astronomy

Southern African Large Telescope

Southern African Large Telescope is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Southern African Large Telescope rather than just read about it. In short: The Southern African Large Telescope (SALT) is a 9.2-metre optical telescope designed mainly for spectroscopy. 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.

Key takeaways

Southern African Large Telescope belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Southern African Large Telescope to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Southern African Large Telescope from memory before moving on to harder problems.

Reference excerpt

The Southern African Large Telescope (SALT) is a 9.2-metre optical telescope designed mainly for spectroscopy. 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. However, its effective aperture is only 9.2 m. It is located close to the town of Sutherland 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 enables spectroscopic and polarimetric analysis and imaging of the radiation of astronomical objects that are not viewable from Northern Hemisphere telescopes. It is closely based on the Hobby–Eberly Telescope (HET) at McDonald Observatory, with some changes in its design, especially to the spherical aberration corrector. The main purpose for these changes was to improve the telescope's field of view. It shares the same fixed mirror altitude design, which limits access to 70% of the visible sky. 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. 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 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, the United States, the United Kingdom, and New Zealand.

General information SALT is located on a hilltop 1837 m above sea level in a nature reserve in the Hantam, Karoo 370 km (230 mi) 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, Japan, Poland and Google 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. 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 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-expansion Sitall glass and 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 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 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 fixed zenith 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 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 long-slit 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 is connected to the SAAO site in Cape Town via a 1 Gbit/s fibre connection over the SANREN network. The SAAO has a 1 Gbit/s connection to the SANREN network with 30 Mbit/s of that link being the international portion.

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.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Southern African Large Telescope

Start with the simplest possible case. Write down what Southern African Large Telescope claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Southern African Large Telescope before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Southern African Large Telescope ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Southern African Large Telescope

In research: Southern African Large Telescope appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Southern African Large Telescope in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Southern African Large Telescope is common in secondary-school and first-year university syllabi. It links to neighbouring topics 21st-century architecture in South Africa, Astronomical observatories in South Africa, Buildings and structures completed in 2005, so understanding it makes those chapters shorter.
In everyday life: Look for Southern African Large Telescope outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Southern African Large Telescope” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Southern African Large Telescope in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Southern African Large Telescope means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Southern African Large Telescope out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Southern African Large Telescope in simple terms?

Why does Southern African Large Telescope matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Southern African Large Telescope?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Southern African Large Telescope.