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BY D R DAVID MADDISON
Image source: www.flickr.com/photos/nasawebbtelescope/37988427785/
The James Webb Space Telescope (JWST) is the newest and most advanced space
telescope. Launched on December 25th, 2021 on an Ariane 5 rocket from
French Guiana, it officially entered service on July 12th, 2022. While much
has been said and written about it in the press, this article will concentrate
on the amazing technology behind it.
T
he James Webb Space Telescope
has the largest mirror of any telescope launched into space. It can
see ‘back in time’ right up to the time
of the first star and galaxy formation
after the Big Bang (the presumed beginning of the universe).
It also has more light gathering ability than any other space telescope,
allowing it to see very faint objects. It
can see in the infrared, meaning it can
image objects that are not visible using
the visible light spectrum.
The JWST mission objectives are
to explore the early universe, examine the evolution of galaxies over
time, examine the star life cycle and
look for and examine other planets.
That includes our own minor planets,
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Kuiper Belt objects and the suspected
Planet Nine in our solar system.
The JWST project is led by NASA
in collaboration with the European
Space Agency (ESA) and the Canadian Space Agency (CSA). Their academic and industry partners include
Who was James Webb?
The telescope was named after
James E. Webb, NASA’s second
director, from 1961
to 1968. He oversaw
the Mercury, Gemini
and Apollo missions
with a total of 75
launches.
Image source: NASA
Australia's electronics magazine
the University of Arizona, Ball Aerospace, L3Harris Technologies, Lockheed Martin, Northrop Grumman and
The Space Telescope Science Institute (see https://jwst.nasa.gov/content/
meetTheTeam/team.html).
Design started in 1996, and in 1999,
there was an expectation of a US$1
billion cost and a 2007 launch. The
JWST has cost NASA US$9.7 billion
($14.1 billion) over the last 24 years,
the ESA €700 million (A$1.02 billion)
and the CSA C$200 million (A$224
million).
JWST vs Hubble
The JWST is a successor to the Hubble Space Telescope (HST), which
entered service on May 20th, 1990 and
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is still in operation today. However,
the JWST does not replace the HST.
Hubble has an uncertain remaining
lifetime partly because NASA’s Space
Shuttle fleet was retired in 2011, so
there is no longer any way to service
it. Despite that, it will be kept in service as long as possible.
A fundamental difference between
the JWST and the HST is the size of
the primary mirror. The HST has a
2.4m diameter mirror while JWST’s is
6.5m in diameter (see Fig.1). They are
also designed to image different light
wavelength ranges (Fig.2).
The HST has an effective light gathering area of 4m2 and the JWST 25m2,
so the JWST has 6.25 times the light
gathering capability of the HST.
The HST was designed for the visible and ultraviolet part of the light
spectrum, plus some infrared, while
the JWST is designed to work mainly
in the infrared. All objects with a temperature above absolute zero (-273°C)
emit infrared radiation, making them
visible to JWST as long as they give
off enough infrared light.
Specifically, HST images wavelengths of 100nm to 800nm with some
parts of the infrared spectrum from
0.8μm (800nm) to 2.5μm, while the
JWST images from 0.6μm to 28μm.
The infrared spectrum extends from
0.75μm to a few hundred microns, so
the JWST works mainly in that area
with a small capability in the visible
range from 600-750nm (orange is 590
to 620nm and red is 620 to 750nm).
As infrared radiation comes from
all objects, it is essential to keep the
JWST as cool as possible. Hence its
vast multi-layer sunshield, its remote
orbit away from the Earth and the
Moon and onboard cooling systems.
It must be kept below -223°C to keep
it from interfering with itself from
self-emission of infrared. The electronics onboard operate at higher temperatures than that, though.
JWST can detect objects 100 times
fainter than the HST. JWST can also
see objects as old as 180 million years
after the Big Bang, compared to 400
million years for HST.
Physical structure
The JWST consists of four major
sections (see Fig.3):
1. The spacecraft bus, which is like
a chassis but also houses the following subsystems:
• Electrical Power Subsystem
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Fig.1: a size comparison of the
Hubble and JWST primary mirrors.
Source: https://jwst.nasa.gov/
content/observatory/ote/mirrors/
Fig.2: a comparison of the light spectrum coverage of the HST & JWST. Source:
www.nasa.gov/content/goddard/hubble-vs-webb-on-the-shoulders-of-a-giant
THE JAMES WEBB SPACE TELESCOPE
Science Instrument Module (ISIM)
Houses all of Webb's cameras and
science instruments
Primary Mirror
18 hexagonal
segments made of
the metal beryllium
and coated with
gold to capture
faint infrared light
Optical Telescope Element (OTE)
Secondary Mirror
Reflects gathered
light from the primary
mirror into the science
instruments
Trim Flap
Helps stabilise
the satellite
Multilayer Sunshield
Five layers shield the
observatory from the light
and heat of the Sun and Earth
Solar Power Array
Always facing the
Sun, panels convert
sunlight into
electricity to power
the observatory
Earth-pointing
Antenna
Sends science
data back to Earth
and receives
commands from
NASA's Deep
Space Network
Star Trackers
Small telescopes that use star
patterns to target the observatory
Spacecraft Bus
Contains most
of the spacecraft
steering and
control machinery,
including the
computer and
reaction wheels
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Fig.3: the crucial parts of the JWST.
