Radome of Arecibo telescope. Interior of the 90- tonne dome housing the gregorian subreflector system installed in 1997 for the Arecibo radio telescope. Lynn Baker, co-developer of this sub- reflector system, is seen preparing alignment targets on the tertiary reflector (lower right). The secondary reflector mirror is at top, with the aperture at lower centre. The dome is suspended and steered 130 metres above the 305m diameter fixed primary dish (partly seen). It can send and receive signals to examine planets and asteroids, and also analyse Earth's upper atmosphere. Arecibo is the world's largest radio telescope situated in a natural crater in the mountains of Puerto Rico.