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

The next generation of ground based telescopes require many hundreds of metre scale off-axis mirrors. In this paper the grinding of a 1.45 m scale Zerodur® mirror segment for the European Extremely Large Telescope (E-ELT) is introduced. Employing an R-theta grinding mode with a multi stage grinding process material removal rates of up to 187.5 mm3/s are achieved, whilst typically removing up to 1 mm depth of material in total. Results show a RMS form error of <1 μm, with subsurface damage <10 μm, and a production cycle time under 20 h.