SALT is modelled on the Hobby-Eberly Telescope (HET) at McDonald Observatory in Texas, whose innovative design enables it to be built for a fraction of the cost of conventional telescopes. The HET -- and now SALT -- has a spherical array of 91 identical hexagonal mirrors of 1m width, which can be produced more easily and cheaply than larger mirrors. SALT's sponsors say the 11m telescope, with an expected life of 50 years, will be built for the same price as a 4m conventional telescope.

The trade-off is flexibility: where conventional telescopes can track the object being observed, SALT's primary mirror will remain fixed at an angle of 37 degrees to the horizontal. To follow a star's movements over a period of 48 minutes to two hours, the imaging instruments will move across a square 12 degrees wide. Distortions introduced by the spherical mirror will be eliminated by four additional mirrors called a "spherical aberration corrector". The telescope will be powerful enough to register the light of a candle flame on the moon.

Sutherland is a remote town in South Africa's semi-desert Karoo region: its clear, dark skies make it an ideal astronomical research site and the town hopes SALT and the associated visitors' centre will attract tourists and much-needed additional income.

The Southern African Large Telescope (incorrectly named above) is to be completed by the end of 2004. SALT is the brain child of Bob Stobie (deceased 2 May 2002) former director of the South African Astronomical Observatory.

Currently three instruments will form part of SALT:

  • PFIS (Prime Focus Imaging Spectrograph) - This instrument will be located at SALT's prime focus and is primarily being built at the University of Wisconsin, Madison. PFIS will exploit the improved blue/UV throughput of SALT as well as access to an 8 arcmin diameter field of view. It will also be capable of multi-object spectroscopy (MOS) to resolutions of R ~ 12000, and narrow-band imaging, as well as having polarimetric capability.
  • HRS (High Resolution Spectrograph) - This instrument will be fed by optical fibre from a 45° fold mirror located in the prime focus. The concept for an HRS instrument is centred around a single object high resolution echelle spectrograph, somewhere in the region R = 30,000 to 100,000. The University of Canterbury (New Zealand) is taking the lead in building this instrument.
  • SALTICAM - This instrument will be fed with a 45° fold mirror from primary focus. It was conceived as a multi-purpose device capable of performing as either an acquisition camera or an imager/photometer for scientific purposes. Frame transfer, science grade, CCDs, with fast readout times, will allow for science imaging and high-time resolution photometry. The instrument is being designed and built by the South African Astronomical Observatory.

Comparing SALT and HET
One of the main pros of having the Hobby-Eberly telescope design to work from is that we have been able, with their advice, to improve the mirror alignment system (currently HET can only align 60 odd of its 91 segments as of end-2002). This is mainly due to the center of curvature measurement system and the sensors located on the edge of each mirror that measures the misalignment between each mirror.

The facility containing the telescope has also been improved to keep the difference between the inside and outside temperature to a lower differential thus allowing for higher quality imaging.

Quick Facts:

Current happenings:
(Live webcam -
December 2003 - First guided observation of a star. Due to fluctuations in the temperature of the atmosphere and other climactic effects a star makes minute jumps on sky (the same thing causes stars to "twinkle"), to adjust for these jumps the verification instrument is used to supply dynamic offsets to the telescope to correct the star track. This process is generally referred to as guidance. Again this is only a proof of concept and not a final test, but it's amazing seeing a star picture stabilize on the instrument's monitor.
October 2003 - First succesful track of a star! Using some initial verification instruments we were able to track our first star (Theta Hydri). This is a proof of concept not a final test of the telescope, but it shows that with the real SALT instruments and some calibration we should be able to complete SALT by the end of next year.
September 2003 - As mentioned by consumagenerica above SALT's instruments (referred to as the Payload) move across the primary mirror to track the movement of the stars. This is acheived by a supporting the payload on six struts (they look kinda like big shock absorbers) which can tip, tilt and piston them, and then two drives that can take the payload up, down, left and right. This assembly is called the Tracker and was successfully installed towards the end of September.
July 2003 - The first mirror sensors were installed. Each mirror has a set of three sensors that allow it to be accurately aligned with adjacent mirrors. This system runs dynamically during the night allowing the 91 segments to form as good a single mirror surface as possible. Since this system doesn't interfere with observation it can be used inbetween centre of curvature alignments (see below) during observations.
June 2003 - First centre of curvature alignment done. A very sensitive laser instrument which sits at the mirrors' centre of curvature allows the mirrors to all be aligned. This procedure needs to happen during the night (hence interfering with observations), so it will only be done approximately once a week.
May 2003 - The building management system which ensures that the telescope chamber temperature is kept as close to the ambient temperature as possible has been tested and is currently being fine tuned. This is very important for high quality data - think about the heat waves you see on a desert road - if the temperature of the mirrors and the surrounding air differ to much you get a similar effect that causes light degredation on the telescope.
April 2003 - The first version of the server that controls the telescope has been installed and tested.
January 2003 - The structure on which the mirror truss and instruments rest has been tested. This structure rotates on 8 air bearings which can be thought of as 8 hovercraft cushions that pick up the structure.
December 2002 - The first three mirrors arrived from Kodak and were coated at Sutherland and temporarily placed in the mirror truss. The mirror truss is the main support structure for the mirror segments and consists of 564 interlocking triangles. Six of these triangles form the base for each of the 91 six-sided mirror segments - see diagram below for a representation.

        /\  /\
       \  /\  /

Personal involvement

Log in or registerto write something here or to contact authors.