This is a flight qualification model of a filter wheel used on the Mid-infrared instrument,
or "MIRI". It looks like a piece of modern techno art, but it's a critical component
of what is literally the coolest instrument aboard the James Webb Space Telescope.
MIRI is one of Webb's four science instruments. They're housed inside
the Integrated Science Instrument Module, or ISIM, located behind Webb's primary mirror.
Although they weren't designed to be swapped out and replaced like on Hubble,
Webb's instruments partially duplicate some of each other's capabilities,
and in some cases have fully redundant copies of their own hardware on board. The instruments
view a portion of Webb's science field of view. This is the part of Webb's total field of view
that's optimized for maximum image quality and avoids as much of the stray light as possible.
We talked about how Webb deals with stray light in a previous video, but
what's important to keep in mind is that all 18 of Webb's mirror segments are contributing
to the science field. It's not like any of Webb's mirrors are going unused or anything.
Still, if the instruments are already restricted to a portion of Webb's total field of view,
well, why then should we restrict them further by having them share this field?
Why not let each instrument take turns viewing the science field?
Well, some telescopes do just that. The Very Large Telescope uses a rotating diagonal mirror
to send the field into just the instrument needed for the observation.
But rotating mirrors introduce additional complexity and risk. So there's really no reason
to take such an approach with Webb unless the science requirements demand it, which they don't.
But this approach allows Webb to operate multiple instruments at once.
It's called Parallel Observing, which was pioneered with the Hubble Space Telescope
in the 1990s. And it's a great way of maximizing science. All of Webb's instruments must be very
cold in order to detect faint infrared signals coming from across the universe.
The near-infrared instruments alone will need to reach about 30 Kelvin before they can be used.
They will need a few months to reach those temperatures for a couple of reasons. For one,
it takes that long to cool because the colder something gets, the longer it takes to radiate
away its remaining heat. But another reason is that the instruments themselves were kept warm
with heaters for about a month after the sunshield was deployed. Now, if you're like me, you're
of the strong conviction that heaters are the wrong device to use when cooling something down.
However, the ISIM is made up of carbon fiber, which likely has water vapor trapped within its
structures. Once exposed to the vacuum of space, the water vapor starts to outgass.
If outgassing water vapor condenses on the instruments, mirrors and electronics,
it will quickly freeze into place and probably render the instruments useless. And that's bad.
So those heaters kept the instruments just warm enough to prevent contamination as the rest
of the ISIM structure cooled. When the ISIM's temperature fell below 140 kelvin,
any remaining moisture was either completely trapped inside the structure or had already frozen
solid onto non-critical surfaces. So even though the sunshield was deployed over a month ago, the
instruments are only now finally being allowed to cool down to their operating temperatures.
So while we're waiting, let's take a closer look at these instruments and what they'll show us.
First up is the Near-Infrared Camera, or NIRCam. This is Webb's workhorse
imager that's now being used for the initial mirror alignment process.
NIRCam consists of two fully redundant modules that can be used simultaneously.
Each module uses a beam splitter to send the light into a pair of short and long wavelength channels.
So NIRCam is really two cameras side by side in two completely redundant modules, and each module
has two channels for short and long wavelengths because of images at shorter wavelengths.
NIRCam will produce Webb's highest resolution images, all of which are grayscale. You see,
rather than taking fixed color images, astronomical cameras take multiple images
of the same target through different filters. These images can be combined later to produce
a color image, and this approach allows the camera to devote 100% of its sensitivity to
just one color at a time, rather than dividing it up among multiple colors simultaneously.
To that end, NIRCam uses a pair of pupil and filter wheels in each module. But these wheels
allow NIRCam to do more than just take pictures. For example, in the Wide-Field Spectroscopy mode,
light is passed through a prism to create a spectrum
of every source in the field. NIRCam can also operate in Time Series modes as well.
And this is where it takes a series of short exposures to precisely measure tiny variations in
a source's brightness, or even variations in its spectrum. So one early use of time-series imaging
will be to monitor the exoplanet HAT-P-18 b as it passes behind its host star. As it does so,
starlight reflecting off the planet is gradually cut off.
The shape of the resulting light curve gives an insight into the density and thermal structure
of the planet's atmosphere. Both of NIRCam's modules include an array of coronagraph masks.