Note how the telemetry components,
which must face the Earth, are on the
opposite side of the sun shade from
the telescope. Source: www.nasa.gov/
mission_pages/webb/observatory/
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• Attitude Control Subsystem
• Communication Subsystem
• Command and Data Handling
Subsystem
• Propulsion Subsystem
• Thermal Control Subsystem
2. The optical telescope element
(OTE), comprising the various mirrors.
3. The Integrated Science Instrument Module (ISIM), containing the
cameras and other instruments such as
NIRCam, NIRSpec, NIRISS and MIRI.
4. The Sunshield.
Size and weight
Fig.4: the JWST primary mirror
during assembly. The left & right sides
are folded to fit inside the rocket.
Source: https://jwst.nasa.gov/content/
observatory/ote/mirrors/
Fig.5: a mock-up of the JWST
at Goddard Space Flight Center
in Maryland, USA. Source:
www.flickr.com/photos/
nasawebbtelescope/8518326611
Actuator
JWST Primary Mirror Segment
Strut
When the center actuator moves
up or down, it pulls or pushes
on the six struts, which in turn
correctly curves the mirror.
The actuators are tiny
mechanical motors that
move the mirrors into proper
alignment and curvature with
each other.
Each mirror has seven
actuators – six at the hexapod
ends and one in the center.
Hexapod
Beryllium Substrate
Beryllium was chosen for the mirror's
“skeleton” because it is strong and light,
and will hold its shape in the extreme cold
of space.
The substrate was machined in a honeycomb
pattern to remove excess material and thus
decrease its weight, yet maintain its strength.
When the actuators at the hexapod
ends pull or push on the hexapod,
it pulls or pushes the mirror into
correct alignment with the other
mirrors.
Electronics Box
Every mirror segment has one
electronics box. This box sends
signals to the actuators to steer,
position and control the mirrors.
The electronics boxes are located
within the backplane – the
structure that holds all the mirrors.
Fig.6: the structure of a mirror segment, showing the six mirror actuators plus
the central one to control its curvature. Three beryllium ‘whiffles’ are located
between the hexapod and substrate, measuring 60cm long by 30cm wide, helping
to spread the load. Source: https://jwst.nasa.gov/content/observatory/ote/mirrors/
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According to the ESA, the launch
mass of the observatory was 6200kg,
including the observatory, on-orbit
consumables and launch vehicle adaptor. Its overall height is about 8m. The
705kg mirror is 6.5m in diameter and
the focal length of the telescope optics
is 131.4m.
The mirror
The JWST mirror and the rest of the
spacecraft were far larger than could
be accommodated by the Ariane 5
launch vehicle, so it had to be folded
for launch, as partially shown in Fig.4.
This was particularly challenging for
the mirror, given the high level of precision required.
The mirror comprises 18 hexagonal
gold-coated beryllium metal segments
(Fig.6), each weighing about 20.1kg
and 1.32m across, with a total diameter of 6.6m and a total area of 25m2.
Each mirror segment forms a primary mirror segment assembly
(PMSA), weighing 39.48kg with actuators and other accessories. 48g of gold
is used to coat the mirror, about the
volume of a marble and the mass of
a golf ball. Gold is used because it is
highly reflective in the infrared.
The primary mirror segments each
have six actuators to adjust their
alignment, as does the secondary
mirror. Primary mirrors also have a
central actuator to adjust the mirror
curvature.
Each segment had to be aligned
with an accuracy of 7nm or one ten-
thousandth the thickness of a human
hair.
The actuators can move to positions
as accurate as 1nm or one-millionth
of a millimetre. In use, the mirrors are
realigned every 10 to 14 days. There
are a total of 132 actuators, including 126 for the primary mirror. The
mirrors are ground to a mean surface
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accuracy of better than 25 nanometres.
Diffraction spikes
Most images of stars make them
look like a point of light or a disc with
four or more radial spikes in a specific pattern. These spikes are called
diffraction spikes – see Fig.7. They
are a common phenomenon in reflector telescopes (like the JWST) and are
partly related to the support vanes of
the secondary mirror.
They are also common in any
camera or telescope aperture that is
non-circular, including the iris diaphragm of a traditional camera. In
the case of the JWST, they also derive
from the fact that the primary mirror
is not circular.
They occur because light interacts
and diffracts around the edges of the
aforementioned structures. So the
JWST has two sources of diffraction
spikes. These are designed so that they
do not overlap with each other and
remain as narrow as possible. Fig.8 is
a comparison of the diffraction spikes
between the HST and JWST.
