ELT design progress: The Dome structure

The Dome of the Extremely Large Telescope (ELT) is under construction at Cerro Armazones, in the Chilean Andes. It is constituted by a concrete pier, with an 86 m diameter concrete wall, and a rotating enclosure on it; the maximum height is about 80 m. The Dome will protect the 40 m class optical telescope, inside it, and must withstand wind speeds of over 40 m/s, as well as strong earthquakes. The whole structure is seismically isolated at the base, for an overall seismic mass of about 35000 t. The rotating enclosure main elements are a truss steel structure, having a base ring and a series of arch girders. It has a hemispherical shape to enhance the aerodynamic behavior and it weighs close to 6500 t. Two slit doors allow the telescope observation, guaranteeing a 42 m wide and 64 m long opening. The enclosure's Azimuth Rotation Mechanism is constituted by 36 trolleys, installed on the top beam of the concrete pier, on a diameter of 86 m. Cladding covers the Dome structure and it is designed in order to provide proper thermal insulation and to withstand the harsh environmental site conditions. A windscreen, made of four permeable panels, having 42 m span and 10 m height each, protects the Telescope during observation and controls the airflow around it, together with a series of 89 louvers, placed both on the rotating and the fixed part of the Dome. In the Auxiliary Building, which is a ring surrounding the pier, technical rooms to operate and maintain the telescope are hosted. A custom HVAC system controls the temperature with a ±2 °C precision inside the Telescope chamber having about 300000 m3 volume.