New eye on the sky
It is more than eight years since planning first began for arguably the most ambitious and most prestigious project to date by the Canaries Astrophysical Institute (IAC). The location of the 75 million Euro project, the Gran Telescopio Canarias (Grantecan, GTC) is the Roque de los Muchachos on La Palma. The observatory is financed by the Spanish state and the regional government of the Canaries. In addition considerable resources have been made available from the European Structural Fund.
SCHOTT was awarded the contract for the primary mirror, the “heart” of the telescope – in the face of competition from the USA and Russia – because of the superior quality of its “ZERODUR®” material.
High demands on material
The segmented primary mirror has a diameter of 10.4 meters (ca. 11.4 yards) and a surface area of 82 square meters (ca. 98 sq. yards). It comprises 36 hexagonal “ZERODUR®” glass ceramic segments. Each 1.9 meter (2 yards) diameter segment is 85 millimeters thick (3.4 inches) and weighs nearly 500 kilograms (1100 pounds). The high demands made on optical quality are just one of the features that make the primary mirror the most sophisticated part of the telescope. The support system for the individual segments and its electromechanical devices calls for the highest degree of precision. The “active optics” allow the most minor geometrical irregularities in the total system to be corrected, providing optimum alignment and superb image quality.
Comparable with top telescopes
The first two mirror blanks were finished in January 2000 from stock material. In all, SCHOTT will be supplying 42 mirror carrier segments, which includes six “spares”, by the end of 2001. Before delivery, demanding specifications have to be met – not only regarding material quality but also the geometrical precision after machining. This calls for the use of modern measurement technology such as a laser tracker.
From Mainz, the mirror carriers go to the French company REOSC near Paris where they are polished. The GTC is expected to carry out its first observations at the end of 2002 with a temporary primary mirror comprised of eight to ten segments The remaining segments will be installed in the course of 2003 and the whole system will finally come into full service in 2004.