Fig.7: the contribution and shape of diffraction spikes from the combination of
the JWST struts and mirror shape. Source: https://webbtelescope.org/contents/
media/images/01G529MX46J7AFK61GAMSHKSSN
Sunshield
Apart from the mirror, the sunshield
is the most prominent feature with a
deployed size of 21.2m x 14.2m or
about the size of a tennis court – see
Figs.5 & 9. It shields the telescope from
heat and light from the Sun, Earth
and Moon. It is made of thin aluminium and doped-silicon coated plastic
called Kapton E, with five separate
layers each 0.025mm thick.
‘Rip stop’ structures are built into
the shield material to prevent a tear
catastrophically propagating through
an entire layer.
You can check the sunshield and
instrument temperatures at the website: siliconchip.au/link/abgu
At the time of writing, the layer on
the sun side has a temperature of 13°C
and 50°C (measured at two locations),
while the innermost layer has temperatures between -231°C and -236°C. The
five instruments were at temperatures
from -235°C to -267°C.
Fig.8: a comparison of the refractive image spikes between the HST and JWST.
Scientific instruments
The Integrated Science Instrument
Module (ISIM) is behind the main mirror and holds the four scientific instruments plus the Fine Guidance Sensor,
a camera for aligning the observatory.
It also has power supplies, computers
and instrument cooling – see Fig.10.
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Fig.9: the JWST’s sunshield comprises
five layers of Kapton film. Source:
https://jwst.nasa.gov/content/
observatory/sunshield.html
Australia's electronics magazine
Fig.10: the ISIM compartment.
Source: www.flickr.com/photos/
nasawebbtelescope/30785200072/
December 2022 17
►
Fig.11: the NIRCam configuration. Source: www.
astro.princeton.edu/~jgreene/ast303/NIRCampocket-guide.pdf
Fig.12: the NIRCam instrument (near-infrared
camera). Source: www.flickr.com/photos/
nasawebbtelescope/albums/72157627248683106
The four scientific instruments are
the Near-Infrared Camera (NIRCam),
Near-Infrared Spectrograph (NIRSpec),
Near-infrared Imager and Slitless
Spectrograph (NIRISS) and MidInfrared Instrument (MIRI)
As their names imply, these all work
in the infrared. See Fig.13 for their specific wavelength ranges. We will go
over each instrument in detail:
NIRCam (0.6-5μm wavelength
range, Figs.11 & 12) is an infrared
camera that has 10 mercury-cadmium-
telluride (HgCdTe) detector arrays,
each with four megapixels (4MP;
2048 × 2048 pixels). It is also used for
“wavefront sensing” to align the mirror segments.
In addition, it has coronographs to
block light from stars with associated
exoplanets (planets outside our solar
system). It operates at -236°C while its
electronics operate at 17°C.
NIRSpec (0.6-5.3μm, Figs.16 & 18)
is a spectrometer that can be used to
analyse the chemical composition of
objects. It has several operating modes,
including the ability to take spectra
of 100 objects simultaneously. The
instrument runs at -235°C.
Fig.14: there are four arrays, each containing
62,000 shutters (measuring 0.1 × 0.2mm).
Source: https://jwst.nasa.gov/content/about/
innovations/microshutters.html
Multiple simultaneous spectra are
taken with the aid of 248,000 micro
shutters (see Fig.14). They can be
individually opened or closed to
allow light from the objects of interest through to the spectrometer via
gratings and a prism to split up the
light into its component wavelengths
– see https://w.wiki/5hex
NIRISS (0.6-5um, Fig.17) is used for
imaging and spectroscopy. It is combined with the Fine Guidance Sensor
(FGS) used to guide the telescope. The
FGS (Fig.19) finds pre-selected guide
stars from a database and uses those
for guidance.
Together, the instrument is known
as the FGS-NIRISS (Fig.15); they
are optically separate but contained
within one assembly.
NIRISS was built by the Canadian
Space Agency.
The detector for NIRISS is a 2048 ×
2048 pixel (4MP) HgCdTe array with
18 × 18μm pixels. NIRISS is used for
near-infrared imaging, wide-field slitless spectroscopy, single object slitless
spectroscopy and aperture masking
interferometry.
Fig.13: the JWST instrument light detection wavelength ranges, mainly in the
infrared part of the spectrum. Source: www.nasa.gov/mission_pages/webb/
news/geithner-qa.html
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Fig.16: a
schematic
view of the
NIRSpec
instrument.
Source:
https://w.
wiki/5gyc
Fig.15: a photograph of the completed
FGS-NIRISS assembly. Source: John
Brebner, Communications Research
Centre
Spectroscopy enables the chemical
composition and physical structure of
distant objects to be determined from
their emission spectra. Slitless spectroscopy is used in sparsely-populated
star fields to determine the spectrum of
many objects at once. Aperture masking interferometry is used to resolve
closely spaced objects such as a binary
star system.
MIRI (4.9-28.8μm, Fig.20 overleaf)
is a camera and imaging spectrometer.
It can see longer wavelength infrared
light than the other instruments and
thus needs to be kept much colder.
