Hexagon Metrology participates in the control of the mirrors of the giant European telescope E-ELT (European Extremely Large Telescope) being built by the European observatory ESO (European Southern Observatory). The E-ELT is a terrestrial telescope that will be 42 m diameter and will be composed of 1000 hexagonal mirror segments each measuring 1.50 m wide and barely 5 cm thick. Four to five times the size of the largest optical telescopes in use today, the E-ELT will also collect 15 times more light. The involvement of Hexagon Metrology in this program is through Cranfield University (United Kingdom), which is working on the machining of 7 telescope mirror segments. Each polished part must have a surface roughness with an accuracy of 1-2 nanometers rms (root mean square error) and a shape accuracy of 10 nanometers. The university has developed the BoX (Big OptiX), a measuring and smoothing system specially designed for the production of these mirrors. This system is controlled by a Leitz PMM-F 30.20.10 CMM from Hexagon Metrology . The combined action of the BoX and the Leitz PMM-F machine should allow each mirror segment to be smoothed in 20 hours, a performance advertised as 10 times faster than the competition. Surface polishing control is meanwhile ensured by an optical test tower of 8 m. The Leica Absolute Tracker AT901 from Leica Geosystems (Hexagon Metrology) , which is integrated into the tower structure, serves to ensure the precise alignment of the tower. The laser tracker system monitors the position of the main optics of the tower during measurements and detects any movements due to thermal effects, which can reach several micrometers.
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