The sixth anniversary of the "China Sky Eye"! What achievements has this unique spherical radio telescope made?

On September 25, 2016, the Five-hundred-meter Aperture Spherical radio Telescope (FAST), known as the "China Sky Eye", was completed and put into use. As of July this year, the "China Sky Eye" has discovered more than 660 new pulsars. On the sixth anniversary of the completion of the "China Sky Eye", let us follow FAST's chief engineer Jiang Peng to learn about the original intention of FAST's construction, what scientific research results FAST has achieved so far, and what FAST may bring us in the future.

01

What is FAST observing?

FAST is a radio telescope that observes space in the radio band. The discovery of the radio band was due to a very accidental event. The study of radio astronomy has led to four very important new discoveries, namely pulsars, interstellar molecules, microwave background radiation, and quasars. Among them, the observation of pulsars won two Nobel Prizes, including the discovery of neutron stars and gravitational waves.

02

What is the “driving force” behind the birth of FAST?

To obtain more and more detailed astronomical data, a telescope with a larger receiving area is needed, which is true for both optical telescopes and radio telescopes. A larger receiving area means stronger dark and weak signal detection capabilities, which can expand the number of observation samples, increase the probability of discovering strange astronomical phenomena, and also allow for the observation of more distant astronomical phenomena. Generally speaking, the more distant astronomical phenomena represent earlier astronomical phenomena. For example, we saw the sun 8 minutes ago, the edge of the Milky Way may be hundreds of thousands or two hundred thousand years ago, and the neighboring galaxy M31 may be tens of thousands of years old. The quasars we see may be more than 10 billion years old. So what if we want to see earlier cosmic phenomena? We can only look in a farther direction.

Therefore, a larger telescope will be of great help to us in tracing back the history of the universe, and building a larger telescope is an endless pursuit of scientists. It has been so in the past, it is so now, and it will be so in the future. The driving force behind the construction of FAST is to be able to see farther and to trace back the more distant history of the universe.

03

What are the characteristics of FAST?

As a spherical telescope, FAST is different from traditional telescopes. Traditional telescopes all use parabolic surfaces. By placing the receiver at the focus of the parabola, signals can be collected. So what is the difference between a spherical surface and a parabola? How far will they deviate? This is a question that many people have never imagined. In the 1990s, Chinese astronomer Nan Rendong and his colleagues calculated that as long as the appropriate cross ratio is selected, a 300-meter parabola will only deviate from a spherical surface by 0.47 meters, which is about 1.5‰. This is unexpected to many people. It is this 0.47-meter deviation that became the cornerstone of the establishment of FAST.

With this concept, we can first build a reference sphere, and slightly change the shape of the reflector (0.47 meters), and then we can form a 300-meter parabola on this sphere. When we observe different sky regions, we only need to form parabolas at different positions, and the remaining problem is how to place the receiver at the intersection position with the correct attitude.

Although the concept is simple, it took about ten years to perfect it. How does this parabola deform? What carrier is used for deformation? How does the receiver control the intersection? These are all being explored. Finally, the FAST system is like this: a cable net formed by nearly 10,000 steel cables is hung on a 500-meter-diameter ring beam. This cable net has more than 2,000 main cable points. Each main cable node is equipped with a control cable, which is fixed to the actuator on the ground. Scientists can change the shape of the reflector through actuator control, forming a 300-meter parabola in its local area. There are 6 sets of cable drives on it to control a 30-ton feed cabin, and a receiver is installed in the cabin to control it to the intersection. At the same time, 20 total stations were set up to measure the shape of the reflector and the position and attitude of the receiver to ensure the control of the receiver's attitude. Finally, the extremely complex telescope system of FAST was formed.

04

How big is FAST?

We all know that the aperture of FAST is 500 meters, but how big is it exactly? How can we quantify the concept of "500-meter aperture" for people who have not seen FAST in person? A colleague proposed this idea: Let's imagine FAST as a pot filled with water. Each of the 7 billion people in the world can get 4 bottles of mineral water from this pot.

05

What are the difficulties in building FAST?

When quantitative changes accumulate to a certain level, they will produce qualitative changes, and the engineering difficulty brought about is also extremely great. The construction of FAST's cable net is a difficulty.

The cable net used to support the sphere is a deformable carrier, just like a balloon, and the "cable" needs to be used as a rubber band or spring. For a rubber band, it can be stretched to one or even two times, but the limit deformation of the steel cable is only 3‰-4‰, which is very small.

Part of the parabola of FAST is below the spherical surface, and part of it is above the spherical surface, which means that the cable net connecting the parabola must be able to be pulled down and loosened upward. However, the elastic deformation of the cable is only 3‰-4‰. It cannot be broken when pulled down, and it cannot be loose when loosened upward. We need to achieve both functions of "pulling down" and "loosening upward" within the deformation range of 4‰, and it must be just right, leaving very little room on both sides.

