The next generation of ground based telescopes under development require many hundreds of metre scale off-axis mirror segments. These are made in low expansion glass or glass ceramics such as ULE~?, Zerodur~?, Astroitall~? or Clearceram~?. Their successful fabrication places extreme demands on manufacturing process chain operations: including grinding with high form accuracy, low sub surface damage and roughness, and critically faster processing times. At 42m in diameter the ESO European Extremely Large Telescope (E-ELT) primary mirror is constructed from 984 hexagonal segments, each 1.45m wide and just 50mm thick.
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