the mirror blanks from Schott, and polish one mirror segment per day to meet the 7-year deadline. During this process, each segment will be polished until it has no surface irregularity greater than 7.5 nm RMS. Afterwards, Safran Reosc will then mount, test, and complete all optical testing before delivery. This is the second largest contract for the ELT construction and the third-largest contract ESO has ever signed. The segment support system units for the primary mirror are designed and produced by CESA (Spain) and VDL (the Netherlands). The contracts signed with ESO also include the delivery of detailed and complete instructions and engineering drawings for their production. Additionally, they include the development of the procedures required to integrate the supports with the ELT glass segments; to handle and transport the segment assemblies; and to operate and maintain them. Secondary mirror Making the secondary mirror is a major challenge as it is highly convex, and aspheric. It is also very large; at 4.2 metres in diameter and weighing 3.5 tonnes, it will be the largest secondary mirror ever employed on an optical telescope and the largest convex mirror ever produced. In January 2017, ESO awarded a contract for the mirror blank to Schott AG, who will manufacture it of Zerodur. Complex support cells are also necessary to ensure the flexible secondary and tertiary mirrors retain their correct shape and position; these support cells will be provided by SENER. The pre-formed glass-ceramic blank of the secondary mirror will then be polished, and tested by Safran Reosc. The mirror will be shaped and polished to a precision of 15 nanometres (15 millionths of a millimetre) over the optical surface. Tertiary mirror The 3.8-metre concave tertiary mirror, also cast from Zerodur, will be an unusual feature of the telescope. Most current large telescopes, including the VLT and the NASA/ESA Hubble Space Telescope, use just two curved mirrors to form an image. In these cases, a small, flat tertiary mirror is sometimes introduced to divert the light to a convenient focus. However, in the ELT the tertiary mirror also has a curved surface, as the use of three mirrors delivers a better final image quality over a larger field of view than would be possible with a two-mirror design. Quaternary mirror The 2.4-metre quaternary mirror is a flat adaptive mirror, and only 2 millimetres thick. With up to 8000 actuators, the surface can be