The‘Eye of Heaven’

Mao+YUAN

On the morning of October 10， 2017， the National Astronomical Observatory of the Chinese Academy of Sciences held a press conference to publicize the achievement made by the "Eye of Heaven"--Five-hundred-meter Aperture Spherical Radio Telescope， FAST for short）. Construction on the FAST project began in 2011 and it achieved first light in September 2016. Gratifying results have been made in 2017. This telescope has aroused the attention of scientists all over the world since the beginning. It is estimated that FAST will hold the lead in 10 to 20 years.

Introduction on FAST

Radio telescopes are telescopes that are mainly observed in the radio wavelength band （the wavelength band of some microwaves and radio）. There is a variety of electromagnetic waves from the universe， but different components in the atmosphere will block the cosmic electromagnetic wave. In order to see that blocked part， the scientists send a lot of telescopes to the outer space with rockets， hence the name space telescopes. The optical telescopes and radio telescopes are the ones left on the ground.

The "five-hundred-meter aperture" means the single reflector diameter is 500m. FASTs surface is nearly spherical rather than parabolic. You will get a spherical surface when cutting off a ball and turn it into a parabolic surface after squashing the spherical surface. For example， the famous American "Very Large Array" is a parabolic telescope， each each of which has a dish diameter of 25 meters （Imagine how large FAST is）. Each telescope has a device in the center above the reflector， called the feed. What is it used for？ It turns out that the parabolic reflector will assemble the radio signals to a point， which is on the feed， so the feed is used to collect the signal.

‘Heavenly Eye -- The Champion

the world's largest filled-aperture radio telescope

One of the most important parameters of a telescope is the aperture. The larger the aperture， the greater the area of the received electromagnetic wave， the more abundant the information， and the farther the celestial body to be observed. Some major radio telescopes with large aperture in the world are shown in Fig.

Interestingly， compared with Arecibo in the United States， the Effelsberg in Germany has been dubbed the "biggest machine on the ground" because it can turn around 360 degrees up and down while Arecibos reflector is basically fixed on the hill， with only the the feed cabin on the top moveable. However， Effelsberg is no longer the largest since the FAST is built.endprint

World Leading Innovation

Among the world-renowned telescopes described above， a common feature of the smaller aperture telescopes（except Arecibo and FAST） is that they are parabolic rather than spherical. What is the difference when reflecting the signal？ The parabolic reflector can concentrate the signal on one fixed point， which is convenient to collect the signal， while the spherical reflector will reflect the signal to several points， not fixed.

Why are there two different kinds of telescopes？ Because the stars are moving all the time， the directions of the signal will also change， so the reflector （which resembles a "pot"） must also be followed together to ensure that the signal is from the same star. The telescope's tracking function is achieved by the rotation axis of the telescope's supporting structure， but for a large telescope with several hundred-meter aperture， what support structures and power systems are capable of supporting this time-to-time tracking ？ The answer is no！

FAST is innovative in solving this problem--its "pot" will automatically chase the star and then transform. First， FAST is located in a natural sinkhole and the reflector is supported by a mesh of steel cables hanging from the rim， so that it can maintain distance from the ground. Second， there is a spherical reflector composed of 4600 triangular panels with an accurate size of millimeter. Finally， the winches located underneath make it an active surface， pulling on joints between 6 panels.

When the star moves towards the FAST， the small panel is pulled down by computer-controlled winches， and the entire sphere is flattened and turned into a parabola， and when the star leaves， the paraboloid is restored to a spherical surface， and is pulled into a parabolic surface in another direction. The whole spherical surface is continuously deformed and restored following the motion of the star. In this way， FAST can complete the tracking with large aperture and good observation. This is an invention in the world， which comes from the wisdom of countless scientists and engineers on the project！

As time goes by， FAST will see more mysteries of the universe and may even receive signals from outer space！ Here， we want to express gratitude to the chief scientist and chief engineer Rendong NAN who passed away on September 15 for his contribution to Chinese astronomy！endprint