Giant Magellan Telescope
The telescope is unique because it will use seven mirror sections, each  in diameter.  These segments will then be arranged to form a single optical surface. This is difficult as the surface of the outer six mirror segments is not radially symmetrical (that is, they are off-axis). This requires a small change in the way mirrors are usually polished. All seven mirrors will be arranged so that the reflecting surface of each of them is on a common axis. One mirror will be in the center and the other six arranged symmetrically around the center. A focal plane will be on-axis with the entire assembly.  The plan is to build seven identical off-axis mirrors. The spare mirror has been made to replace a mirror when it needs recoating, a 1–2 week (per segment) process required every 1–2 years.