The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of 4,145 metres (13,600 ft) near the summit of Mauna Kea in Hawai'i. The primary mirrors of each of the two telescopes are 10 metres (33 ft) in diameter, making them the second largest optical telescopes in the world, slightly behind the Gran Telescopio Canarias. The telescopes can operate together to form a single astronomical interferometer. In 1985, Howard B. Keck of the W. M. Keck Foundation gave $70 million to fund the design and construction of the Keck I Telescope. The key advance that allowed the construction of the Keck's large telescopes was the ability to operate smaller mirror segments as a single, contiguous mirror. In the case of the Keck each of the primary mirrors is composed of 36 hexagonal segments that work together as a single unit. The mirrors were made from Zerodur glass-ceramic by the German company Schott AG. On the telescope, each segment is kept stable by a system of active optics, which uses extremely rigid support structures in combination with adjustable warping harnesses.