The optical and science segment of the largest space telescope ever now stands complete in the largest clean room on the planet, at NASA's Goddard Space Flight Center. Now poised to undergo rigorous pre-flight testing ahead of its 2018 launch, the James Webb Space Telescope will span the size of a tennis court when fully assembled and will look back over 13.5 billion years to help unravel the mysteries of the early universe, such as the formation of the first stars and galaxies, while also studying planets around distant stars.

The James Webb Space Telescope (JWST) has been almost two decades in the making and as the successor to NASA's Hubble space telescope, will boast seven times the light-collecting capacity of that long-orbiting spacecraft and will be sensitive enough to spot a single firefly a million kilometers away.

NASA and its collaborators on the project, the European Space Agency and the Canadian Space Agency, have quite the laundry list of cosmic queries they hope these capabilities can help answer, and the completion of the telescope element marks a significant milestone on the journey.

The primary mirror is made up of 18 hexagonal segments coated in gold that when placed together make up a 21.4-ft (6.5-m) reflector. A mirror this big has never been fired into space before, so the segments will need to actually be folded up and crammed inside the rocket prior to launch, and then unfurled again after reaching orbit.

The completed telescope segment was stood upright after technicians completed a high-precision optical measurement of its primary mirror, something called a center of curvature test. With the telescope set to be put through a number of violent testing phases mimicking the stresses of a launch and a journey into space, it is important that the team grabs before and after snapshots so they know how its shape might be affected throughout.

So optical measurements are taken before and after the simulated launch, and then the results compared to gauge how the mirror might be altered, degraded or possibly even ruined. The team does this by using an instrument that takes measurements of light reflecting off the mirror at a rate of 5,000 "frames" per second – a speed that's faster than even the smallest of vibrations in the clean room that may otherwise interfere with measurements, while avoiding physical contact lessens the risk of scratches and other damage.

Initially, the shape and position of the mirror will then be compared to a computer-generated hologram bearing the ideal dimensions, so the team can confirm everything is as it should be before testing commences, and then again afterwards to gain insights into how the telescope will work after launch.

Though the telescope segment is complete, there are still some fairly important bells and whistles to be attached before it enters operation. After the upcoming launch testing phase is complete, it will be ferried off to NASA's Johnson Space Center in Houston for cryogenic testing, before traveling to Northrop Gunman in LA. Here it will be fixed to its spacecraft bus and sunshield, before launching on the ESA's Ariane 5 rocket in 2018.

The video below provides an overview of the mission's progress.