These are tiny disks or bars that can block out the light from a bright object to reveal
fainter objects next to them. One of its early uses will be to block out the star Beta Pictoris
to study its surrounding protoplanetary disk.
But if NIRCam is Webb's workhorse camera, NIRSpec is Webb's workhorse spectrograph. Spectrographs
work by passing light through a prism or reflecting it off of a diffraction grating.
In fact, most spectrographs use a combination element called a "grism" for this very purpose.
But NIRSpec carries a fundamental technology that takes spectroscopy to a whole new level.
Let me show you how this works.
A pick-off mirror sends the light from the telescope's optics into the instrument.
The beam is directed to a curved mirror that prepares the image. It's then sent through the
filter wheel and then refocused before entering the spectrograph. Now, here's the amazing part: as
it's entering the spectrograph, the image reaches an array of tiny windows called "microshutters".
Each shutter is only 100 by 200 microns. That's about the thickness of only a few human hairs.
Prior to an observation, pre-selected shutters are given a small electric charge. A magnetic
arm sweeps past and the charged shutters swing open. Open shutters allow light from
a particular part of the field to pass through, while the closed shutters block everything else.
Now, the fact that these tiny shutters can operate at cryogenic temperatures without fatiguing
is mind-boggling. They're made of silicon nitride, which was chosen because of its high strength and
its resistance to fatigue. And this allows the shutters to be opened and closed again
without the risk of breaking off or getting stuck. There's about 250,000 microshutters,
so tiny portions of each field can be precisely targeted while blocking everything else.
And this allows NIRSpec to take the spectra of up to 100 targets
simultaneously, without risking contamination from the background or other objects in the
field. Another of Webb's spectrographs is the Near-Infrared Imager and Slitless Spectrograph,
or NIRISS. It's actually bundled with the Fine Guidance Sensors.
FGS, as they're called, is a pair of cameras that Webb uses to lock onto guide stars
so the telescope can track its science target. That means the FGS's are not available for
science, at least for the time being. But the NIRISS side is very much a science instrument.
Now, earlier we talked about how NIRCam is the main camera with some spectroscopic capability.
Well, NIRISS is a high-performance spectrograph with some imaging capability.
NIRISS covers the same wavelength range as NIRCam and NIRSpec,
but its spectroscopy modes were designed for two very specific purposes.
In its Wide-Field Spectroscopy mode, NIRISS is optimized for studying several high-redshift
galaxies at once. And this allows near us to study starbirth in the very first
galaxies to form in the universe. In Single Object mode. NIRISS captures the spectra of
exoplanets as they transit in front of their host stars. As the planet transits, its atmosphere
absorbs certain wavelengths of light while allowing other wavelengths to pass through.
By comparing the star's spectrum before and during the transit,
the composition of the planet's atmosphere is revealed.
NIRISS also has an Aperture Masking Interferometry mode or AMI. And this mode uses a mask that only
allows light from just seven of Webb's mirror segments through. And this causes the light waves
to interfere with each other and reveal details that would otherwise be too faint to detect.
For example, here are simulated nearest images of two stars. On the left is what is almost likely a
binary star system, and on the right is a single reference star. The binary system is too close
together to be resolved. They're separated by just 207 thousandths of an arc. Second,
at this scale, both stars in the system are just blocks of pixels.
But which pixels correspond to the actual positions of each star?
Well, the two stars are too close together for Webb's coronagraphs to be of any use.
But when NIRISS operates in AMI mode, the exact position of the companion star is revealed.
Finally, NIRISS' imaging mode allows high-resolution, multi-wavelength imaging over
its entire field of view. That's with an imaging resolution of just 0.065 arcseconds per pixel!
And, it can operate as a second or technically a third camera in parallel with NIRCam.
Not bad for a spectrograph! NIRCam, NIRSpec, and NIRISS are all near-infrared instruments,
so they're sensitive from around 0.6 to about 5 microns. But the Mid-Infrared Instrument, or MIRI,
picks up where the others leave off and continues all the way out to 28.5 microns. But looking
deeper into the infrared means that mirror has to be even colder than its near-infrared siblings.
