How will JWST see the beginning of our universe?

What instruments will be on board the James Webb Space Telescope?
23 January 2018

Interview with 

Gillian Wight, UK Astronomy Centre

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There’s no point in having a perfect mirror to collect light if this information has nowhere to go. There are four science instruments that do this are at the heart of the James Webb Space Telescope (JWST). On board, we have the near infrared camera that initially scans the skies. Then comes the near infrared spectrograph to reveal what chemicals are hiding there. There’s also the find guidance sensor, which allows Webb to point precisely to create crystal clear images, and, as Izzie Clarke hears from Gilliant Wright, there’s MIRI…

Gillian - MIRI just stands for Mid Infrared Instrument. It’s not a very imaginative name I suppose.

Izzie - That’s Gillian Wright the Principal Investigator for MIRI and the Director of the UK Astronomy Technology Centre in Edinburgh…

Gillian - The mid infrared is a very important part of light especially if you want to study planets and also if you want to study the first galaxies that formed. It’s very, very red light so it’s head radiation, and one of the advantages of that is that you can see better through dust. We know that planets form in very dusty systems and so one of the things we would like to do with MIRI is to understand a bit more about the systems the planets are forming from.

Izzie - Okay. So MIRI is looking into the mid infrared region. How does it do that exactly?

Gillian - MRI and JWST are like a camera. JWST is like the most ginormous lens you could imagine and it’s gathering all that light from the very faint dust or the very faint galaxy. Then the mid infrared instrument, MIRI, is like the body of your camera so it’s where the detectors are. They’re not CCDs, but it’s the same type of idea, an electronic detector that records the light, and then what we can do with MIRI is that we can use filters or an optical element called a spectrometer and an image slicer or a coronagraph to analyse that light in different ways so give us a way of understanding more about what’s coming from that astronomical source.

Izzie - One way to do that is by splitting up the light from a star into its different wavelengths with a spectrograph…

Gillian - Most spectrographs would have something like a very narrow slit to only emit a very narrow beam of light into the spectrograph. Then the light is split up into its constituent parts by an optical element that we call a grating and it’s similar to a prism. If you put a prism in a beam of light, in front of a lamp or something, you would see a spectrum of colours like a rainbow. So what we’re doing with this grating is splitting the light up into its rainbow of colours so that we can look for the unique ones that are associated with specific molecules.

Izzie - That’s because different molecules have different wavelengths. It’s a bit like having a unique fingerprint. From this light Gillian and her team will be able to work out what molecules are hiding in these galaxies or planetary atmospheres to keep an eye out for signs of life…

Gillian - For example, we might be able to see methane in their atmospheres or carbon dioxide, and if we can understand a bit better about what the materials are in those planets then, again, that helps us to understand better how planets form and how planetary systems form around their stars.

Now everyone knows it’s really hard to see anything if the sun is shining right in your eyes. Well that’s a problem that all the researchers are facing when looking for planets around other stars. Luckily, Gillian and her team found have a few ways around this!

Gillian - As the planet goes round the star, some of the time what you see is the light from the star plus the planet, and some of the time what you see is the light from just the star. So, if you subtract the two, what you should be left with is the light from the planet.

The other technique is to use is what we call a chronograph. A chronograph is a little black spot that blocks out the light from the star. So you can think of this as like shading your eyes to look at something that’s very bright or, perhaps, putting your thumb or a spot in front of your eyes to block out the light from something. That’s exactly exactly how a chronograph works and if we were, for example, to take an image we’d be able to take an image directly of just the planet in the mid infrared and nobody’s ever done that before.


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