Southern African Large Telescope

The Southern African Large Telescope (SALT) is a 10-metre class optical telescope designed mainly for spectroscopy. It consists of 91 hexagonal mirror segments each with a 1-metre inscribed diameter, resulting in a total hexagonal mirror of 11.1 m by 9.8 m. It is located close to the town of Sutherland in the semi-desert region of the Karoo, South Africa. It is a facility of the South African Astronomical Observatory, the national optical observatory of South Africa. SALT is the largest optical telescope in the southern hemisphere. It enables imaging, spectroscopic, and polarimetric analysis of the radiation from astronomical objects out of reach of northern hemisphere telescopes. It is closely based on the Hobby-Eberly Telescope (HET) at McDonald Observatory, with some changes in its design, especially to the spherical aberration corrector. It shares the same fixed mirror altitude design, with access to 70% of the visible sky. The main driver for these changes were desired improvements to the telescope's field of view. First light with the full mirror was declared on 1 September 2005, with 1 arc second resolution images of globular cluster 47 Tucanae, open cluster NGC 6152, spiral galaxy NGC 6744, and the Lagoon Nebula being obtained. The official opening by President Thabo Mbeki took place during the inauguration ceremony on 10 November 2005. South Africa contributed about a third of the total of US$36 million that will finance SALT for its first 10 years (US$20 million for the construction of the telescope, US$6 million for instruments, US$10 million for operations). The rest was contributed by the other partners - Germany, Poland, the United States, the United Kingdom and New Zealand. SALT is located on a hilltop in a nature reserve 370 km (230 mi) north-east of Cape Town, near the small town of Sutherland. In March 2004, installation of the massive mirror began. The last of the 91 smaller mirrored hexagon segments was put in place in May 2005. Korea and Japan have telescopes at the site and South Africa has at least five optical telescopes there. The University of Birmingham has a solar telescope to help monitor the Sun. SALT will probe quasars and enable scientists to view stars and galaxies a billion times too faint to be seen by the naked eye. Both SALT and HET have an unusual design for an optical telescope. Similar to the Keck Telescopes, the primary mirror is composed of an array of mirrors designed to act as a single larger mirror; however, the SALT mirrors produce a spherical primary, rather than the paraboloid shape associated with a classical Cassegrain telescope. Each SALT mirror is a 1-meter hexagon, and the array of 91 identical mirrors produces a hexagonal-shaped primary 11 x 9.8 meters in size. To compensate for the spherical primary, the telescope has a four-mirror spherical aberration corrector (SAC) that provides a corrected, flat focal plane with a field of view of 8 arcminutes at prime focus.