So that's why MIRI uses a cryocooler to bring its temperature all the way down to less than
seven degrees above absolute zero. Previous spacecraft, like Spitzer and Herschel, used
liquid helium cryostats to bring their instruments down to similar temperatures. As the liquid helium
boils off, it carries away heat. But once all the liquid helium is gone, the instruments warm up.
MIRI avoids this problem with a first-of-its-kind, 4 stage closed-cycle cooling system. In other
words, a killer refrigerator I'd love to nerd out on just the cryocooler alone, but I know
this video was getting a bit long. But the point is, is that MIRI's lifespan is not limited by any
expendable cryogenics. So it should continue to operate for the duration of the mission.
Because MIRI is Webb's only mid-infrared instrument, it basically does a little bit
of everything else that the other instruments can do. To that end, MIRI is physically set up so that
one half of the instrument handles the imaging and the other half handles the spectroscopy.
MIRI's imaging field of view takes up the largest section of its focal plane.
It gets a resolution of about 0.11 arcseconds per pixel. And that's about half of NIRCam
and NIRISS's resolution. But we have to remember that the longer the wavelength,
the lower the resolution gets. Still with a six and a half meter primary mirror,
MIRIary achieves a much sharper resolution than Spitzer ever could.
But resolution is only half the story - just by virtue of seeing deeper into the Mid-infrared.
MIRI will be able to look through even more dust and gas to reveal the
interiors of stellar nurseries. It will be our clearest look yet at watching embryonic stars
forming inside their dusty cocoons. It's also going to be a great camera for
imaging icy bodies in the Kuiper belt and understanding how our solar system formed.
The optical system of the camera consists of five mirrors and a filter wheel.
The wheel consists of 18 elements, nine of which are filters for wideband
imaging. But the wheel also has four coronagraph imaging filters as well.
And these are special filters that combine with mirrors coronagraph masks
to block out a star and let MIRI directly image the surrounding planets. There's actually two
kinds of choronagraph that MIRI supports. There's the traditional mode called a Lyot coronagraph,
which places a small circular mask that's suspended by a bar and blocks out the star.
And this approach works well for investigating Ddiffuse objects
near bright sources. So, for example, the outer regions of a protoplanetary disk,
the ejected dust cloud around old dying stars or the central regions of active galactic nuclei.
But the mask always imposes a limit on just how close you can get to the star. And that's
why MIRI allows for a new kind of coronagraphy called Four Quadrant Phase Masking, or 4QPM.
It's a special mask that splits the field into four quadrants.
Light waves passing through two of the quadrants are shifted 180 degrees out of phase
with the waves passing through the other two,. The two sets of waves destructively interfere
and cancel each other out. This lets MIRI study exoplanets that are up to three
times closer to their host stars than it could with the old school coronagraph.
And there's actually three different 4QPM masks, each of which are optimized for a different
wavelength of infrared light. But all of that is just half of what MIRI can do. The top half, to be
precise, the bottom half of the instrument houses MIRI's Medium-Resolution Spectrograph. Initially,
light enters through the top half of the instrument where the imaging hardware is located.
After passing through the initial formatting mirrors, the beam is sent to the bottom where the
spectrograph is located. After light enters the spectrograph, it is split out into four channels,
each of which are sensitive to different parts of the spectrum.
The images are sliced into segments which are then fed into a maze of relay mirrors and grisms
that disperse the image into a final spectrum.
The result is that MirI will see through the surrounding dust belched out by evolved supergiant
stars and take the spectrum of the water forming in their convulsing atmospheres. Or perhaps
understand how the ejecta from supernovae interact with the interstellar medium to produce new stars.
Or even investigate the cryovolcanoes of Neptune's moon Triton
and understand how they contribute to its atmosphere.
If a picture is worth a thousand words, spectra are worth a thousand pictures.
And Webb is going to give us so many of both. Now, I know I've said this before, but
the skills required to just design and build these instruments is nothing short of mind-blowing.
The good news, though, is that skills can be learned, whether there are technical skills
like optical engineering, or creative skills like video editing and Photoshop.
And that's why I'm so thrilled to have Skillshar'se support for today's video.
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It's a new year, so I'm making a point of finding ways of creating content more efficiently while
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