It operates at -267°C and has its own
‘cryocooler’ cooling system, shown in
Fig.21 (also overleaf).
The cooling system is spread over
several regions of the ISIM, all of
which are at different temperatures.
Like NIRCam (0.6-5μm), it has four
coronagraphs to block starlight when
►
Fig.18: a
photograph
of the
NIRSpec
instrument.
Source:
https://w.
wiki/5gyd
Pick-Off Mirror
Kinematic Mounts (3 pairs)
Collimator
Pupil / Filter Wheel
Fig.17: a schematic
view of the NIRISS
instrument (Nearinfrared Imager and
Slitless Spectrograph).
Source: https://w.
wiki/5gye
Camera
Detector
Guider POM
Guider Relay TMA
Fig.19: the FGS optical
assembly; this is the other
side of the NIRISS assembly
shown in Fig.17. Source:
https://w.wiki/5gyf
Guider ICP-1
Optical
Alignment
Cuber (1 / 2)
Fine Focus
Mechanism
with fold
mirror
Guider
SIDECar
ASICs (2)
Detector
Assembly with
2 FPAs
Kinematic Mounts
(2 of 3)
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December 2022 19
Fig.20: a schematic view of the MIRI (Mid Infrared Instrument). Source:
https://w.wiki/5gyg
►
Table 1 – JWST instruments and their detectors
NIRCam
HgCdTe H2RG (0.6-2.5μm)
8
HgCdTe H2RG (0.6-5μm)
2
NIRSpec
FGS/NIRISS
2
3
Arsenic doped silicon (5-2.8μm)
MIRI
3
Shows the number of different types of IR detectors used in each instrument.
observing exoplanets. It uses arsenic-
doped silicon arrays as its infrared
sensors.
Infrared detectors
The infrared detectors are essential for the operation of the scientific
instruments described above. There
are two types, 4MP HgCdTe arrays
for the 0.6-5μm ‘near-infrared’ and
arsenic-
doped silicon (Si:As) detectors of about 1MP for the 5-28μm ‘mid-
infrared’ wavelength range.
These are all extremely sensitive
as they must detect incredibly faint
light. HgCdTe sensors can be tuned
to the wavelength range of interest by
adjusting the Hg-to-Cd ratio; two different compositions are used, one for
0.6-2.5μm and the other for 0.6-5μm.
See Table 1 for the specific detectors
used in each instrument.
The basic layout of one of the detectors is shown in Fig.22, while an
actual detector is shown in Fig.23.
There is an HgCdTe or Si:As absorber
layer on a silicon readout chip. When
a light photon strikes the absorber,
one or more electron-hole pairs are
created. The electrons and holes move
under the influence of an electric
field and can be sensed by the readout circuitry.
Folding the observatory
With such a large mirror and sunshield, the spacecraft could not fit in
any rocket and so needed to be folded.
That is why the mirror has multiple
segments. The sunshield, mirror, solar
panels and antenna were all folded –
see Figs.24 & 25.
According to Mike Menzel of
NASA, the unfolding process involved
hundreds of possible “single points of
failure”. JWST has 344 known possible single-points-of-failure, about 80%
related to the unfolding process.
There were 144 release mechanisms
for the unfolding process, all of which
had to work perfectly. Naturally, all
such mechanisms got special attention during design, assembly and testing to ensure they would work. There
were also contingency plans for any
deployment failure that might have
occurred, some as simple as re-sending
a command.
The most important thing that had to
work first was the solar array deployment.
For a video of the unfolding (deployment) sequence, see the video titled
“James Webb Space Telescope Deployment Sequence (Nominal)” at https://
youtu.be/RzGLKQ7_KZQ
Fig.21: the cooling arrangement for MIRI; it is kept at -267°C or 6K. Source: https://w.wiki/5gyi
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The JWST’s orbit
JWST orbits a Lagrange point. These
are five special points in space in the
Earth-Moon system. At the L4 and L5
Lagrange points, there is an equal gravitational pull from the Earth, Moon and
Sun, meaning that (in theory) an object
can remain there indefinitely (asteroids are known to accumulate there).
The L1, L2 and L3 points are only
metastable; for an object to stay there, it
must expend minimal fuel for station-
keeping. Objects can reside there for a
long time, but not indefinitely.
Unlike most satellites but like some
other space telescopes, the JWST orbits
in a ‘halo orbit’ around the L2 Lagrange
Point (see Fig.26). It is way beyond the
orbit of the Moon, 1,500,000km from
the Earth. In contrast, Hubble orbits
the Earth at an altitude of only 550km.
The reason for orbiting L2 is to
avoid the heat radiated from the Sun,
Earth and Moon, which would swamp
its sensitive infrared instruments.
JWST can maintain the same orientation, so its sunshield will continue
to protect the telescope. JWST’s view
will never be blocked by the shadow
of the Earth or the Moon, unlike Hubble, which is in Earth’s shadow every
90 minutes.
