The
European Southern Observatory’s Extremely Large Telescope (ESO’s ELT), currently being built in Chile, will be the world’s largest visible/near-infrared telescope when completed in 2028. Its 39m-wide main mirror comprises 798 separate hexagonal pieces, attached to a base structure using steel flanges and
SKF Vibracon ‘adjustable chocks’.
Marco Colussi, a structural engineer at the Italian engineering firm
Cimolai, which is responsible for assembling the mirror, said: “Each flange sits on four adjustable chocks, and the ability to readjust each mirror segment is key to the success of the telescope; and while each low-profile Vibracon unit typically allows up to 12mm of adjustment, this application needed up to 15mm and necessitated several design tweaks.”
First, Cimolai inserted extra washers, allowing the adjustment range to be extended to 15mm. “This had to be performed with a positioning accuracy of tenths of millimetre, across the 39m span of the mirror. The chocks also had to fit into a limited space that was restricted to 25mm in height — and they had to be installed upside-down, to help meet the stiffness requirements of the system and optimise the telescope’s stability and performance.”
Overall, the main mirror will use more than 9,500 SKF Vibracon SM 16 ELP-ASTR low-profile chocks, 12 for each mirror segment. Chocks are individually surface treated, to give them consistent quality and extend their performance in the ELT’s demanding environment.
Mr Colussi concluded: “The ELT will cost an estimated 1.4 billion euros to build and will, among other things, search for exoplanets and study early galaxies. It is hard to think that, for its vast size and cosmic ambitions, its success will rely partly on adjustable chocks just 3in in diameter.”