An industrial team led from Astrium in Germany has completed the build of the Near-Infrared spectrometer, one of four instruments that will go in JWST.
NirSpec's job will be to determine the age, composition, movement and distance of the objects in its field of view.
The expectation is that some of these targets will include the very first stars to shine in the Universe.
That would mean picking up light signals that have travelled across space for perhaps 13.6 billion light-years - something Hubble cannot do.
JWST will make it possible with a suite of next-generation technologies, including a 6.5m primary mirror (more than double the width of Hubble's main mirror), and a shield the size of a tennis court to guard its keen vision against the light and heat from the Sun.