being revalidated to be ready for construction. One year after signing the contract, and after the laying of the first stone ceremony in May 2017, the site was handed over to ACe, signifying the beginning of the construction of the dome's main structure. Astronomical performance In terms of astronomical performance the dome is required to be able to track about the 1-degree zenithal avoidance locus as well as preset to a new target within 5 minutes. This requires the dome to be able to accelerate and move at angular speeds of 2 degrees/s (the linear speed is approximately 5 km/h). The dome is designed to allow complete freedom to the telescope so that it can position itself whether it is opened or closed. It will also permit observations from the zenith down to 20 degrees from the horizon. Windscreen With such a large opening, the ELT dome requires the presence of a windscreen to protect the telescope's mirrors (apart from the secondary), from direct exposure to the wind. The baseline design of the windscreen minimises the volume required to house it. Two spherical blades, either side of the observing slit doors, slide in front of the telescope aperture to restrict the wind. Ventilation and air-conditioning The dome design ensures that the dome provides sufficient ventilation for the telescope not to be limited by dome seeing. For this the dome is also equipped with louvers, whereby the windscreen is designed to allow them to fulfill their function. Computational fluid dynamic simulations and wind tunnel work are being carried out to study the airflow in and around the dome, as well as the effectiveness of the dome and windscreen in protecting the telescope. Besides being designed for water-tightness, air-tightness is also one of the requirements as it is critical to minimise the air-conditioning load. The air-conditioning of the dome is necessary not only to thermally prepare the telescope for the forthcoming night but also in order to keep the telescope optics clean. The air-conditioning of the telescope during the day is critical and the current specifications permit the dome to cool the telescope and internal volume by 10 °C over 12 hours. Science goals The ELT will search for extrasolar planets – planets orbiting other stars. This will include not only the discovery of planets down to Earth-like masses through indirect measurements of the wobbling motion of stars perturbed by the planets