Sky and Telescope - February 2017 - 29

BOT TO M: FAST PROJECT; CHINESE ACA DEM Y OF SCIENCES / LING XIN; TOP RIG H T: CHINESE ACA DEM Y OF SCIENCES / LING XIN & CH AO ZH A NG; DIAGR AM: SK Y & TELESCOPE / LE A H TISCIONE

Incoming
radio energy

Spherical reflector

Paraboloidal reflector

S SHAPE SHIFTING A spherical reflecting surface, like that used
at Arecibo Observatory, can't bring all incoming radio energy to a
single focus. However, a portion of FAST's spherical surface can be
deformed (red arrows), effectively making a paraboloidal sub-reflector
that's 300 m (1,000 feet) across.
X PULLING TOGETHER The shape of FAST's reflective surface
can be modified by more than 2,200 mechanical actuators that are
anchored to the ground and connected by cables to the dish.

overall surface must be controlled to within 3 mm. In fact,
each panel has a curved surface that conforms closely to the
paraboloid's shape. This actuator-controlled design allows the
surface to be reshaped continuously, so it can track specific
celestial targets for several hours as they cross the sky or shift
from one aim point to another.
However, such a shape-changing operation creates up to
500 megapascals of stress in the underlying network of steel
cables, and the plan is to do so often during observations.
So the engineering team had to install specially built cables,
developed in China, that can survive this much stress for 2
million fatigue cycles - ensuring reliability of the reflecting
surface for the facility's planned 30-year lifetime.
T MAJOR UNDERTAKING The FAST effort transformed a large,
natural depression known locally as Dawodang ("grand puddle") from
a tiny hamlet with 65 villagers to the world's largest single-dish radio
telescope. Before construction could really begin, crews first removed
nearly a cubic kilometer of rock.

The Receiving End
All of the radio energy collected during an observation is
directed to a feed cabin that is 13 m long and weighs only 30
tons. (By contrast, Arecibo's very large receiver platform has a
total mass of 900 tons.)
Designing an overhead feed cabin that can be supported
and positioned precisely via cables became a real challenge.
The team built quite a few scale models and carried out many
simulations to verify the feasibility of the concept.
The chosen cabin-suspension system connects three
opposed pairs of cables to six towers around the circumference that average more than 100 m tall. Instead of moving
the receivers along a fi xed track on the central platform, the
cable lengths are adjusted at the towers to position the entire
feed cabin to within 100 mm in a 206-m-wide "focal plane."
The cables also adjust the cabin's tilt so that it closely parallels the curved reflector surface below.
After this rough positioning, robotic motors within the

s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

29


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Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
Sky and Telescope - February 2017 - 4
Sky and Telescope - February 2017 - 5
Sky and Telescope - February 2017 - 6
Sky and Telescope - February 2017 - 7
Sky and Telescope - February 2017 - 8
Sky and Telescope - February 2017 - 9
Sky and Telescope - February 2017 - 10
Sky and Telescope - February 2017 - 11
Sky and Telescope - February 2017 - 12
Sky and Telescope - February 2017 - 13
Sky and Telescope - February 2017 - 14
Sky and Telescope - February 2017 - 15
Sky and Telescope - February 2017 - 16
Sky and Telescope - February 2017 - 17
Sky and Telescope - February 2017 - 18
Sky and Telescope - February 2017 - 19
Sky and Telescope - February 2017 - 20
Sky and Telescope - February 2017 - 21
Sky and Telescope - February 2017 - 22
Sky and Telescope - February 2017 - 23
Sky and Telescope - February 2017 - 24
Sky and Telescope - February 2017 - 25
Sky and Telescope - February 2017 - 26
Sky and Telescope - February 2017 - 27
Sky and Telescope - February 2017 - 28
Sky and Telescope - February 2017 - 29
Sky and Telescope - February 2017 - 30
Sky and Telescope - February 2017 - 31
Sky and Telescope - February 2017 - 32
Sky and Telescope - February 2017 - 33
Sky and Telescope - February 2017 - 34
Sky and Telescope - February 2017 - 35
Sky and Telescope - February 2017 - 36
Sky and Telescope - February 2017 - 37
Sky and Telescope - February 2017 - 38
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Sky and Telescope - February 2017 - 40
Sky and Telescope - February 2017 - 41
Sky and Telescope - February 2017 - 42
Sky and Telescope - February 2017 - 43
Sky and Telescope - February 2017 - 44
Sky and Telescope - February 2017 - 45
Sky and Telescope - February 2017 - 46
Sky and Telescope - February 2017 - 47
Sky and Telescope - February 2017 - 48
Sky and Telescope - February 2017 - 49
Sky and Telescope - February 2017 - 50
Sky and Telescope - February 2017 - 51
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Sky and Telescope - February 2017 - 53
Sky and Telescope - February 2017 - 54
Sky and Telescope - February 2017 - 55
Sky and Telescope - February 2017 - 56
Sky and Telescope - February 2017 - 57
Sky and Telescope - February 2017 - 58
Sky and Telescope - February 2017 - 59
Sky and Telescope - February 2017 - 60
Sky and Telescope - February 2017 - 61
Sky and Telescope - February 2017 - 62
Sky and Telescope - February 2017 - 63
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Sky and Telescope - February 2017 - Cover3
Sky and Telescope - February 2017 - Cover4
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