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Visiting SALT at the SAAO in Sutherland

| September 3, 2010
By Elizabeth Danckwerts

Sutherland is in the middle of the Karoo and is ideally situated for SALT (Southern African Large Telescope) at the SAAO (South African Astronomical Observatory).

At 1 759 metres Sutherland is only just below Johannesburg in altitude, has the cleanest and clearest air and is seismologically stable. It’s also far away from city lights and pollution. SAAO scientists work at night and sleep during the day, their windows blackened. When they drive or cycle from their lodgings to the telescopes, they can only use the hazard lights of the car and drive using the centre line of the road as a guide. Additional light outside and around the telescopes degrade the quality of the data obtained by researchers using the telescopes.

A complex and beautiful telescope

Construction started in 2000 and SALT commenced operation in 2005. The total telescope cost, including instruments and operations, was approximately US$32 million, of which South Africa contributed a third and international partners provided the balance. The design was based on a modified version of the Hobby-Eberly Telescope in Texas, USA. SALT support facilities include a primary mirror alignment tower (CCAS tower), a control room, instrument room, electrical room, computer room,
workshop, storeroom, mirror coating facility and a visitor’s gallery. The primary mirror consists of 91 interchangeable hexagonal mirror segments, with a spherical figure. Each mirror segment is one metre wide and weighs approximately 100 kilograms.

Three actuators (precision ‘pistons’) on each of the 91 mirror mounts are used to position the individual mirror segments accurately. An optical wavefront sensor positioned in the CCAS tower, about 26 metres from the mirrors, is used to align all of the mirrors to an ideal spherical surface roughly once a night. Edge sensors, once operational, will detect changes in the relative positions of the mirror segments and provide data on mirror misalignment approximately every 10 seconds. The detected displacements can be corrected in a closed loop control system, which adjusts the mirror actuators accordingly, to an accuracy equivalent to onethousandth the thickness of a millimetre.
The ringwall, on which the dome is supported, is 17 metres high, with a diameter of 26 metres. The telescope dome is a hemisphere structure with a shutter opening of 11 metres. The height to the top of the dome is approximately 31 metres, and to the top of the CCAS tower is 34 metres. The first-generation instrumentation includes a low- to medium-resolution imaging spectrograph/spectropolarimeter, imaging camera and a fibre-fed highresolution spectrograph.
This flagship project demonstrates that the frontiers of science are not entirely reserved for the developed world. SALT provides a first-class facility for fundamental research in Africa, in a field where South Africa has a long history of excellence. It will strengthen ties with researchers around the world, and provide opportunities for young scientists and engineers in a stimulating, high-technology environment.

SALT allows us to see into the far reaches of the universe

With a giant mirror gathering about 25 times as much light as the previous largest African telescopes, SALT allows astronomers to explore:
• the scale and age of the universe
• the earliest galaxies and quasars
• the life and death of stars
• planets orbiting other suns
• extreme environments
SALT was built at less than a fifth of the cost of a conventional 10 metre telescope. This was made possible by drastically simplifying the telescope optics and mounting, and optimising for competitive observational modes (e.g. spectroscopy, polarimetry and imaging photometry). In conventional telescopes, massive, expensive precision engineering allows the telescope to point anywhere in the sky at any time, and moves the whole telescope to track the stars as the earth rotates. SALT moves only in azimuth, rotating into position on air bearings and remaining stationary during each observation. Precise pointing and tracking are handled by the moving optical corrector assembly and instrument payload at the top of the telescope tube, which allows
SALT to reach objects in a circular band in the sky 12 degrees wide and centred 37 degrees from the zenith.

Education at SAAO

The SAAO is committed to promoting Science and Mathematics at primary, secondary and tertiary levels, offers Science education workshops and resources for educators and learners, and has participated in developing curriculum standards nationally.
Students from the various universities of technology learn the practical side of scientific support through in-service training programmes in SAAO’s mechanical and electronics engineering workshops. Postgraduate students can complete advanced degrees under the joint supervision of an SAAO scientist and a faculty member at the student’s home university. The SAAO was instrumental in the establishment of a collaborative postgraduate training programme by a number of South African universities.

The National Astrophysics and Space Science Programme (NASSP) aims to produce highly qualified African scientists who, among other things, will make use of the SALT facility. A tour of the SAAO Cape Town begins at 20:00 on the second and
fourth Saturday of each month and lasts for approximately two hours. Weather permitting, visitors can look through the telescopes on site, including the historic McClean refractor – the largest telescope in the Western Cape.
The tour starts at the Visitors Centre and a guide then escorts visitors through the telescopes, including SALT. These tours must be booked. Accommodation for visiting school learners is provided at the local school hostel and arranged by SAAO. It is
advised that schools take buses of about 20 students at a time, to allow for tours of SAAO and SALT to follow one after another, with one-hour intervals. Tours and star-gazing for students is free of charge at SAAO. Star-gazing at night can also be done, but must be booked well in advance.
Sutherland is an ideal stop over on the way to Oudtshoorn via Prince Albert, Meiringspoort or the Swartberg Pass (depending on your vehicle) and other places of interest.

Contact SAAO Sutherland on:

Tel: +27 (23) 571 2436
Websites: or

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Category: Featured Articles, Spring 2010 Edition

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