In the mountainous province of Guizhou in southwestern China, there is a place that looks as if a giant meteorite had crashed into it. In a natural karst depression lies a metallic dish 500 meters in diameter — about the size of 30 football fields. This is FAST, the world's largest radio telescope with a single filled aperture. It is often called China's “Heavenly Eye,” but in essence it is not an eye, but a supersensitive ear tuned to the almost imperceptible whisper of the Universe.

An optical telescope captures visible light. FAST captures radio waves — the same form of electromagnetic radiation that includes, for example, radio broadcasts and Wi‑Fi, but arriving from pulsars, interstellar gas, distant galaxies, and fast radio bursts. The problem is that over millions and billions of light-years, a cosmic signal becomes incredibly weak: according to estimates by the U.S. National Radio Astronomy Observatory, a mobile phone signal can be a billion billion times stronger than the natural radio waves detected by instruments like this.

That is why astronomers need a huge collecting area capable of gathering as much energy as possible. FAST solves this with sheer scale, but its defining feature is not only its size.

A dish that changes shape

At first glance, FAST resembles a stationary stadium with no field in the center. It cannot be turned to follow an object in the sky: the 500-meter structure is built into a natural basin and remains in place. However, the telescope still needs to “look” at different points in the sky. To achieve this, engineers made not the whole dish move, but only its working section.

The reflecting surface of FAST consists of about 4,500 metal panels. Beneath them operate thousands of actuators: they pull on elements of the cable mesh and slightly deform the surface. In this way, within the giant spherical dish, the section needed for observation appears — a 300-meter parabolic “mirror.” It is this active zone that, at any given moment, reflects radio waves to a single point.

In other words, FAST does not rotate the dish toward the signal source. It creates, inside the fixed half-kilometer dish, a new 300-meter dish that is virtually turned in the required direction. This is an important engineering distinction. A conventional parabolic antenna focuses incoming radio waves at one point thanks to the constant shape of its surface. In FAST, the original surface is spherical, and computers continuously reshape the required fragment into a parabola. The receiving system is aimed at it.

The receiver hangs above the void

Above the FAST dish, a lightweight cabin with the feed — the antenna that receives radio waves after they are reflected from the metal surface — is suspended on cables. This cabin must be positioned exactly at the focus of the created parabola: any error would mean losing the weak signal.

A system of six cables and servomechanisms moves the cabin through space, while a robotic platform fine-tunes its position with high precision. When the telescope changes its observing direction, two movements happen simultaneously: thousands of surface elements form a new 300-meter parabolic section, and at the same time the suspended cabin shifts to its new focal point.

It was precisely this combination that made it possible to build a telescope noticeably larger than the previous engineering limit for classic movable antennas. The official FAST website explicitly describes this scheme as a new approach to building large radio telescopes.

From radio wave to scientific discovery

Collecting radio waves is only the beginning. At the focus of the reflector is a receiver that isolates the required frequency ranges and converts the oscillations into digital data. Radio astronomers then search these data for repeating pulses, spectral lines of molecules, or barely noticeable changes in a source’s brightness.

The logic is similar to that of a camera with a very long exposure. The telescope can observe one region of the sky for hours, repeatedly accumulating the signal. Random noise averages out over time, while a faint cosmic source becomes distinguishable. Modern systems can simultaneously divide a wide frequency range into thousands of channels, making it possible not just to detect a signal, but to determine its structure and origin.

For FAST, the key areas of work include the search for pulsars, observations of neutral hydrogen, the study of interstellar molecules, participation in radio interferometry networks, and the search for unusual radio signals.

Pulsars are especially important: these are rapidly rotating neutron stars that send regular radio pulses into space. They can be used as extraordinarily precise natural clocks. A network of observations of such objects helps researchers study extreme physics, test models of gravity, and search for traces of gravitational waves on ultra-long timescales.