Walker

Davies

Fox-Leonard

et al. 2014

AMR

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This paper describes the application of loose-abrasive processes to the manufacture of 1.4 meter, off-axis aspheric, hexagonal mirror-segments. These are prototypes for the 39 m European Extremely Large Telescope (E-ELT). The application of active forces to correct the overall form of segments in the telescope, means that the overall form-accuracy achieved in polishing can be less critical than for a non-active mirror. However, it is a requirement that the base-radii and conic constants of mirror-segments are very closely matched, so that the combined image is not degraded. This means that abrasive processes have to operate with respect to anabsoluterather thanrelativedatum. Furthermore, there are stringent requirements on mid-spatial frequency defects on segment surfaces, and on edge-roll. These control stray-light, and ultimately detectability of faint astronomical targets. We describe the CNC abrasive techniques we have developed in response. We then demonstrate the success of the approach, which represents the first time ever that segments have been processed entirely in the hexagonal shape:- a milestone in loose-abrasive processing. Finally, we address up-scale for the unprecedented number of segments required for the E-ELT build-phase.

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“…The proposed process chain starts with ultra-precision CNC hard-grinding the off-axis aspheric form directly into the hexagonal segment [3,4]. Cranfield University upgraded their BoX machine from 1m to 1.5m capacity for this purpose.…”

Section: The Technical Approach Adoptedmentioning

confidence: 99%

Fast manufacturing of E-ELT mirror segments using CNC polishing

et al. 2013

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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 TM 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.

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Grinding metre scale mirror segments for the E-ELT ground based telescope

Cited by 68 publications

References 7 publications

Edge-control and surface-smoothness in sub-aperture polishing of mirror segments

Edge-control and surface-smoothness in sub-aperture polishing of mirror segments

Advanced Abrasive Processes for Manufacturing Prototype Mirror Segments for the World’s Largest Telescope

Fast manufacturing of E-ELT mirror segments using CNC polishing

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