When we say the words “radio telescope,” the well-known Arecibo observatory immediately resonates with us, yet it is far from being the only one in the world of astronomy. Indeed, through slightly different technologies and specifications, many others around the world enhance and complement much of its work. The topic of our article is one of these radio telescopes, located in south-western China and known as “FAST,” shot for “Five-hundred-meter Aperture Spherical Telescope.” 

FAST was built from March 2011 to July 2016 in a natural basin called the Dawodang depression. The terrain enabled the Chinese to build the telescope dish into the landscape whose 500m diameter claims the title of “world’s largest filled-aperture radio telescope.” The telescope then entered a testing phase, after which it was declared fully operational in January of 2020. However, this rather long testing phase did not halt FAST’s scientific pursuits. On the contrary, it made some exciting discoveries in the meantime. The most recent one is information on Fast radio bursts or FRBs, which we will cover later in this article.

An aerial view of FAST
An aerial view of FAST | Image: Ou Dongqu/Xinhua/ZUMA

For what purpose was the FAST telescope commissionned ?

Radio telescopes, as indicated by their name, investigate the radio frequency portion of the electromagnetic spectrum. Radio waves are on the lower end of the spectrum, energetically speaking. All astronomical objects such as stars, nebulae, planets, and galaxies emit radio waves. However, they are very faint and thus require large disks to be intercepted, a characteristic of radio telescopes. 

In the case of FAST, it was commissioned with a few scientific missions in mind. First of all, it will be studying pulsars, observing them, and running correlations on the timings of their rotations. Secondly, FAST will be studying interstellar molecules, more specifically, conducting a large-scale neutral hydrogen survey. Neutral hydrogen is an electrically neutral hydrogen atom with one proton and one neutron. In the cosmos, it is located throughout galaxies, and its detection helps scientists determine the structure of our galaxy.

FAST will also be leading a network of telescopes that perform VLBI, “very long baseline interferometry”, a technique allowing the network to all simultaneously collect and aggregate data from an astronomical radio source, such as a quasar, and using the time delay in the detection between the telescope and the distance separating them, to simulate a much larger radio telescope. Finally, FAST will be joining the SETI project, the Search for Extraterrestrial Intelligence. We can now confidently state that it is no coincidence that FAST is commonly nicknamed “Tianyan” or “Heaven’s Eye.”

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History and geographical considerations

The location of FAST is very convenient, the shape of the terrain assimilating itself to that of the telescope disk, a perfect place to receive it—however, an advantage to the detriment of the local population, which had to be relocated. The total cost of the telescope was 180 million dollars. Indeed, several difficulties were encountered during its construction; the site was remote and thus had poor access. 

Specifications and technology overview

Accompanying the previously mentioned 500 meters reflecting surface of the telescope is a plethora of technology that together build a precise and innovative telescope. The main reflector disk, made of 4450 triangular aluminum panels and supported by steel cables, works by focusing the received radio waves on a receiving antenna thanks to its incurved convex shape (that of a geodesic dome). This antenna is suspended 140 meters above the disk and finds itself in a so-called feed cabin that can be moved around in the desired direction of interception. The underlying mechanism is complex and relies on six support towers and a cable robot to move the cabin around and compensate for any movement with astonishing precision.  

How The 'Heaven's Eye' Is Searching For Signs of Life Beyond Our Planet. 1
Concept of the adaptive cable-net structure, the supporting structure for the FAST reflector. | Image: NAOC/CAS

Although the telescope’s aperture is officially 500 meters, circular sections of 300 m are used or illuminated by the receiver at a time. This allows for a wider range and several positions at which the telescope can be pointed.

How The 'Heaven's Eye' Is Searching For Signs of Life Beyond Our Planet. 2
Schematic view of the feed cabin suspension system | Image Credit: NAOC/CAS

What makes FAST so special?

At the beginning of the article, we mentioned the Arecibo telescope, the former grand and notorious radio telescope. So how does FAST distinguish itself from it? Indeed, in terms of situation and composition, both are pretty similar. However, we can find some significant differences which make FAST unique. Firstly, FAST’s disk technology is pretty novel. It is suspended and moves around automatically, which is an improvement from the manual adjustment that was required in Arecibo.

Another difference from Arecibo to FAST is the depth of the main disk. Indeed, that of FAST is much deeper, allowing for a wider field of view. And furthermore, Arecibo’s receiving platform was fixed in place, limiting its range of motion. 

On the other hand, FAST also has its limitations. Indeed, due to the size of its primary reflector, the frequencies that FAST can receive are more limited than those of Arecibo. In addition, Arecibo hosted additional radar technology, which allowed it to track and study solid objects.

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Vladimir Vega
Vladimir Vega

I keep saying “amazing” to the Chinese strong resolve to follow the Universe´s behavior through innovative technologies. I have been interested in Astronomy since my early teen years and I appreciate what these dedicated professionals are doing about it. Thank you Anja for this wonderful report. It is a shame that the Puerto Rican telescope crashed not long ago, but now we have a more capable one. Have a great day people!

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