The 500-meter Aperture Spherical Telescope — known as FAST, the world's largest radio telescope in Pingtang county, Guizhou province — has been upgraded and maintained many times after making some great achievements.

The so-called China's sky eye has a 250,000-square-meter "retina". With more than 2,200 hydraulic actuators acting as neurons to precisely control the "retina", how do engineers keep it healthy?

The active deformable reflector is a significant technological innovation that distinguishes the FAST from traditional radio telescopes. Those hydraulic actuators receive instructions from the control room to flexibly adjust the position of 4,450 reflection units to achieve a specified position for observation.

During astronomical observations, it is difficult for naked eyes to see the deformations — tiny changes of the huge reflector from the surface — but the noise of thousands of hydraulic actuators roaring together can be heard. Because of their heavy workload, the hydraulic actuators get the most care and maintenance.

"The chief engineer, Jiang Peng, proposed that a safety assessment system must be built for the super reflector," said Li Hui, an engineer responsible for the structure during debugging.

Then, the team applies the integrated simulation of whole process technology to completely digitize the entire reflector through a mechanical model to show the influence on the telescope inserted by bugs during observation. Alarms go off automatically when it's beyond its limits.

With the help of the system, engineers can do "eye exercises" for the telescope to inspect and maintain normal operations. If a problem arises on the reflection panel, engineers will mimic a "spacewalk" by using a helium balloon 7.6 meters in diameter to greatly reduce their weight to stand on the panel airily to make repairs.

In this way, the world's largest telescope undergoes the world's most delicate and intelligent "eye exercises".

Wang Jin contributed to this story.