Everything about The Southern African Large Telescope totally explained
The
Southern African Large Telescope (
SALT) is a ~10 metre (~33 feet) diameter
optical telescope, located in the semi-desert region of the
Karoo,
South Africa. It is a facility of the
South African Astronomical Observatory, the national optical
observatory of South Africa.
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 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.
General information
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.
Korea and
Japan have telescopes at the site and South Africa has at least five optical telescopes there. The
University of Birmingham has a solar telescope to help monitor the
Sun around the clock.
SALT will probe
quasars and enable scientists to view stars and galaxies a billion times too faint to be seen by the
naked eye.
Primary Mirror
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.
Instrumentation
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.
Internet connectivity
The telescope has a 1.5Mbit internet connection, feeding to what is termed the 'Beach-head', from where other institutions can access the data.
Science working group
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
Partners
In 2007, the following new partners joined the SALT consortium:
American Museum of Natural History
Inter-University Centre for Astronomy and Astrophysics (India)Further Information
Get more info on 'Southern African Large Telescope'.
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