SALT - Southern African Large Telescope

SALT System Overview


The SALT telescope collects light from astronomical objects and accurately focuses it onto the telescope focal plane. From there the light proceeds into an optical instrument while the telescope tracks the relative movement of the object across the sky to maximise exposure time. (see SALT Key Performance Criteria for more details).

The SALT design is based on the Hobby-Eberly Telescope (HET) in Texas, but deviates in many ways to accommodate requirements specific to SALT and to take advantage of the opportunity to optimise aspects of the design where difficulties have been identified with the HET performance. It is being constructed at the existing SAAO site at Sutherland using South African companies as far as possible.

SALT has a fixed elevation angle and can rotate only about its azimuth axis. This important design aspect allows a significant cost saving in exchange for added complexity of the optical and control systems.

SALT has an 11m diameter spherical primary mirror with an optical axis tilted to 37 degrees from the vertical. It can rotate through 540 degrees in azimuth. Positioned ~13m above the mirror, is a tracker and an optical payload, looking down at the mirror. The tracker moves across the mirror on a virtual spherical focus surface, allowing sky-objects to be "followed" as the earth rotates, without adjusting the azimuth angle for a period of up to two hours. This gives the telescope an annulus-shaped observing area in the sky, 12 degrees wide between declination angles of approximately -75 degrees and +10 degrees.

There are three basic types of science instrument that will be used:

  • Both during commissioning and subsequently, a science-grade CCD camera located at the prime focus will be used to perform imaging. This camera will also be the system acquisition camera.
  • SALT is optimised for an imaging spectrometer located at prime focus which has good performance in the UV region.
  • An optical fibre system pipes light down to an instrument room where various fibre-fed instruments (e.g. a high-resolution spectrograph) will be located.

SALT will be used primarily as a queue-scheduled telescope, where observations are planned well in advance. A secondary application (a dedicated mode), will allow a single observation program to occupy an entire night.