ELT design status: the most powerful ground telescope

The ELT, Extremely Large Telescope, a 40m class optical, near and mid-infrared telescope that will be installed on Cerro Armazones, on the Chilean Andes, will be characterized by a an alt-azimuthal steel structure mounting weighting about 3700 tons. The telescope design consists of a highly optimized, space-frame structure, whose deflection characteristics have been carefully tuned to facilitate the performance of the oil film associated with the hydrostatic bearing system and the performance of the rotation associated with the drive and encoder systems. The Altitude structure design incorporates the M1 Mirror cell and hosts all the telescope optics. The major challenges in the design are the need to keep the primary mirror segments within a reasonable range from the prescribed locations and the need to minimize the static and dynamic deflections of the secondary mirror. The telescope rotates on tracks fixed to a concrete pier with a diameter of about 52m, completely isolated from the ground by means of special seismic devices. Power, data, control cables and fluid hoses follow the azimuth range of 550 degrees by means of the 18m diameter cable wrap, and the altitude range of 96 degrees with two lateral cable drapes. The rotation of the Azimuth and Altitude axes are possible thanks to the Direct Drive System and kept in place by high precision incremental tape encoders. The telescope is equipped with a very performing Control System that implements state of the art automation technologies, such as isochronous real time fieldbus, communication protocols, absolute time synchronization and safety, state of the art software engineering methods, such as object oriented design and iterative AGILE software development methodology.