Progress in 4m class ZERODUR mirror production

The first monolithic ZERODUR® 4 m class mirror was ordered by the German Max Planck Institute for Astronomical Physics in 1968. A ratio of 1:6 for thickness to diameter ratio ensured the necessary stiffness to minimize deformation under gravity load. The technological ability to actively compensate the bending of the mirror substrate under gravity initiated the development from heavy non active thick mirror substrates to ever thinner thicknesses starting with the NTT, the New Technology Telescope of ESO. The thinner the mirror substrates are becoming the more demanding are the requests on homogeneity of material properties to ensure best performance over the clear aperture at every spot. In this paper we present results on material properties achieved for the 4 m class mirror substrates recently delivered by SCHOTT. The CTE homogeneity, the internal quality regarding striae, bubbles and inclusions as well as stress birefringence data are reported. Improvements in CNC processing and overall manufacturing process for the very thin 4 m class blanks are discussed.