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South African Large Telescope

At 11 metres in diameter, the Southern Africa Large Telescope (SALT) is the largest single optical telescope in the southern hemisphere, able to detect stars one billion times dimmer than the faintest visible to the unaided eye. It is sited at the Southern Africa Observatory site near Sutherland, Northern Cape.

The main mirror, however, is not one single piece of glass, but made up of 91 hexagonal mirror segments which are all carefully joined and aligned to reflect the light as if they were one mirror.

South African Large Telescope - © SALT

Some facts about the telescope:

  • Observatory location: South Africa
  • Height above sea level: 1,759 metres (5,770 feet)
  • Moving Mass: 82 metric tonnes
  • Mirror segments: 91 hexagonal segments
  • Mirror diameter: 11 metres
  • Mirror thickness: 22 cm

Hexagonal sections used in mirror - © SALT

SALT is of a slightly different design to most large telescopes in that it points at a fixed angle of 37° (i.e. cannot move up and down), so can only rotate in a circle.

To compensate for its limited ability to point at the whole sky, there is a sophisticated tracker system that can move around over the main mirror and capture light from 70% of the night sky. And because the Earth rotates, this means that all objects are visible at some point during the night.