The $10 Billion eye-piece — James Webb Space Telescope (JWST)
James E. Webb (1906- 1992) — NASA’s second administrator. Webb is best known for leading Apollo, a series of lunar exploration programs that landed the first humans on the Moon.
— On 10 September 2002, the Next Generation Space Telescope was named in the honor of James E. Webb.
“So, why all the hassle and what’s with the hype?”
Nicknamed the “Next Generation Space Telescope”(NGST), the primary objective of this humble garage project is to study the formation of the earliest stars, planets and galaxies of the Universe and bask in the awe of the infant Universe’s glow (okay, I will stop!).This formation period roughly from 3,70,000 — 100 million years after the Big Bang is termed as “The Epoch of Reionization”. And also, of course, to gain precision in determining the age of the Universe, charting an atlas, finding exo-planets potentially containing life and so on. But this time with much improved wavelength coverage, stable-pointing, observational efficiency and a lot of gold than its predecessor Hubble Space Telescope (HST).
If a 5 watt bulb glowing on the Moon’s surface were to be observed from Earth, the bulb will still be 20 times brighter than the early stars that are to be observed.
Why does it look so different and golden?
This telescope design is unlike any other conventional telescopes that operate in visible light spectrum and looks more like one of those large radio telescopes that can get into stealth mode in case of alien attack. Well, that’s partially true. The aim is to detect very faint Infrared light from the oldest and early stars distant in space and time scattered in the background with utmost precision and stability. The celestial objects emitting such light are colder and glow only in the weak orange, red and mid-Infrared spectrum which cannot be detected from the ground or by the HST. The pre-requisites to be met by this eye-piece were —
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to be set in a specific orbit at around 1.5 million km — Sun-Earth L2 Lagrange point*,
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to be extremely efficient owing to finite fuel amount — required to run the reaction wheels to adjust minute position changes,
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to not involve human intervention so as to reduce lag in communication or require periodic servicing or replacements,
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to avoid and withstand the infrared pollution and heat from our favorite fire-ball — operating temperatures range roughly from −223 °C to 83 °C (50K to 360K) and above all
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to fold this 6,161.4 kg ; 20m x 15m structure into a rocket that can drive it to the L2 orbit and be able to unfold by itself while keeping all parts intact and operational — withstanding high acoustic, thermal, vibrational, and physical stresses.
What kind of telescope is JWST? A three mirror Anastigmat Telescope.
The primary mirror is concave, the secondary is convex, and it works slightly off-axis. The tertiary removes the resulting astigmatism and also flattens the focal plane. This also allows for a wider field of view.
Now, about the golden stuff — Primary Mirror
The Optical Telescope Element (OTE) Primary Mirror’s (concave) purpose is to act like a bucket and collect as much faint photons as possible while operating in the cryogenic temperatures (-220 degrees C) and reflect all of them on the Primary focus point, where the Secondary Mirror (convex) is installed. More about it —
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Made of Beryllium — (atomic no.=4) owing to its high strength-to-weight ratio, good at holding its shape across a range of temperatures, good conductor of electricity and heat and is not magnetic.
Mirror without the actuators weighs around 20kg.
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Gold coating — Gold is a poor reflector of visible light but the best reflector of Infrared light.
“Vacuum vapor deposition” : the mirrors are put inside a vacuum chamber and a small quantity of gold is vaporized and is deposited on the mirror (thickness ~ 100 nanometre).
A thin layer of Glass (amorphous SiO2) is deposited on top of the gold to protect it from scratches in case of handling or if particles get on the surface.
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Spanning 6.5m across (2.7 times bigger than HST’s) — this was decided as a mirror of this size was required to study the light coming from the target stars and galaxies.
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18 Hexagonal-shaped mirror segments — each of the size 1.32 m and slightly curved to obtain one primary focus.
It was designed in a way to fold up like drop-leaf table.
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Why Hexagon? — Ideally a large circular mirror is needed; however, as the mirrors have to be folded, out of all shapes Hexagons have “high filling factor” and “six-fold symmetry.”
High filling factor means the segments fit together without gaps. If the segments were circular, there would be gaps between them.
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Adjusting these pieces using Actuators — The primary mirror segments and secondary mirror are moved by six actuators that are attached to the back of each mirror piece.
— Primary mirror approximately will bounce back roughly 1 photon per second coming from the faintest celestial objects.
— A little more than 48 grams of gold are used in the Webb mirror.
— Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair.
The Sunshield
The sunshield separates the observatory into a hot sun-facing side and a cold side — modelled max and min temperature resp., of the outermost layers are 383K/110°C and 36K/−236°C. In order to be able to detect those faint heat signals, the telescope itself must be kept extremely cold and stable to avoid frequent mirror alignments thereby reducing fuel consumption.
This giant parasol, the size of a tennis-court (roughly 20m x 14m), with its 5-layers of shade — always faces the Sun and protects the telescope from external sources of light and heat (the Sun, Earth, and Moon) as well as from heat emitted by the observatory itself. The shiny silver material of the sunshield is a complex and innovative feat of material science and engineering.
Why doesn’t it have just one good thick layer instead? Heat radiates between the layers in a zig-zag pattern due to the curvature and vacuum is a good insulator.
One big thick sunshield would conduct the heat from the bottom to the top more than five layers separated by vacuum.
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Membrane : Each successive layer of the sunshield, made of “kapton”, is cooler than the one below and is coated with aluminium, and the sun-facing side of the two hottest layers also have a “doped-silicon” coating to reflect the sun’s heat back into space.
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Thickness — sun-facing layer = 0.05 millimeter ; rest = 0.025 millimeter.
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Kite-like shape — The shape and design also direct heat out the sides, around the perimeter, between the layers.
The fifth layer is mostly for margin against imperfections, micro-meteoroids holes, etc.
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Special Seaming — The membrane material is tough, but if it gets a small tear or hole, the hole could become much larger.
Reinforcing strips of Kapton are Thermal Spot Bonded to the parent membrane about every 6 feet or so, forming a grid pattern of “rip-stops”.
The sun-shield is used to block the heat energy from the Sun, that can be upto 2,00,000 watt on sun-facing side to merely less than 1 watt on the other side.
Where is Webb?
Launched on 25th December, 2021 at 12:20 UTC on the Russian-made Ariane-5 rocket (streak of 82 consecutive successful launches), which is the European Space Agency’s (ESA) contribution to the mission. As I was typing this section, the telescope had just arrived at its desired L2 position to set itself into the halo-orbit facing away from the Sun, after a month of travel since launch. The deployment of telescope mirrors and the shield was already done along the way.
The telescope and scientific instruments will take several weeks to cool all the way down and reach stable temperatures. The next five months will be all about aligning the optics and calibrating the scientific instruments.
*Lagrange Point —
These are positions in space where the gravitational forces of a two body system like the Sun and the Earth produce regions of equilibrium.
These can be used by spacecraft to remain in position without using fuel.
By - Shravankumar Setlur
