The Mid-InfraRed Instrument (MIRI) of the James Webb Space Telescope (Webb) sees light in the mid-infrared region of the electromagnetic spectrum, at wavelengths that are longer than our eyes can see.

To support the whole range of Webb’s science goals, from observing our own Solar System and other planetary systems, to studying the early Universe, MIRI allows scientists to use multiple observing techniques: imaging, spectroscopy and coronagraphy.

To pack all these modes in a single instrument, engineers have designed an intricate optical system in which light coming from Webb’s telescope follows a complex 3D path before finally reaching MIRI’s detectors.

This artist’s rendering shows the path for MIRI’s Medium Resolution Spectroscopy module, or MRS. Whereas the Low Resolution Spectroscopy mode is designed to observe single, compact objects like very distant galaxies or single stars, the MRS can produce spectra of its entire field of view. As a result, this part of MIRI is ideally suited for observations of more complex and extended objects, such as closer-by galaxies, crowded fields of stars and galaxies, or nebulae.

We first take a look at its mechanical structure with its three protruding pairs of carbon fibre struts that will attach it to Webb’s instrument compartment at the back of the telescope.

The beam of light coming from the telescope is then shown in deep blue entering the instrument through the pick-off mirror located at the top of the instrument and acting like a periscope.

Then, a series of mirrors redirect the light toward the bottom of the instruments where a set of 4 spectroscopic modules are located. Once there, the beam of light is divided by optical elements called dichroics in 4 beams corresponding to different parts of the mid-infrared region. Each beam enters its own integral field unit; these components split and reformat the light from the whole field of view, ready to be dispersed into spectra. This requires the light to be folded, bounced and split many times, making this probably one of Webb’s most complex light paths.

To finish this amazing voyage, the light of each beam is dispersed by gratings, creating spectra that then projects on 2 MIRI detectors (2 beams per detector). An amazing feat of engineering!

Learn more about spectroscopy with the Webb telescope.

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