Webb Telescope's scientists and engineers determined that a primary mirror 6.5 meters (21 feet 4 inches) across is what was needed to measure the light from these distant galaxies. Building a mirror this large is challenging, even for use on the ground. A mirror this large has never before been launched into space!

If the Hubble Space Telescope's 2.4 meter mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb team had to find new ways to build the mirror so that it would be light enough - only one-tenth of the mass of Hubble's mirror per unit area - yet very strong.

The Webb Telescope team decided to make the mirror segments from beryllium, which is both strong and light. Each segment weighs approximately 20 kilograms (46 pounds).

The Webb Telescope team also decided to build the mirror in segments on a structure which will fold up, like the leaves of a drop-leaf table, so that it can fit into a rocket. The mirror would then unfold after launch. Each of the 18 hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter, flat to flat. (Webb's secondary mirror is 0.74 meters in diameter.)