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E-ELT will have a five-mirror optical design: three-mirror on-axis anastigmat and two-fold mirrors for adaptive optics

The primary mirror will be 39 metres in diameter, comprised of 798 hexagonal 1.4m segments

E-ELT will gather 100 million times more light than the human eye

Each adaptive mirror is supported by 6,000 actuators that can distort its shape 1,000 times per second

The E-ELT site is at an altitude of 3,060 metres on Cerro Armazones, a mountain in the central part of Chile’s Atacama Desert

The telescope is due for completion in 2022; the tender for primary mirror segments is due in 2014/15

The whole telescope is considered to be an on-axis system because everything is axis- symmetric, but any individual optic by itself is off-axis; i.e. the optical axis and the mechanical centre of the aperture are not aligned. John Filhaber, internal astronomical systems expert at optical metrology company Zygo, explains: ‘When you say off-axis and asphere in the same sentence it strikes fear into the hearts of optical engineers and optical manufacturers everywhere.’ In terms of manufacturability, there are two processes involved in making these off-axis segments: ‘Vibration insensitive aspheric metrology is essential to be able to handle the task of measuring the off-axis aspheric segments,’ explains Filhaber. ‘But, you also need advanced computer controlled polishing (CCP) equipment to finish these mirrors to within a few tens of nanometres.’ The team in Glyndwr has developed technologies to overcome these challenges. This was achieved with a new process chain, which was demonstrated on subscale parts, and then developed on full-scale equipment. Walker says: ‘We have conquered for the first time the challenge of controlling the edges of the segments in polishing.’ The UK National Facility is polishing the segments as hexagons all the way through. Polishing large parts and maintaining good edges was previously deemed to be unfeasible, because polishing tends to roll edges down. This hexagonal shape avoids intermediate handling of the segment, reducing


Four segments of the giant primary mirror of the E-ELT undergoing testing together for the first time

risk of accidental damage, and circumvents the need for ion-figuring. Therefore, it has the potential to be easily scaled up for integrated segment mass production, with a high level of automation. The process starts with ultra-precision grinding of the off-axis asphere on the pre-cut hexagonal segment. This is carried out with the BoX machine, designed and developed by Cranfield University and manufactured by Cranfield Precision. Then, the segment is polished and corrected on a 1.6m-capacity IRP1600 Zeeko Computer Numerical Control (CNC) polishing machine – the first machine built of this size. Bonnet polishing with variable spot-size capability was used, in combination

with a local smoothing technique. Together, these have demonstrated highly-effective control of overall form, mid-spatial frequencies, edges and local texture. A major metrological issue is that the segments have a very long radius of curvature – 84m – ruling out a direct test to measure the centre of curvature. To tackle this problem, the Glyndwr team has compacted full aperture interferometry into a 10m high test tower directly over the IRP1600 machine, allowing the segments to be measured in situ. This is complemented by sub-aperture stitching interferometry and white-light texture interferometry, also conducted on-machine. Although the segments are being made over a

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European Southern Observatories (ESO)

European Southern Observatories (ESO)

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