In addition, the machining accuracy of the steel cable is also very important. If each index is one millimeter long, the cable will not be able to loosen any more. In addition, temperature changes will also eat up some of the elastic deformation, so the machining accuracy of the entire cable structure is extremely high, and the margin left is very small. The entire design is like walking on a single-plank bridge. If you deviate a little, the project will fail.

In addition, in order to ensure that there is no interference from other radio signals, FAST was located in a remote mountainous area in Guizhou. Many large equipment could not enter the site, so a large amount of construction work was done manually, including a tower over 170 meters high, 38 meters deep manually dug piles, and nearly 10,000 steel cables that were scattered in the air. This is a large-scale project that can only be completed with the wisdom of our working people and ingenious construction methods.

06

What are the difficulties in debugging FAST?

The FAST system is different from traditional telescopes. The parabola of a traditional telescope is naturally matched with the receiver at the intersection, no matter which direction it points. But for FAST, its reflective surface is a 500-meter-span cable net, a complex cable net system woven with nearly 10,000 steel cables, with a 600-meter-span cable drive structure on top and a feed cabin weighing more than 30 tons on the outside. The two systems are completely independent, and both systems are flexible control systems. To achieve millimeter-level positioning accuracy on a kilometer-level scale, this is something we have never encountered in traditional industrial stadiums. For traditional engineering precision, a difference of 1 meter in 500 meters will not have any visual impact or safety impact. But for FAST, an error of a few millimeters will reduce the efficiency of the antenna.

In addition, FAST works in the rainy and foggy environment of Guizhou. All measurement and control must be able to be implemented around the clock, which means that a large number of optical measurement experiments cannot be used around the clock, and everything must be reconsidered. In order to ensure the all-weather operation capability of FAST, we finally adopted the fusion measurement technology of satellite positioning measurement system, inertial navigation and laser total station. Although the laser total station basically cannot work in rainy and foggy days, satellite navigation and inertial navigation can still ensure the all-weather operation capability of FAST.

On the one hand, this system combines different measurement methods to achieve complementary advantages; on the other hand, it also improves accuracy, and more importantly, improves FAST's weather resistance. Later, we also replaced the laser total station with a microwave rangefinder. In addition, we often use GPS "invertedly", using the base station as a satellite in the sky and inverting it to illuminate the feed cabin, thus achieving the goal of all-weather high accuracy.

07

What has FAST accomplished?

From 1937 to the present, radio telescopes have been developed for nearly 90 years. Our "China Sky Eye" has made a significant mark in the history of the development of cosmic telescopes. For the first time, we have stood at the commanding heights, and the lead is not just a little bit, but at least 2.5 to 3 times higher in accuracy.

FAST's sensitivity, all-weather operation capability, and more than 6,000 hours of observation time per year were all beyond our imagination at the beginning.

FAST is not only a radio telescope with excellent performance, but also a very easy-to-use radio telescope. As of today, scientists have discovered more than 660 pulsars through FAST, which is more than five times the total number of pulsars discovered by all radio telescopes in the world during the same period. At the same time, during its two years of operation, FAST has also helped scientists achieve some important scientific results, including 5 papers published in Nature and 1 paper published in Science. Some of these scientific results were selected as the top ten breakthroughs and discoveries by Nature in 2021. In addition, the top ten scientific advances selected by the academicians of the two Chinese academies in 2021 also include the results of FAST.

Among these scientific achievements, the most interesting one is the fast radio burst, which is a new astronomical phenomenon discovered in 2007. Fast radio burst is a phenomenon that emits energy equivalent to that of the sun for several days or even a whole year in a few thousandths of a second. Scientists are very curious about the source of fast radio bursts, but they have not been able to detect it clearly until the ultra-highly sensitive FAST clearly measured the change in the deflection angle of electromagnetic waves - this means that the magnetosphere structure plays a very important role in this type of celestial body, and it is basically predicted that it comes from a distant magnetar.

FAST can be used not only in scientific research, but also in practical applications. For example, if FAST is combined with active radar, it will be possible to see millimeter-level objects in a thousand-kilometer orbit, that is, to observe objects within 50 mm in the geosynchronous orbit. You should know that now space launch missions rely on the flux model of debris, and you need to avoid debris and find a reasonable launch window. However, the flux model used to be a foreign model, but with FAST, we can completely establish an autonomous and controllable flux model.

In addition, FAST can also help us with early warning of near-Earth objects. For example, in 2019, an asteroid passed by us at one-sixth the distance between the Earth and the Moon. The reason why humans did not detect it was because it came from the direction of the Sun, and the strong sunlight made it impossible for us to clearly observe the asteroid with an optical telescope. However, if we use the radio band to observe it, it is possible to detect it earlier, so it will be an important strategic support for near-Earth object defense in the future.

In the future, FAST will bring us countless imaginations. For example, adding a launch device to FAST will allow it to do more interesting things. Let us look forward to the "China Sky Eye" bringing us more novel things.

Finally, let us pay tribute to the builders of the "China Sky Eye"!

This article is based on the speech given by FAST Chief Engineer Jiang Peng at the Star Forum