Free Astronomy Magazine September-October 2016

Arecibo, where it is counteracted through more complex solutions. Obviously, radio telescopes of these dimensions are not ad- justable, because they don't have a mount similar to that of optical te- lescopes or smal- ler radio tele- scopes. In theory, it should there- fore be possible to observe only celestial objects that pass the ze- nith. Indeed, by acting on the orienta- tion of the device receiving the waves collected by the disk and, in the case of FAST, also acting on the shape of the disk itself, it is possible to widen the re- gion of sky to be investigated up to a few tens of de- grees from the zenith (up to 40° for FAST, almost 20° for the Arecibo instrument). Whatever the position chosen for the ob- servations, the op- tic geometry of FAST implies that only a 300-meter wide portion of the disk will ac- tually be used each time. It is the so-called “il- luminated aper- ture”. The same goes for the more static radio tele- scope of Arecibo, of which is used every time an area not much bigger than 200 meters. Leaving out other technical details that might be boring, what has been said till now is enough to sense the conside- rable progress in T he final stages of the instal- lation of the disk, with the as- sembling of the last panel and the party of the technicians, which released a lot of colorful balloons. [NAOC, Xinhua/ REX/ Shutterstock]