The Southern African Large Telescope

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The Southern African Large Telescope
Engineering a Giant Eye
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• There are two main reasons for building big
  telescopes like SALT
• 1. They can catch lots of light
• 2. They can resolve more detail

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The light aspect opens up new areas of
research It enables SALT to see faint things.
These could have a low surface brightness or be
far away or both. For example low surface
brightness galaxies In enables SALT to take
quick exposures and reveal what is happening in
rapid events. For example interacting binary
stars
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Designing a giant eye like this sets difficult
challenges to scientists and engineers.
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Lets look inside the dome to see how they have
met these challenges.
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SALT has a new design that gives it all of the
advantages of large telescopes for about a tenth
of the cost.
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SALTs light collecting mirror is made up of 91
hexagonal elements, each 1m across. The segments
are spherically concave. Using lots of small
mirrors rather than one makes it much lighter and
easier to handle. All these mirrors have to be
precisely aligned to get them acting as one
mirror.
11.1 meters (36.4 feet)
9.8 meters (32.2 feet)
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Thats what the 28m tall tower is all about. It
houses a laser that is fired down into the
telescope.
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Through the dome opening
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past the instrument payload
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and then reflects from one mirror back into the
tower.
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Motors attached to the mirrors adjust their
position until they are all acting together as
one giant mirror.
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To point this huge mirror accurately would
require precise engineering on an equally huge
scale. This is very expensive.
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Instead, the mirror is locked at a vertical angle
of 53 degrees. It is moved horizontally into
the area to be observed by revolving it on
bearings. These are made of hovercraft-like
rubber skirts. The bearings ride on one of the
smoothest concrete surfaces in the world .
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• The instrument payload of the telescope is
  accurately scanned over the main mirror. The
  engineering of these precise movements is very
  challenging but far less expensive than pointing
  the main mirror.

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The instrument payload runs with extreme
precision on two sets of rails. One set lets it
move vertically and the other, horizontally.
Vertical
horizontal
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This animation shows the movement as viewed from
the main mirror below.
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This animation shows the movement as viewed from
the main mirror below.
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Together, these movements give SALT the ability
to track objects accurately through 12o in both
directions.
12o
12o
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59o
47o
12o
When combined with the rotation of the telescope,
this gives SALT a 12o wide circular view of the
sky. The window extends 6o either side of
53o - i.e. between 47o and 59o to the horizon.
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SALT uses the rotation of the Earth to bring
objects into its 12o tracking window.
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This image, a long photographic exposure, shows
the clockwise rotation of the sky around the
southern celestial pole.
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South
The combination of the revolving of the
telescope, the scanning of the instrument payload
and the movement of the sky results in SALT being
able the see objects that pass through a circular
band around the sky between 47o and 59o to the
horizon.
59o
47o
North
Horizon
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South
Compare the time an object stays in the upper 12o
tacking window (near to the southern celestial
pole) to the time it spends in the lower 12o
tracking window (further from the southern
celestial pole).
59o
47o
North
Horizon
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The objects nearer to the pole move more slowly
and so can be tracked for longer. The two grey
patches represent the Magellanic Clouds
satellite galaxies to the Milky Way. SALT is
designed to be able to track these objects for
the longest time.
South
59o
47o
North
Horizon
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The objects nearer to the pole stay for more than
three hours in the tracking window. Those at the
most northerly latitudes can be tracked for less
than an hour. About 10 of the sky, for example
the area close to the pole itself, never enters
SALTs view.
South
59o
47o
North
Horizon
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Here is another way to show the information
with the X axis representing the time. The
width of the shaded areas show the maximum and
minimum times that objects can be tracked for.
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Lets sum up the three ways SALT is engineered to
point at objects in the sky

• Horizontally positioning by rotation on air
  bearings
• 12o of precision horizontal and vertical tracking
  by the instrument payload.
• Movement of the sky due to the rotation of the
  Earth.