Free Astronomy Magazine September-October 2016

100 meters tall, al- lows to contain the alignment error within 100 mm with respect to the ideal “opti- cal” path. Finally, additional devices in the cabin and at the base of the disk reduce the error to just 10 mm, ensuring a resolu- tion of 2.9’ arc (L- Band) and a pre- cision in pointing equal to 8 arcsec- onds. Thanks to the de- formability of the disk, which in fact passes from spher- ic to parabolic, FAST resolves in a brilliant way the problem of the spherical aberra- tion, that instead affects the disk of cular trellis, nearly 1.6 km long, the top of which was used as a rail for car- rying and mounting the various components of the radio telescope. The cable-mesh is flexible and, within certain limits, can be deformed to give to the reflective disk the most suitable configura- tion for the observation of certain areas of the sky. A focusing and signal control cabin (a kind of “illuminator”) is placed 140 meters above the centre of the disk and through servomecha- nisms connected to 6 pe- rimetrical towers, about A group of technicians assembles one of 4450 panels be- fore the crane transports it into place on the steel cable-mesh. On the side, FAST al- most completed. Below, a cut view of the huge struc- ture highlights the strong curva- ture of the disk and the impres- sive system of py- lons and cables that hold it in place. [NAOC, Xinhua, VCG via Getty Images]