Figure: Working on Hubble's Main Mirror

Workers study Hubble's main, 2.4 m mirror. Hubble, like all telescopes, plays a kind of pinball game with light to force it to go where scientists need it to go. When light enters Hubble, it reflects off the main mirror and strikes a second, smaller mirror. The light bounces back again, this time through a 0.6 m hole in the center of the main mirror, beyond which Hubble's science instruments wait to capture it. In this photo, the hole is covered up.

Credit: NASA

http://hubblesite.org/gallery/spacecraft/22/