There is a growing requirement for larger quantities of large sized, accurate, free-form optical surfaces. One example of such a demand is the new generation of ground-based Extremely Large Telescopes (ELTs). Together with Cranfield University Hembrug has been engaged in the manufacture and metrology of seven prototype segments for the E-ELT primary mirror. The mirror segments are ground to a form accuracy < 1 micrometre (µm) RMS as the input quality to figure corrective processes. The 42m primary mirror has 984 hexagonal segments, each finished segment an off-axis ellipsoid, around 1.5 metres across corners, with ~84 metres base radius of curvature and typically ~100 micrometres p-v departure from nominal sphere. Each segment is ground to final shape from a hexagonal blank, using the Hembrug BoX® ultra-precision grinding and measurement system in a grinding time under 20 hours, with subsurface damage restricted to less than 10 µm. Optical figure is measured in-situ by the machine, and then the optic is fully characterised off-machine with traceable metrology, for all its geometric features.
Below: there is a growing need for large mirror segments for extremely large telescopes.