Hexagon inspects mirrors for the European Extremely Large Telescope

The current production is for prototype mirror segments. Once these are signed off Cranfield University, as part of an as yet un-named UK production company, will be able to bid for the manufacturing of more segments. The production and measurement challenges for this project are significant challenges for both Cranfield University and Hexagon Metrology. "Cranfield University is uniquely placed in the UK with the capability to undertake various stages of machining the mirror segments to the accuracy required", explains Professor Paul Shore, head of Cranfield University precision engineering. "We have developed Cranfield's BoX (Big OptiX), a specialised grinding and measurement system, at Cranfield for realising these mirrors." A Leitz PMM-F 30.20.10 CMM from Hexagon Metrology is used to verify the performance of the Cranfield BoX grinding machine and measures the mirror segments. With the Cranfield BoX and the Leitz PMM-F it is expected that each mirror segment can be ground within 20 hours, said to be up to ten times faster than the competition. After processing at Cranfield, the mirrors are sent for polishing. The segments are polished utilising error surface maps generated from the Leitz PMM-F. These identify high and low points for initial corrective polishing. The polished quality requirement is a surface roughness of 1-2 nm RMS (root mean square) and form accuracy of 10 nm RMS. An 8 m optical test tower is used to verify surface finish accuracy and to ensure accurate alignment of the tower a Leica Absolute Tracker AT901 from Hexagon Metrology is integrated into the tower structure. The laser tracker system measures the positioning of the tower's main optics during measurements, tracking any movement due to thermal effects which can be some microns over such a large structure. Built by the European Southern Observatory (ESO) the E-ELT, a ground-based telescope, will be 42 m in diameter and made up of 1,000 hexagonal mirror segments, each 1.5 m wide and just 5 cm thick. The E-ELT is four to five times larger and will gather 15 times more light than the largest optical telescopes operating today.