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| Alternative names | SALT |
|---|---|
| Location(s) | Sutherland,Karoo Hoogland Local Municipality,Namakwa District Municipality,Northern Cape, RSA |
| Coordinates | 32°22′33″S20°48′39″E / 32.375918°S 20.810757°E /-32.375918; 20.810757 |
| Altitude | 1,798 m (5,899 ft)[1] |
| Wavelength | 320 nm (940 THz)–1,700 nm (180 THz) |
| Built | 2005 |
| Diameter | hexagonal array of ~11.1 m × 9.8 m 9.2 m (effective aperture) |
| Angular resolution | EE(50) ≤ 0.6" |
| Collecting area | 79 m2 (91 × 0.87 m2) 66.5 m2 (effective aperture) |
| Mounting | 45 ton steel structure |
| Enclosure | 25 m spherical |
| Website | www |
  Location of Southern African Large Telescope | |
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TheSouthern African Large Telescope (SALT) is a 9.2-metreoptical telescope designed mainly forspectroscopy. It consists of 91 hexagonal mirror segments each with a 1-metre inscribed diameter, resulting in a total hexagonal mirror of 11.1 by 9.8 m.[2] However, its effective aperture is only 9.2 m. It is located close to the town ofSutherland in the semi-desert region of theKaroo,South Africa. It is a facility of theSouth African Astronomical Observatory, the national opticalobservatory of South Africa.
SALT is thelargest optical telescope in theSouthern Hemisphere.[3][4] It enablesspectroscopic andpolarimetric analysis and imaging of the radiation from astronomical objects that are out of reach ofNorthern Hemisphere telescopes.
It is closely based on theHobby–Eberly Telescope (HET) atMcDonald Observatory, with some changes in its design, especially to thespherical aberration corrector. The main purpose for these changes was to improve the telescope'sfield of view. It shares the same fixed mirroraltitude design, which limits access to 70% of the visible sky.[5]
First light with the full mirror was declared on 1 September 2005, with 1-arc-second resolution images ofglobular cluster47 Tucanae,open clusterNGC 6152,spiral galaxyNGC 6744, and theLagoon Nebula.[6] The official opening byPresidentThabo Mbeki took place during the inauguration ceremony on 10 November 2005.[7]
South Africa contributed about a third of the total of US$36 million that will finance SALT for its first 10 years (US$20 million for the construction of the telescope, US$6 million for instruments, and US$10 million for operations). The rest was contributed by the other partners:Germany,Poland, theUnited States, theUnited Kingdom, andNew Zealand.[8]
SALT is located on a hilltop 1837 m above sea level in anature reserve in the Hantam, Karoo 370 km (230 mi) north-east ofCape Town, near the small town ofSutherland. In March 2004, installation of the massivemirror began. The last of the 91 smaller mirroredhexagon segments was put in place in May 2005.
Korea,Japan,Poland andGoogle[citation needed] have telescopes at the site and South Africa has at least five optical telescopes there. TheUniversity of Birmingham has a solar telescope to help monitor theSun.SALT will probequasars and enable scientists to view stars and galaxies a billion times too faint to be seen by thenaked eye.
Both SALT andHET have an unusual design for an optical telescope. Similar to theKeck Telescopes, theprimary 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 hexagonally shaped primary 11 by 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 is made of low-expansionSitall glass and can be adjusted in tip,tilt andpiston 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 is 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 theArecibo 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 fixedzenith angle of 37 degrees, optimised for the Magellanic clouds, 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 theentrance 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 long-slit and multi-object imaging spectrograph and spectropolarimeter, designed and built by theUniversity of Wisconsin–Madison,Rutgers University, and the SAAO; and a fiber-fed High Resolution Spectrograph (HRS), designed by theUniversity of Canterbury (New Zealand). SALTICAM was installed in early 2005, while the RSS was installed on 11 October 2005.
The telescope is connected to theSAAO site in Cape Town via a 1 Gbit/s fibre connection over theSANREN network. The SAAO has a 1 Gbit/s connection to theSANREN network with 30 Mbit/s of that link being the international portion.
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:
Research using SALT at the South African Astronomical Observatory has led the facility to important discoveries. By using the Southern African Large Telescope, SAAO has the ability to take "snapshots" of stars in very quick succession. It is optimized for wavelengths and observing modes not available on other very large telescopes. As a result, astronomers can study rapidly changing properties of compact stars, primarily as they pull in gas from their companion stars or surroundings. The significance of this discovery allows us to detect black holes. The gravitational field of a compact star commonly pulls in gas from a companion star, thusradiation (especiallyX-ray) is emitted. Scientists used this as an indirect way to locate black holes. Another phenomenon that SALT has helped astronomers investigate is the way that masses build up on some compact stars until supernova explosions blow them apart, which gives scientists a "Type Ia" supernovae used to show that the expansion of the universe is speeding up.[9]
Other note-worthy research the South African Astronomical Observatory has achieved using SALT include the discovery of a class of stars known as "polar", or a pair of stars. The "polar" binary star system, where a compactor star called a "white dwarf" whose volume has shrunk about one millionth of a star like the Sun. Studies using SALT concluded that these polar binary star systems take only an hour and a half to complete an orbit. Also, the SALT telescope allows scientists to study the rapid brightness changes in exotic stars.
More research using SALT has aided astronomers to investigate the structure and evolution of ourgalaxy, such asquasars,Magellanic clouds, the galactic structure and stellarastrophysics.[10] SALT released its first color images, which marked the achievement of the "first light". This also marked the debut of the fully operating SALTICAM, which is a $600,000 digital camera designed and built for SALT.
Despite initial estimates by SAAO that SALT would bring up to 30,000 tourists to Sutherland, the telescope has so far only resulted in about 14,000 annual visitors, which has nevertheless resulted in the creation of at least 300 jobs in the town of 5,000.[8]
Operational wavelengths: 320nm to 1700nm
The spherical primary mirror has a master radius of curvature of 26 165 mm. It consists of 91 interchangeable hexagonal mirror segments, each of 1 m inscribed diameter, forming a hexagon of ~11.1 x ~9.8 m.
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