not be individually radially symmetrical, necessitating a modification of the usual polishing and testing procedures. The mirrors are being constructed by the University of Arizona's Steward Observatory Richard F. Caris Mirror Lab. The casting of the first mirror, in a rotating furnace, was completed on November 3, 2005, but the grinding and polishing were still going on 6 years later when the second mirror was cast, on 14 January 2012. A third segment was cast in August 2013, the fourth in September 2015, the fifth in 2017, and the sixth in 2021. The casting of each mirror uses 20 tons of E6 borosilicate glass from the Ohara Corporation of Japan and takes about 12–13 weeks. After being cast, they need to cool for about six months. Polishing of the first mirror was completed in November 2012. As this was an off-axis segment, a wide array of new optical tests and laboratory infrastructure had to be developed to polish the mirror. The intention is to build seven identical off-axis mirrors, so that a spare is available to substitute for a segment being recoated, a 1–2 week (per segment) process required every 1–2 years. While the complete telescope will use seven mirrors, it is planned to begin operation with four mirrors. The primary mirror array as a whole will have a focal ratio (focal length divided by diameter) of f/0.71. For an individual segment – being one third that diameter – this results in a focal ratio of f/2.14. The overall focal ratio of the complete telescope will be f/8 and the optical prescription is an aplanatic Gregorian telescope. Like all modern large telescopes it will make use of adaptive optics. Scientists expect very high quality images due to the very large aperture and advanced adaptive optics. Image resolution should exceed that of the Hubble Space Telescope. Support structure The telescope structure is an alt-azimuth design and it will stand on a pier that is 22 meters in diameter. In late October 2019 GMTO announced the signing of a contract with German company MT Mechatronics (subsidiary of OHB SE) and Illinois-based Ingersoll Machine Tools, to design, build and install the GMT's telescope structure. The structure will weigh 1,800 tons without mirrors and instruments. With mirrors and instruments it will weigh 2,100 tons. This structure will float on a film of oil (50 microns thick), being supported by a number of hydrostatic bearings.