The telescope has presented some special engineering problems.
When light emitted by these stars and galaxies reaches us it has been shifted to the infrared part of the spectrum and so it will be observed with four very sensitive infrared instruments.
But a very large mirror and very low operating temperatures are crucial for these infrared measurements of very distant stars and galaxies.
If the mirror were built in a single piece it would be so large as to be too heavy to launch.
To get around this, engineers came up with a design for a foldable mirror composed of 18 hexagonal-shaped segments constructed from beryllium, a lightweight yet strong metal.
Each segment will be coated with a thin layer of gold, which is an excellent material for reflecting infrared light, protected by a thin layer of glass. The amount of gold needed for all of the JWST mirrors is equivalent in a size to a golf ball.
The mirror segments are mounted on to a black framework that forms the spine of the JWST, folded to fit inside the launcher.
Once in space, the JWST will unfold and spread its wings to make it the largest astronomical telescope in space.
JWST is an international collaboration between NASA, ESA and the Canadian Space Agency (CSA).
For more information about the exciting project, have a look at the Cosmos Magazine feature by Dan Falk, Pulling back the curtain on the Universe
Originally published by Cosmos as Engineers test James Webb Space Telescope mirrors that will gaze back 13.5 billion years
Bill Condie is a science journalist based in Adelaide, Australia.
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