infrastructures, such as the Astronet Science Vision and Infrastructure Roadmap and the ESFRI Roadmap. The telescope underwent a Phase B study in 2014 that included "contracts with industry to design and manufacture prototypes of key elements like the primary mirror segments, the adaptive fourth mirror or the mechanical structure (...) [and] concept studies for eight instruments". Design The ELT will use a novel design with a total of five mirrors. The first three mirrors are curved (non-spherical), and form a three-mirror anastigmat design for excellent image quality over the 10 arcminute field of view (one third of the width of the full Moon). The fourth and fifth mirrors are (almost) flat, and respectively provide adaptive optics correction for atmospheric distortions (mirror 4) and tip-tilt correction for image stabilisation (mirror 5). The fourth and fifth mirrors also send the light sideways to one of two Nasmyth focal stations at either side of the telescope structure, allowing multiple large instruments to be mounted simultaneously. ELT mirror and sensors contracts Primary mirror The 39-metre primary mirror will be composed of 798 hexagonal segments, each approximately 1.4 metres across and with 50 mm thickness. Two segments will be re-coated and replaced each working day, to keep the mirror always clean and highly reflective. Edge sensors constantly measure the positions of the primary mirror segments relative to their immediate neighbours. 2394 position actuators (3 for each segment) use this information to adjust the system, keeping the overall surface shape unchanged against deformations caused by external factors such as wind, gravity, temperature changes and vibrations. In January 2017, ESO awarded the contract for the fabrication of the 4608 edge sensors to the FAMES consortium, which is composed of Fogale and Micro-Epsilon. These sensors can measure relative positions to an accuracy of a few nanometres, the most accurate ever used in a telescope. In May 2017, ESO awarded two additional contracts. One was awarded to Schott AG who will manufacture the blanks of the 798 segments, as well as a maintenance set of 133 additional segments. The latter allow segments to be removed, replaced, and recoated on a rotating basis once the ELT is in operation. The mirror will be cast from the same low-expansion ceramic Zerodur as the existing Very Large Telescope mirrors in Chile. The other contract was awarded to the French company, Safran Reosc, a subsidiary of Safran Electronics & Defense. They will receive