Pass the SALT !
Dr David Buckley, SALT
Astronomy research will get a major boost when the Southern African Large
Telescope (SALT) is completed. We take a look at what we can expect.
 The Southern African Large Telescope (SALT) is well on track for completion
in about 18 months time. When it is finished it will be the largest single
optical telescope in the southern hemisphere.
SALT is based on the pioneering Hobby Eberly Telescope (HET), which
represented a new paradigm in telescope design. However, significant problems
were encountered with HET, primarily associated with the control of the 91
mirror segments, for which the SALT designers have been fully cognizant. As a
result, with SALT following several years behind HET, it has managed to avoid
these pitfalls and at the same time improve on the design in various areas. The
major improvement is the redesigned prime focus aberration corrector, which will
deliver a four times larger field of view compared to the HET. In addition, the
image quality and efficiency are all greatly improved. From the outset SALT has
incorporated a so-called 'active optics' control system. This use about 480
capacitive edge sensors to sense the relative motion of mirror segments and the
correct for these by moving 273 precision actuators (small encoded motors), 3
per mirror.
Once the 91 individual 1-m hexagonal spherically figured segments are
aligned, using a wavefront sensor at the centre of curvature, a closed loop
control system (operating at 0.05-0.1 Hz) will maintain the alignment for many
days at a time. Daytime air conditioning and nighttime natural ventilation,
using controllable louvres, plus the use of a false floor with fan-forced
ventilation, will ensure dome seeing is kept to a minimum.
Likewise, an aggressive policy of removing all sources of heat inside the
telescope chamber has been employed, with all heat generating equipment,
electronics racks, etc, being housed in sealed insulated cabinets ('igloos')
with plumbed-in glycol to remove heat.
 Although the SALT design limits some types of observations that can be
undertaken, it is still well suited to many science programs for 10-m class
telescopes, particularly spectroscopic surveys. SALT will access over 70% of the
southern sky, from declination +10° to -75°, and will be able to observe
suitably placed targets from 1 to 3 hours at time (twice a night for optimal
positions), depending on their declination.
SALT will be operated entirely as a queue-scheduled telescope by SALT
operations staff, avoiding the expense of sending observers to the facility.
This modus operandi is well suited to synoptic or monitoring observations and
survey follow-up science. SALT's pre-eminent role will be as a spectroscopic
telescope, operating initially in the optical regime (320-900 nm), but with
capability to the near infrared (about 2 microns). However, SALT will also have
imaging and photometry (measuring berightness) capabilities.
In addition, SALT is designed to target some niche areas, with the three
first generation instruments (being built by some of the SALT partners) capable
of the following types of observations:
ˇ Low and medium resolution spectroscopy down to the atmospheric UV cutoff at
~320 nm. Important for almost all branches of astronomy.
ˇ Multiple Object Spectroscopy (~100 objects at a time) with laser cut slit
masks. Well suited for cosmological studies of the Universe and dynamical
studies of galaxies.
ˇ Polarimetry and spectropolarimetry (linear, circular and 'all-Stokes').
Important for the study of accretion phenomena, mass outflows, star formation,
etc.
ˇ Optical fibre-fed high resolution (R 17,000 to 80,000) single-object
spectroscopy over 370-850 nm in one exposure. Ideal for searching for planets
around other stars.
ˇ Fabry-Perot imaging spectroscopy (up to R 13,000). Targeted for studying the
mass distributions in galaxies and stellar kinematics.
ˇ High-speed spectroscopy and photometry (10Hz observations with minimal
deadtime). Ideal for studies of accretion physics, black holes and neutron
stars.
The UV capability is possible through the use of new high efficiency
multi-layer reflective coatings and optical materials with good UV throughput.
Custom made so-called frame transfer Charge Coupled Devices (CCDs) are used two
prime focus instruments, mounted atop the telescope, which will enable high time
resolution observations.
SALT has recently achieved two significant milestones: the 'first light' of
the imaging camera CCDs (on another telescope) and the installation and active
control of the first seven segments. Over the next few months the prime focus
tracker - the heart of the telescope - will be installed, which will lead to the
commencement of the first on-sky engineering tests before the end of this year.
Commissioning will take place during 2004, which will see installation of the
remaining subsystems, the delivery of the prime focus instruments and the final
batch of mirror segments by the end of the year. Science verification
observations will then begin late in the year or early in 2005. The final
first-generation instrument, HRS, will be commissioned in 2006.
Salt's green light
The 'green light' for the SALT project was given by the South African
government in late 1999, after sufficient matching funds had been committed by
the SALT partners, which now number 11 institutions in 6 countries.
At a cost of about US$32M, including the first generation instruments and
operations costs for the first 10 years, SALT represents a cost effective means
for access to 10-m class astronomy. Currently the project is 90% funded, with
the remaining US$2.7M required primarily for the High Resolution Spectrograph
and also to cover some cost escalation in the telescope subsystems. The
following are the SALT shareholding institutions, with the addition of the Hobby
Eberly Telescope Board as a non-shareholding partner:
South African National Research Foundation : 34.4%
University of Wisconsin-Madison : 15.1%
Nicholas Copernicus Astronomy Centre (Poland) : 11.0%
Rutgers University : 10.8%
Dartmouth College : 9.6%
Göttingen University (Germany) : 4.9%
University of Canterbury (New Zealand) : 4.1%
United Kingdom SALT Consortium : 3.9%
Carnegie Mellon University : 3.1%
University of North Carolina : 3.1%
More information
David Buckley SALT Project Scientist (dibnob@saao.ac.za)
SALT website: www.salt.ac.za
|