JWST takes six months to complete
its halo orbit. In this orbit, JWST is in
continuous contact with NASA’s Deep
Space Network with stations in Australia, Spain and California.
For more on the orbit, see the video
titled “Animation: The James Webb
Space Telescope’s Orbit” at https://
youtu.be/6cUe4oMk69E
Fig.22: a schematic view of an
infrared detector sensor used in
several JWST instruments. Source:
https://jwst.nasa.gov/content/about/
innovations/infrared.html
Fig.23: an infrared detector as used
in the NIRCam instrument. Light is
collected on the mauve HgCdTe film.
Source: https://jwst.nasa.gov/content/
about/innovations/infrared.html
Fig.24: this shows how the JWST was folded inside the fairing of the Ariane 5
launch vehicle. Source: https://jwst.nasa.gov/content/about/launch.html#
postLaunchDeployment
Comparing images from
JWST and HST
►
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►
The ‘Deep Field’ images shown in
Figs.27 & 28 (overleaf) are the same
area of space taken using the JWST
and HST. In astronomy, Deep Field
means a very long exposure.
The JWST Deep Field image was
its first, taken with its Near-Infrared
Camera (NIRCam) using several wavelengths and a 12.5 hour exposure time.
It shows the galaxy cluster SMACS
0723 which, due to its great mass,
acts as a ‘gravitational lens’, distorting light from galaxies behind it into
a circular pattern.
The HST image needed to be
exposed for weeks. Despite that, it
shows much less detail due to its
smaller mirror and inability to image
objects in the infrared.
Australia's electronics magazine
Fig.25: the deployment sequence of
JWST. LV is launch vehicle, UPS is
Unitized Pallet Structure, PMBA is
Primary Mirror Backplane Assembly
and SMSS is Secondary Mirror
Support Structure.
Source: https://w.wiki/5gyj
Fig.26: this shows the Lagrange points
around the Earth-Sun-Moon system
and the location of the JWST in a halo
orbit around L2. Source: https://jwst.
nasa.gov/content/about/orbit.html
December 2022 21
Fig.27: the first and iconic Deep Field
image from the JWST. Source: www.
nasa.gov/image-feature/goddard/2022/
nasa-s-webb-delivers-deepestinfrared-image-of-universe-yet
Fig.28: a Deep Field image from the
HST taken in 2017 of the same area
shown in Fig.27. Source: https://
archive.stsci.edu/prepds/relics/color_
images/smacs0723-73.html
The area of sky covered by these
images is equivalent to a grain of sand
held at arm’s length.
smallness. The known laws of physics
cannot describe the singularity but do
more-or-less apply for periods starting
10-43 seconds after the Big Bang.
It is important to realise that it was
not an explosion in the conventional
sense, but a sudden expansion of the
very fabric of space-time itself for reasons not fully understood. It might
have been due to some sort of quantum fluctuation.
Light and objects cannot travel faster
than the speed of light, but space itself
expanded much faster than the speed
of light during the early ‘inflationary’
Looking back in time
One objective of the JWST mission
is to ‘look back in time’ at the early
universe. What does that mean? To
understand, we first have to consider
the beginning of the universe.
According to accepted theories of
cosmology, the universe started in a
‘Big Bang’; it came into being suddenly from a ‘singularity’ of infinite
temperature and density and infinite
Waves Imprint Characteristic
Polarization Signals
Density Waves
Earliest Time
Visible with Light
0
−32
10 s
1 µs
Cosmic Microwave Background
Nuclear Fusion Ends
Nuclear Fusion Begins
Inflation
Big
Bang
Protons Formed
Quantum
Fluctuations
Radius of the Visible Universe
Free Electrons
Scatter Light
0.01 s
3 min
380,000 yrs
Redshift
Just as a vehicle-mounted siren
appears to rise in frequency as it
approaches and falls in frequency as
it moves away, so too does light. A
light source such as a star or galaxy
moving away from us shifts toward a
lower frequency which is also a longer
wavelength, pushing it toward the red
end of the spectrum. This is called redshift. The opposite, blueshift, occurs
for objects moving towards us.
In 1929, Edwin Hubble discovered that all galaxies were moving
away from us and each other, ie, the
Modern Universe
{
Neutral Hydrogen Forms
Inflation
Generates
Two Types of
Waves
History of the Universe
Gravitational Waves
phase of the Big Bang, before 10-32
seconds had elapsed and where the
early universe grew to enormous size
in an unimaginably tiny fraction of a
second, going through several phases
as shown in Fig.29.
Because of the ongoing inflation of
the universe, objects can be more light
years away than the universe’s age.
We can currently look as far back
in time as the cosmic microwave
background 380,000 years after the
Big Bang (but not with the JWST, as
explained below). In future, it may
be possible to look back in time even
further than that by detecting so-called
primordial gravitational waves, which
current detectors cannot sense (see our
article on Gravitational Waves in the
October 2021 issue – siliconchip.au/
Article/15063).
There are also density waves, like
shock waves, which correspond to the
regions of differing matter density in
the universe that led to the formation
of stars and galaxies.
