SALT - Southern African Large Telescope

SALT System Overview

SALT collects light from astronomical objects and accurately focuses it to one of four selectable focii. 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 and take advantage of technical innovations. SALT is located at the SAAO site at Sutherland.

SALT has a fixed elevation angle (53°) and can rotate only about its azimuth axis to acquire objects. This important design aspect allows a significant cost saving in exchange for added complexity of the optical and tracking systems.

SALT has a spherical primary mirror array with an aperture of approximately 11.1 m x 9.8 m.  The optical axis tilted to 37° from the vertical. It can rotate through 540° in azimuth. Positioned ~13 m 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 "First Generation" SALT science instruments that will be used, of which the first two are completed and the third still under construction:

  • SALTICAM, a fast and sensitive CCD imaging camera, located at the telescope's prime focus, will be used to perform imaging and photometry. It can be operated in a "video" mode, allowing high time resolution (down to ~0.1s) photometry. This camera also serves as the telscope's acquisition camera.
  • SALT is optimised for an imaging spectrometer, the Robert Stobie Spectrograph (RSS, named in memory of Dr Robert S. Stobie, past SAAO Director, and one of the initiators of the SALT project). It is also located at the prime focus and operates in the UV-Visible region (310 - 900 nm) of the electromagnetic spectrum. It features a number of modes:
    • longslit and multi-object low to medium resolution spectroscopy
    • Fabry-Perot imagin spectroscopy
    • Polarimetry
  • An optical fibre feed system will pipe light down to a high-resolution spectrograph (SALTHRS), located in a room underneah he telescope.

SALT will be used primarily as a queue-scheduled telescope, where observations are planned well in advance. Additional details concerning the telescopes, subsystems, and instruments are located under technical information.