The $10 Billion eye-piece — James Webb Space Telescope (JWST) Shravankumar Setlur
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 —
to be set in a specific orbit at around 1.5 million km — Sun-Earth L2 Lagrange point*,
to be extremely efficient owing to finite fuel amount — required to run the reaction wheels to adjust minute position changes,
to not involve human intervention so as to reduce lag in communication or require periodic servicing or replacements,
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
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 —
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.
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.
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.
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.
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.
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.