Introduction
On June 1 1998 Minister L P H M Mtshali announced that the South African government would
support the building of the Southern African Large Telescope, SALT. The announcement
marked the culmination of many years of work by the South African Astronomical Observatory,
SAAO. In the announcement it was stated that the SA government would put up US$10 million
and that the SAAO would find the remaining US$10 million from overseas partners. At present
universities such as Rutgers and Carnegie-Mellon have expressed an interest as have
astronomers in countries such as Poland, Germany and Japan. The SAAO feels confident that
it can find the remaining funds and it is anticipated that construction could start as early as March
1999. A suitable site is at present being identified at the Sutherland observing station.
Southern African Large Telescope - SALT
In the 1980's it became clear that if South Africa was to retain its position in world astronomy it
would need to build a new telescope, substantially larger than the existing instrument. The
original proposal for SALT was to build a general purpose 4m class telescope using thin-mirror
technology with active and adaptive optics. Although this would have enabled South Africa to
retain its international competitiveness, the telescope would have been eclipsed by many larger
telescopes in the southern hemisphere, especially in South America at sites such as La Silla and
Paranal, where several large telescopes in the 8m class have been built or are under
construction. However, recent developments in technology have enabled South Africa to
leapfrog other nations by building an 11m spectroscopic survey telescope at two thirds of the
cost of the original 4m SALT and about one fifth of the cost of a similar sized general purpose
telescope.
The new SALT will be a twin of the Hobby-Ebberly telescope, HET, recently commissioned at
McDonald Observatory in Texas. HET is of an innovative and revolutionary design in optical
astronomy developed by a consortium of five universities, those of Texas, Pennsylvania State
and Stanford in the United States and M0nich and G6ttingen in Germany. Such a revolutionary
telescope will save costs in part by simplifying telescope's design, enabling many South African
companies to participate in the construction, and by using commercially available resources and
components where possible. An added advantage of the project is that where local technology
proves inadequate it will be imported from the US or Europe to the benefit of South African
industry. It is expected that well over 60% of SALT will be constructed by South African industry
using South African scientists, technicians and engineers.
The New Telescope
The new telescope, an 85 tonne structure, will be permanently tilted at 53ø to the horizontal, but
free to rotate on a horizontal concrete rail. This allows about 70% of the sky to be covered while
keeping constant gravity's pull on the mirrors, thereby avoiding distortions which can affect
image quality. The tilt is marginally different to that of HET (55ø) so as to include the Larger and
Smaller Magellanic Clouds.
The primary mirror consists of 91 hexagonal, identical 1m mirrors making an array 11m across,
the largest in the southern hemisphere and with HET the largest in the world. The 91 individual
mirrors will make up a spherical surface with a focal length of 13.08m. Each segment is
positioned under computer control to yield a complete primary mirror with an area of 77.6 square
metres. However when operating only part of the huge mirror will be used, equival.ent to a 9.2m
telescope at best and a 7m telescope at worst. SALT will therefore be classed as a 9m class
telescope.
The truss that holds the mirrors in position is a feat of modern engineering - over 1 700 struts
fit together with an overall deviation less than the thickness of a R1 coin! Each mirror is
supported on a 9 point support. The light gathered by the primary mirror is focussed onto a
complex corrector containing 4 secondary mirrors. This assembly corrects for spherical and
other aberrations and is mounted on a 4.5 tonne tracker beam at the top of the telescope. After
passing through the corrector the light is fed by optical fibre to the spectrographs and other
instruments in the instrument room uniderneath the telescope.
The entire telescope rotates on four air bearings and once correctly "pointed", the telescope
rests on its concrete rail. Tracking is achieved by moving the corrector assembly in a curve up
and down the tracker beam as the beam itself moves across the top of the telescope. This
highly complex motion requires very sophisticated software and a great deal of precision
engineering.
The telescope itself is housed in an aluminium geodesic dome 26m in diameter weighing 52
tonnes. Next to the dome is a 30m double walled tower containing a laser interferometer for
precisely aligning the 91 mirrors. The telescope will be controlled and serviced from a nearby
building.
The light that is eventually received by the instruments will have undergone 5 reflections, and
there will be a light loss at each reflection. To minimize this problem HET has silver coatings on
all mirrors with a protective overcoating to prolong the surface's life. SALT will have an
aluminized primary mirror, whilst all secondary mirrors will be coated with silver and a protective
coating. The resulting 'throughput'of light will be about 75%, ie. three quarters of the incoming
light will reach the detectors in the basement of the telescope.
Scientific Goals
This type of new-generation telescope will take astronomy well into the 21st century. It was
conceived with one main purpose - astronomical spectroscopy; the study of light emitted or
absorbed by celestial objects. The telescope will also be able to record very faint and distant
objects in direct images.
Because of its huge primary mirror and state of the art detectors, the planned research
programme includes the following:
- identification of very distant quasars to investigate conditions of the early universe,
- investigations of the intergalactic medium by the absorption of light,
- search for dark matter in and around galaxies,
- search for planets around other stars,
- optical identification of X-ray and Extreme UV objects discovered by space based
surveys,
- precise measurements of astronomical distances and speeds.
Friends with the Universe
Developing parallel with the construction of SALT will be the Science Education Initiative started
in the middle 1990's. The SALT announcement made a commitment to education and public
awareness and it is anticipated that the present YEAST project "Friends with the Universe" will
be extended and continued in years to come.
With the raised public profile that such a large telescope will bring, ( McDonald Observatory in
Texas has over 100 000 visitors each year! ), it is anticipated that at least one visitors centre will
need to be built at an as yet undecided location. Funding for this and other outreach projects will
need to be raised independently of the SALT funding and plans for this are being implemented.
Future Advantages
The location of such a new telescope in South Africa has advantages not only for astronomy,
but also for science and technology in the future. The construction of a major new high-
technology instrument will extend and broaden the local engineering and technological base; not
only during construction, but also on an ongoing basis to support such a project and to continue
to develop new hardware to keep the telescope competitive for the next 30 - 40 years.
The existence of such a major scientific instrument will provide a world class facility for African
scientists and act as a catalyst for other sciences. Scientists of international standing will
continue to be attracted to South Africa, for the benefit of, not only South Africa, but the entire
sub-Saharan region, so contributing to deputy President Thabo Mbeki's vision of the "African
Renaissance". It will also strengthen existing scientific and technological ties with countries in
Europe and the USA and build new ones with countries in Africa.
Astronomy remains one of the most dynamic of the modern sciences, constantly producing new
and exciting results about our greater environment - the Universe. As pointed out in the White
paper on Science and Technology in 1996,
"It is important to maintain a basic competence in flagship sciences such as
physics and astronomy for cultural reasons. Not to do so would be to take a
negative view of our future - the view that we are a second class nation forever
chained to the treadmill of feeding and clothing ourselves.
The original design concept of SALT - 4m active optics telescope.