A consequence of the Big Bang is
that all parts of the universe are moving away from each other, like dots
painted on the surface of a balloon
as it is inflated. We see these objects
as they were long ago, not as they are
now, because of the time it takes light
to travel to us.
13.8 Billion yrs
Age of the Universe
Fig.29: a timeline of events during the universe’s formation, showing how
the radius of the universe is thought to have changed with time. Note the
gravitational and density waves. Source: https://w.wiki/5gyk (CC BY-SA 3.0).
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universe was expanding. He saw that
the redshift of fainter and presumably
more distant galaxies was greater than
brighter, closer galaxies. Hence, he
concluded that the more distant the
galaxy, the faster it is receding and
that the universe must be expanding.
The rate at which the universe is
expanding is determined by the Hubble constant, which is about 65km/s
for every three million light years an
object is away from us. One light year
is the distance light travels in a year,
about 9,461,000,000,000,000km.
It was also concluded that the higher
the redshift of a galaxy (the same as
saying the more distant it is), the earlier in its life we see it. In other words,
we see it the way light first left the
object millions or billions of years
ago. The object might not even exist
now, but we wouldn’t know that and
would have to wait millions or billions
of years to find out.
The redshift can be so far toward
and beyond the red end of the spectrum that it is beyond the visible light
spectrum, ie, in the infrared.
We can tell how far the light spectrum has been redshifted by reference
to specific markers within the spectra
corresponding to known molecular
and atomic absorption lines. These
characteristic spectral patterns correspond to specific elements or compounds – see Fig.30.
In extreme cases, ie, the most distant
objects, the entire spectrum becomes
invisible as it has entirely shifted into
the infrared.
The Big Bang happened 13.8 billion years ago, and the first stars are
now believed to have formed 100 million years after the Big Bang and the
first galaxies about one billion years
after the Big Bang. The JWST seeks
to detect some of the very first stars
and galaxies.
Redshift is denoted by the letter z,
corresponding to the fractional change
Fig.30: two example
spectra with absorption
lines; our Sun below and
a supercluster of distant
galaxies above. The upper
absorption lines are all
shifted towards the red
end of the spectrum due
to redshift as the cluster
is moving away from us
rapidly. Source: www.ctaobservatory.org/redshiftwhy-does-distance-matterto-cta/
siliconchip.com.au
Why doesn’t JWST have ‘selfie’ cameras?
The JWST doesn’t have any cameras for viewing itself because they would
be an unnecessary source of unwanted heat. Heat would be conducted along
the connecting wires and struts
even if they were turned off. It was a
matter that the designers did carefully
consider. Also, onboard sensors can
detect most malfunctions.
The telescope does have a limited
capability to take a selfie of the
primary mirror.
A ‘selfie’ image of the primary
mirror of the JWST taken during
initial mirror alignment procedures.
Source: https://blogs.nasa.gov/
webb/2022/02/11/photons-receivedwebb-sees-its-first-star-18-times/
in wavelength. For example, if light
were emitted at 120nm (nanometres)
and observed at 150nm, the redshift
factor z would be 0.25 (150 ÷ 120 − 1).
While the HST can see objects no
further back than 400 million years
after the Big Bang (redshift of z ≈ 11.1),
JWST can detect objects even earlier
at 180 million years after the Big Bang
(redshift z ≈ 20).
The earliest stars are now thought to
be from 100 to 180 million years after
the Big Bang (redshift of z ≈ 30 to z ≈
20), and the earliest galaxies from 270
million years after the Big Bang (redshift of z ≈ 15).
Imaging in the infrared
The ability to image in the infrared has several advantages plus some
challenges. Important advantages are:
1. Being able to see through dust
and gas clouds, as they tend to block
visible light but are transparent to IR.
2. Being able to see very distant
objects where the redshift causes
them to be invisible in the visible light
spectrum.
3. Infrared radiation is absorbed in
Earth’s atmosphere, so an IR space
telescope can see things that are very
difficult or impossible to image from
the Earth’s surface.
4. Objects such as planets, local
asteroids and debris discs around
other solar systems being formed emit
more strongly in the infrared than in
visual wavelengths.
One of the most significant challenges is that the telescope has to be
kept as cool as possible because all
matter radiates in the infrared in proportion to its temperature.
The colder something is held, the
less infrared radiation emanates from
it. We all know that metal objects glow
when very hot, but you might not realise that they emit light before being
heated; we just can’t see it because it
is infrared.
If the telescope and its instruments
were not kept cool, the instruments
would not be able to detect infrared
radiation from the universe because
they would be swamped by radiation
from the telescope itself.
The use of infrared telescopes is limited on Earth because water vapour in
the atmosphere absorbs infrared radiation. Such telescopes are placed on
Can the JWST be seen with other telescopes?
Researchers at the Virtual Telescope
Project (www.virtualtelescope.eu)
managed to image the JWST as a
single small dot of light.
The JWST imaged with an amateur
Planewave 17in (43cm) f/6.8
telescope with a 300s exposure.
Source: www.virtualtelescope.
eu/2022/01/25/james-webb-spacetelescope-a-new-image-24-jan-2022/
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December 2022 23
Fig.31: our atmosphere almost completely absorbs infrared energy. That is why
infrared observations are best made from space. Source: https://w.wiki/5gym
high mountain tops with dry environments or are airborne on aircraft or
balloons. Regardless, superior infrared
observations can be made from space
– see Fig.31.
Looking back further in time
While the JWST looks back in time
as far as is possible to see with infrared
light, to about 180 million years after
the Big Bang, we have looked back further in time using microwaves with
the Wilkinson Microwave Anisotropy
Probe (WMAP) to about 375,000 years
after the Big Bang.
This was a time before star and galaxy formation; microwaves were evidence of the “afterglow” of the Big
Bang – see Fig.32.
The time between 375,000 and 400
million years after the Big Bang is
known as the “Cosmic Dark Age”, as
there were (previously) believed to be
no stars or other light sources to generate light. In fact, the end of the Cosmic Dark Age at 400 million years is
now disputed. The JWST has found
galaxies younger than that (see below).
The most distant galaxy
At the time of writing, the most distant and earliest galaxy is believed to
be the candidate object named CEERS93316, discovered using the JWST
in July 2022 – see Figs.33 & 34. It is
believed to have formed just 235.8
million years after the Big Bang. It
was previously believed that the first
Fig.32: looking back in time with microwaves. The cosmic microwave
background was imaged by the Wilkinson Microwave Anisotropy Probe (WMAP)
and depicted as the afterglow pattern in this diagram. The JWST sees back in time
to the first stars. Source: https://map.gsfc.nasa.gov/media/060915/index.html
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Australia's electronics magazine
Fig.33: the galaxy CEERS-93316. It
mightn’t look like much, but it is the
most distant object yet observed by
the JWST. Source: www.ed.ac.uk/
news/2022/edinburgh-astronomersfind-most-distant-galaxy
galaxies formed 400 million years after
the Big Bang.
Light from this object has travelled
for 13.55 billion years, and the distance to the object is 34.68 billion light
years due to the universe’s expansion.
The red shift is z ≈ 16.7.
Imaging exoplanets
JWST will be able to observe certain young, hot planets via a technique
called direct imaging as well as other
methods. JWST will also be able to
detect oxygen and organic molecules
in exoplanet atmospheres, which are
possible indicators of life.
Limited ability for service
The JWST was not intended to be
serviced. Once its fuel is depleted or
there is a major system failure, the mission will be terminated. The minimum
planned mission time is five years, so
service is not expected to be needed,
but compare that to Hubble, which has
exceeded its design lifetime by a substantial margin and has been in orbit
for 32 years.
But the HST was designed to be serviced and was placed in an orbit accessible by the Space Shuttle.
In contrast, the JWST is in a very
hard-to-access orbit. There is no present way to service the JWST, but there
are very limited provisions for a possible manned or robotic servicing operation. Details on that are hard to find.
Among these provisions are a refuelling adaptor and, according to Space.
com, a docking ring (see their 2007
article at siliconchip.au/link/abgo).
Alternatively, the interface ring used
to attach the JWST to the Ariane 5
siliconchip.com.au
Fig.34: a timeline from the Big Bang to the present. The letter z refers to the amount of redshift. The more redshift, the
more distant the object and the older it is. Source: https://w.wiki/5gyn
launch vehicle could be used to grapple the spacecraft.
The JWST has been engineered with
multiple redundant systems so that if
one fails, others can take over, minimising the need for servicing. The goal
is for a ten-year lifespan, ie, twice the
planned mission duration. Ultimately,
if there are no significant failures, the
fuel supply for station keeping will be
the limiting factor.
Because of an excellent initial rocket
burn and trajectory, it used much less
fuel for mid-course correction than
expected, and it is hoped that there is
enough fuel left for perhaps 20 years
of operation.
Electrical power
The JWST has a solar array to provide 2kW of electrical power. JWST
stores power from the array in lithium-
ion batteries, specifically, Enersys
ABSL types in an 8S44P (series and
parallel) 28V, ~66Ah configuration.
Propulsion & attitude keeping
The propulsion system uses thrusters that run on hydrazine fuel (N2H4,
159L tank capacity) with dinitrogen
tetroxide oxidiser (N2O4, 79.5L tank
capacity).
There are four Secondary Combustion Augmented Thrusters (SCATs)
in two pairs. One pair was used to
propel the JWST into orbit, while the
other is for station keeping. It also
has eight monopropellant Rocket
Engines (MRE-1) – see Fig.35. These
use hydrazine decomposition (without oxidiser) and are for attitude control and momentum unloading of the
reaction wheels.
