NASA has finished assembling the primary mirror array on its biggest telescope yet. There's still a lot of work to be done on the telescope, dubbed the James Webb Space Telescope, before it launches in 2018, but this marks a major milestone for the telescope that will ultimately succeed the Hubble Space Telescope.

Installing the mirrors has been a careful process

Installing the mirrors has been a careful process. The 18 hexagonally shaped segments are made of ultra-lightweight beryllium and measure about 4.2 feet across. Each mirror had to be fitted on top of a specialized frame, and they are separated from each other by just a few millimeters. A highly precise robotic arm had to be used to slowly lower each mirror onto its exact location.

The frame that holds the mirrors is designed to fold up, so that the telescope can become a more compact payload for launch. Once in space, the James Webb will travel to its location of up to 1 million miles from Earth. Along the way, the frame will start to unfold, eventually forming a mirror measuring 21.3 feet in diameter. That's nearly three times longer than the diameter of Hubble's primary mirror.

Though all 18 main mirrors have been assembled, the James Webb team still has more to install. More optics technology is needed, along with a secondary mirror measuring 2.5 feet in diameter. All of these pieces will work together so that the James Webb can observe galaxies over 13 billion light-years away.