SALT is the largest optical telescope in the southern hemisphere. It will enable imaging, spectroscopic, and polarimetric analysis of the radiation from astronomical objects out of reach of northern hemisphere telescopes. It was originally planned to be a copy of the Hobby-Eberly Telescope at McDonald Observatory, but while adapting the construction plans, significant changes were introduced to its design, especially to the spherical aberration corrector. The main driver for these changes were desired improvements to the telescope's field of view.
First light (astronomy) 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 $36 million USD that will finance SALT for its first 10 years ($20 million for the construction of the telescope, $6 million for instruments, $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 at , on a hilltop in a nature reserve 370 km (230 miles) 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.
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.
Each of the 91 mirrors can be adjusted in tip, tilt and piston in order to properly align them so as to act as a single mirror. Because the mirror is spherical, light emitted from a position corresponding to the center of curvature of the mirror will be reflected and refocused to the same position. Therefore, the telescope employs a Center of Curvature Alignment Sensor (CCAS) situated at the top of a tall tower adjacent to the dome. Laser light is shone down on all the segments and the position of the reflections from each mirror measured. A process called "stacking" thus allows the telescope operator to optimize the adjustments of the mirrors.
The telescope is also unusual in that during an observation, the mirror remains at a fixed altitude and azimuth, and the image of an astronomical target produced by the telescope is tracked by the "payload", which resides at the position of prime focus and includes the SAC and prime focus instrumentation. This is similar in operation to the Arecibo Radio Telescope. Although this results in only a limited observing window per target, it greatly simplifies the primary mirror mount, when compared to a fully-steerable telescope, transferring the complexity to the smaller and lighter payload tracking system, providing for an overall reduction in total telescope construction cost. SALT has a zenith angle of 37 degrees, but because of the full range of azimuths and the celestial rotation, SALT has access to a good fraction of the sky available at the Sutherland site.
Another consequence of this design is that the entrance pupil varies in size during the tracking of a target.
The first generation instrumentation for SALT includes the SALT Imaging Camera (SALTICAM), designed and built by the South African Astronomical Observatory (SAAO); the Robert Stobie Spectrograph (RSS) (née Prime Focus Imaging Spectrograph), a multi-purpose longslit and multi-object imaging spectrograph and spectropolarimeter, designed and built by the University of Wisconsin-Madison, Rutgers University, and the SAAO; and a fiber-fed High Resolution Spectrograph (HRS), designed by the University of Canterbury (New Zealand). SALTICAM was installed in early 2005, while the RSS was installed on 11 October 2005.
Membership of the SALT Science Working Group :
David Buckley, Gerald Cecil, Brian Chaboyer, Richard Griffiths, Janusz Kałużny, Michael Albrow, Karen Pollard, Kenneth Nordsieck, Darragh O'Donoghue, Larry Ramsey, Anne Sansom, Pat Cote
In 2007, the following new partners joined the SALT consortium: