Download The Southern African Large Telescope*

The Southern African Large Telescope*
R S Stobie·
Director: South African Astronomical ObselVatory, PO Box 9, ObselVatory, 7935, South Africa
E-mail: [email protected]
Abstract: The announcement by the South African government to support the construction of a
large telescope for optical/infrared astronomy has given a new impetus to the development of basic
science on the African continent. The Southern African Large Telescope (SALT) will be a southern
hemisphere equivalent of the Hobby-Eberly Telescope recently completed at McDonald Observatory, Texas. This 9m-class telescope is of revolutionary design and highly cost-effective. SALT will
be a major scientific and technological project with international partners. This new telescope plus
the existing facilities at the South African Astronomical Observatory should be seen as a focus for
the development of basic science on the African continent. It will enable African scientists to be
internationally competitive in astronomy well into the 21 st century.
Reproduced by Sabinet Gateway under licence granted by the Publisher ( dated 2012)
1. SALT gets the 'Green Light'
The motivation for the construction of the Southern
African Large Telescope (SALT) as presented to the
South African government was described in a previous article (MNASSA, 57, Nos. 5 & 6, pp 21-26,
"Why South Africa needs SALT").
On 1 June 1998 the Minister of Arts, Culture,
Science and Technology announced during his budget speech in parliament that South Africa would fund
50% of the construction cost of SALT. This decision
was taken at the highest level by the South African
Cabinet. In approving the construction of SALT Cabinet specifically noted the following advantages:
1. Collaboration with northern hemisphere countries prepared to contribute to capital and operational costs;
2. Utilising South Africa's unique geographical
location and astronomically excellent climate
for exciting new contributions to astronomy in
study of the early Universe, galaxy populations
and planetary searches;
* Delivered as the Presidential Address to the Society at its 1998 Annual General Meetingheld at SAAO,
CapeTown.
3. Promoting the general development of South
African science and the awareness of the importance of science and technology;
4. Providing a focus for astronomical development
and training on the African continent, and technological spin-offs to industry; and
5. The extension of science and technology infrastructure to the Northern Cape as an underresourced Province in this regard.
The remaining 50% of the construction cost of
SALT will be contributed by international partners.
A number of countries and individual institutions have
expressed an interest in collaborating with South
Africa in a Large Telescope. In particular the HobbyEberly Telescope Board has agreed to make available the detailed plans and drawings of the RET telescope in return for telescope time. The universities
of Carnegie Mellon and Rutgers in the USA and
GOttingen in Germany are interested in participating
in the SALT project. Carnegie Mellon has aheady
prepared a promotional video for the purposes of
raising $6 million from potential donors. Countries
that have expressed an interest in participating in
SALT include Poland, New Zealand and Nigeria.
A feasibility study has shown that South African
industry has the capability to construct over 50% of
the telescope. Any funding contributed by South Africa will be to the benefit of South African industry.
-------------------------------81 ------------------------------December 1998
Reproduced by Sabinet Gateway under licence granted by the Publisher ( dated 2012)
tlJ
The Proposed Telescope Facility
SALT
It. Mirror Alignment Tower
Tracker
It. Diameter Dome
Control & Service Building
Telescope azlmulh
rolatlon bearing
Fiber Coupled
Inslrument Room
10 meier. = 32.1 fl.
Figure 1. The proposed Southern African Large Telescope will be based on a southern hemisphere equivalent
of the Hobby-Eberly Telescope at McDonald Observatory, Texas. (Courtesy: HET Board)
In addition there will be technological spin-off to
South African industry from joint ventures 'with industry internationally. The construction time scale and
funding profile for SALT are given in Table 1.
Table l. SALT construction schedule & funding profile
Year
Activity
98/99
99/00
00/01
01/02
02/03
03/04
Concept development/site assessment
Initial procurement/ground breaking
Component development/installation
Integration of mirror & tracker system
First light/commissioning
Completion and science operations
Total
Budget
($ million)
1.5
4.0
6.0
5.0
2.5
1.0
20.0
2. The Telescope Facility
SALT is based on a southern hemisphere equivalent
of the Hobby-Eberly Telescope (RET) recently completed at McDonald Observatory, Texas. The RET
is a radical departure in design of optical/infrared
telescope (Figure 1). The design is based on a tiltedArecibo concept, with the telescope fixed at 35° to
the zenith but capable of full 360° azimuth rotation.
During an observation the telescope remains stationary and a tracker beam at the top end enables an
object to be followed for 12° across the sky. Thus
the telescope can effectively observe an object in an
annulus 12° in a cone centred on 35° from the zenith. Scientific requirements in the southern hemisphere to enable the whole of the Small Magellanic
Cloud to be accessible from the latitude of
------------------------------82-----------------------------MNASSA, Vol. 57, Nos. 11 & 12
Reproduced by Sabinet Gateway under licence granted by the Publisher ( dated 2012)
Sutherland will require this angle of tilt to be increased to 37°.
The primary mirror, of spherical shape, will be
constructed from 91 hexagonal segments, each 1
metre in diameter. The primary mirror measures 11
metres across and has a focal length of 13.08 metres.
The individual mirrors are supported on a metal truss
frame that is kinematically mounted to the telescope
structure. Each mirror has three adjusters to align it. .
The mirror alignment instrument, which is presently
based on a shearing interferometer, is mounted at the
centre of curvature of the primary at a particular azimuth. Thermal control of the environment is essential to keep the metal truss at a uniform temperature
and to be able to model the deformation of the primary mirror as a function of temperature. The calibration of this deformation is obtained by measurements with the centre of curvature alignment sensor
as a function of temperature.
Because the telescope is stationary during an observation the pupil moves across the primary mirror
(Figure 2). The maximum area that is imaged on to
the detector corresponds to a 9.2m mirror - at the
edge ofthe travel of the tracker the pupil is still equivalent to a 7-m telescope. The large amount of spherical aberration is removed over a 4 arcmin field by a
spherical aberration corrector (SAC). The total imaging error budget of the telescope gives a full width
half maximum of 0.6 arcsecs, which is designed to
not degrade significantly the median seeing of 0.9
arcsecs at Sutherland. The mirror surfaces of the 4element SAC are silver with a protective overcoat,
which gives higher reflectivity than aluminium over
the wavelength range of interest 0.351lm to 2.01lm
Much of the complexity of the telescope is contained in the SAC and the precision with which this
must be moved by the tracker beam. At all times the
optical axis of the SAC must remain aligned with the
centre of curvature of the primary mirror whilst the
SAC is moved with high precision over the curved
focal surface. The tracker beam can support a light
weight instrument at the prime focus. However, most
instruments will be housed in an environemntally controlled basement and fibre fed from the prime focus.
Because of a conscious decision not to build a
general-purpose telescope but rather a telescope with
specialised spectroscopic capabilities the whole design is cost-effective. The estimated cost of constructing a southernhemispehere Hobby-Eberly Telescope
~\i
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____
T_he_H_O_bb_y_-E_b_BrI_y_T_BI_Bs_c_OP_B_:41_1_7_/9_6_ _ __
How the telescope tracking works I
Tracker off centar
and pupil partlallf on
primary mlrroranay.
Al worstox1nlme, .UII •
7
wl_opol
m_
Part of pupil off
mirror Is baffled ~
PIU
~,
I--- ,....... ----..t
Figure 2. During an observation the telescope is stationary and the tracker beam follows the object over
a 12° range. As a result the pupil moves across the
primary mirror - when centred it is equivalent to a
9.2m telescope and at the worst extreme still a 7m
telescope. (Courtesy: HET Board)
is US $20 million, comparable to the cost of a general-purpose 4m class telescope, and about one-fifth
the cost of a general-purpose 10m class telescope.
The major cost saving factors are (1) the constant
gravity vector simplifying the support structure (2) a
spherical primary mirror (3) specification of the image error budget to 0.6 arcsec.
The telescope has been designed essentially as·a
spectroscopic survey instrument with a spectroscopic
capability over the wavelength range of 0.351lm to
2.0Ilm. It will be most competitive scientifically when
used on astronomical targets uniformly distributed
on the sky or clustered on a scale of a few arcmins.
On any given night the telescope will observe a range
of different scientific programmes and operate in
queue-scheduled mode in order to optimise its efficiency with the objects accessible in the observable
annulus. Because the telescope will be queue scheduled, it is particularly effective in time variability studies on time scales of a day or longer. Furthermore,
this queue scheduling will open a niche area of time
variability which is not accessible in conventionally
scheduled telescopes.
-------------------------------83------------------------------December 1998
Reproduced by Sabinet Gateway under licence granted by the Publisher ( dated 2012)
There is a wide variety of science that SALT can
tackle. For example, programmes involving the
chemical composition and evolution of the Milky Way
and nearby galaxies, redshifts of high z quasar candidates from imaging surveys, evolution of the gas in
galaxies through quasar absorption lines, the kinematics of gas, stars and planetary nebulae in distant
galaxies, optical characterization of X-ray objects
discoverd by AXAF, follow-up of objects from the
Sloan Digital Sky Survey, spectroscopic study of
objects in the Hubble Deep Field in the South, to name
but a few.
3. A 'Flagship' Science Project on the African
Continent
The Southern African Astronomical Observatory
(SAAO) with its outstation at Sutherland is the major observatory for optical/infrared astronomy on the
African continent. This facility should be available to
other African countries in order to encourage the
development of basic science. It should be seen as
the focus of astronomical development on the continent. Already scientists and engineers have come from
Namibia, Malawi, Zambia, Zimbabwe, Nigeria and
Egypt to train at SAAO. Basic science on the African continent would be encouraged by the provision
of observation time at this premier research facility.
SALT should be seen as the driver of this initiative, as part of the Deputy President of South Africa,
Mr Thabo Mbeki's, African Renaissance. We need to
strengthen the astronomical community in South Mrica especially by developing research groups at the
Historically Black Universities. SALT will enable
South African (and African) scientists to remain internationally competitive in astronomy well into the
21st century. To encourage links with other countries in Africa and promote the development of astronomy groups within physics departments at universities, South Africa will need to help in providing
training as wen as access to the research facility. Training for MSclP~ degrees could be provided at existing departments at some South African universities.
Research ·projects could be carned out jointly with
astronomers at the National Facilities in South Mrica and at the universities. The major question is how
these initiatives can be funded.
For South Africa the impact of SALT is going to
be much greater than purely that of the largest single
telescope in the southern hemisphere enabling the
country to remain internationally competitive in astronomy. It is the impact on the youth of the country
(from where the future scientists and technologists
will come), the development of a culture of science
and technology, and increasing the number of students interested in pursuing careers in science and
technology, that in the end will bring most benefit.
Astronomy is the catalyst that can spark the imagination of the youth to become interested in science and
technology. To maximise this impact we plan to construct a Visitor Centre / Science Education Centre at
Sutherland (with independent funding) which will
have as an integral part of it a viewing gallery for
SALT. This Science Education Centre will be open
every day with hands-on interactive displays on astronomy and related topics. On some nights Star Parties will be held with viewing the heavens through
small telescopes. Because of the remote location of
Sutherland special arrangements will have to be made
to ensure that school pupils from all over South Mrica can visit and experience this facility at first hand.
Provided they feel that this SALT project belongs to
all of South Africa it will engender a source of pride
in what is a flagship science and technology project
for South Africa and the African continent.
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