Polishing of the blanks will be distributed amongst three partner countries, with efforts to polish and machine the blanks already started in the USA. Manufacturing of the primary mirror blanks is very advanced with over half of the segment blanks already fabricated. The segments are made from Clearceram, a zero expansion glass-ceramic and are 45 mm thick. The need for large handling equipment, high-capacity handling cranes and large vacuum coating chambers is also reduced. Primarily, with the segments being of manageable size, many of the difficulties involved in the construction of large telescopes are reduced, including fabrication, testing, risk mitigation and transportation of large mirrors and mirror cells. The segmented primary design provides a number of advantages.
The segments are closely spaced, with segment gaps of only 2.5 mm (0.1 inch). As the name implies, the primary mirror is 30 meters (98 feet) in diameter, and because it is f/1 it has a focal length of 30 meters.įollowing the lead of the highly successful Keck 10-meter telescopes, the TMT primary mirror is segmented, with 492 hexagonal elements, each measuring 1.44 meters (56.6 inches) corner-to-corner. This short focal ratio was chosen to make the telescope compact, which helps to keep the telescope structure and the enclosure affordable. The image is formed 20 meters from the center of the tertiary mirror, with the telescope focal ratio being f/15. The tertiary mirror folds and steers the science beam to any of eight instruments, mounted on the two Nasmyth platforms. Both the primary and secondary mirrors are hyperboloids, and together form a well-corrected image. The TMT telescope optical design is a folded Ritchey-Chrétien.
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