Slewing and then pointing the telescope in the desired direction is done
Fig.35: a schematic view
of the JWST propulsion
system. “GHe” stands
for gaseous helium used
to pressurise the fuel
tanks. Original source:
Hammann, Jeff, JWST
Propellant Budget
Document, Northrup
Grumman, July 19th,
2013 (D40258).
siliconchip.com.au
Australia's electronics magazine
December 2022 25
by the Attitude Control System (ACS)
and the JWST Fine Guidance Sensor
(FGS). The ACS also is responsible for
Delta-V (orbit correction), momentum
unloading (see below), antenna pointing, avoiding pointing at the Sun and
controlling observatory “safe modes”.
The spacecraft flight software
receives data from various sensors,
instructions from the Integrated Science Instrument Module (ISIM) and
JWST ground control and processes
them to send data to either the reaction wheels or the thrusters.
The sensors include sun sensors,
two star trackers and gyroscopes. The
star trackers choose appropriate stars
from a catalog, track their positions
and compare them with the commanded position.
During exposures (taking pictures),
the Fine Guidance System (FGS)
observes the guide star and makes
measurements every 64ms. That data
is sent to the ACS, which corrects any
pointing error using reaction wheels
and the Fine Steering Mirror (FSM).
Momentum management and
reaction wheels
Reaction wheels are used in spacecraft, including the JWST, to control
their attitude (orientation with respect
to a fixed object). They are essentially
motorised flywheels. When the wheel
spins up or down, the spacecraft reacts
by rotating in the opposite direction.
The JWST has six reaction wheels –
see Fig.36.
Their use saves spacecraft fuel and
they can also be used for tiny and
accurate attitude adjustments, more
so than rocket thrusters. They can
only be used to rotate a spacecraft,
not to move it.
Interesting links
1.
2.
3.
4.
5.
Build a paper model of the JWST: siliconchip.au/link/abgq
Make a model of the JWST mirror: siliconchip.au/link/abgr
Links for accessing data from JWST and instrument documentation:
siliconchip.au/link/abgs
Details of all the deployment operations: siliconchip.au/link/abgt
43-part playlist of time-lapse videos of the JWST being built and tested:
siliconchip.au/link/abgp
Photons from the Sun constantly hit
the JWST sunshield. Since photons
can exert a small force, this causes a
force to be applied to the telescope.
The centre of pressure of the sunshield
is not the same as the centre of mass
of the telescope, so this force generates a torque, making the telescope
want to rotate.
The reaction wheels counter this
rotation. As a result, angular momentum accumulates in the wheels (ie,
they keep spinning faster). If this
were not corrected, the wheels would
exceed their design limits. Therefore, the thrusters are fired about 4-8
times per month to allow the reaction wheels to be spun down. This
“momentum unloading” activity takes
several hours.
The JWST has a “momentum flap”,
also known as a “trim flap”, to somewhat minimise the rotation due to photon pressure, saving fuel.
What can JWST image?
Every six months, the JWST can
image almost anything in the celestial sphere as it orbits the Sun and the
Earth. At any one instant, however,
it can see anything with a 50° field
of view. 39% of the celestial sphere
is potentially accessible to it at once.
The only areas permanently inaccessible are imaging of the Sun, Mercury,
Venus, Earth and Moon as these are too
Fig.36: a model RSI
50-220/451 reaction
wheel, similar or
the same as used on
the JWST and built
by Rockwell Collins
Deutschland GmbH
(formerly Teldix). It
features integrated
electronics, spins at
up to 4500 RPM, is
347mm in diameter
and 124mm high,
weighs 9.5kg, runs
on 100V DC and
consumes under
20W. Source:
https://artes.esa.int/
projects/htmod2
26
Silicon Chip
Australia's electronics magazine
hot and would overload its sensors,
possibly damaging or destroying them.
False image colours
Images in visible light have the traditional colours of the visible light
spectrum that we are used to, but pictures from the JWST are also coloured,
even though they were taken in the
infrared.
Beyond the visible light spectrum,
colour is meaningless; however, adding colour to images helps us interpret
them, so, like visible light, colour in
infrared images is based on the wavelength of the light detected.
Colour is arbitrarily assigned to
the various infrared wavelengths to
convey additional information to us;
otherwise, the images would be all
in greyscale and only show intensity
information.
Data and comms
Data from the JWST is sent to the
ground via NASA’s Deep Space Network. The telescope can downlink
a minimum of 57.2GB of data daily
at 28Mbit/s. It has a solid-state data
recorder to store up to 65GB of science data. Downlinks occur twice per
day for four hours, and up to 28.6GB
of recorded data is transmitted per
downlink period.
Comms occur over Ka-band (2740GHz) for the high-rate downlink
of data and telemetry, and S-band
(2-4GHz), which is used for command
uplink, low-rate telemetry downlink
and ranging.
Micrometeroroid impacts
As expected, the JWST mirror has
suffered at least 19 micrometeoroid
impacts at the time of writing, but
these have not caused any significant
performance degradation. One impact
was larger than expected and required
a readjustment of the mirror actuator
to compensate for the damage.
When the JWST passes through
high-risk areas, its mirror will be
turned away from the direction of
travel.
SC
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