Editor at ‘The Secrets Of The Universe’, I have completed my Master’s in Physics from India and I am soon going to join Institute of Space Sciences, Barcelona for my doctoral studies on Exoplanets. I love to write about a plethora of topics concerned with planetary sciences, observational astrophysics, quantum mechanics and atomic physics, along with the advancements taking place in the space industry.
As November is approaching, the world’s excitement for the James Webb Space Telescope launch is touching new heights. A 10 billion dollars mission, the James Webb Space Telescope will be the largest, most powerful, and most complex space telescope ever built and launched into space. Webb will take us on a journey that will help us look back in time, revealing how our universe looked just 100 to 250 million years after the big bang and will eventually help us understand the formation of the first-ever galaxies.
In simple words, the James Webb Space Telescope, named after James Edwin Webb, will redefine our current understanding of the colossal cosmos. The James Webb Space Telescope is nothing less than a marvel of humanity’s innovation. However, what differentiates Webb from all the space telescopes launched so far is its unique mirror system. The composition, geometry, and arrangement of Webb’s mirrors make them one of a kind. Most of us must have heard that Webb’s primary mirror comprises 18 closely packed hexagonal mirror segments, but there’s a lot more to it!
The fabrication of mirrors
For any telescope out there, the primary mirror is like its heart. The primary mirror is the one that captures all the light, which is further processed to produce the dazzling images of the cosmic wonders beyond us. This makes the fabrication of the primary mirror one of the most intricate tasks in developing a telescope.
Many factors need to be taken into account while fabricating a mirror. For example, the mirror must be lightweight yet extremely sturdy enough to withstand the launch exertion and other physical pressures. So to realize these properties in Webb’s mirrors, the usage of beryllium was taken into account. Beryllium (Be) is a light metal that is very strong for its weight. It is a good conductor of electricity and heat and is not magnetic. Moreover, it is extremely good at holding its shape across a range of temperatures. All of this made Beryllium is a desirable choice for manufacturing Webb’s primary mirror.
To construct the mirror, a finely powered class of Beryllium, O-30, was used. The mirror segments were first brought to life at beryllium mines in Utah and were later moved across the USA for processing and polishing. In total, the primary mirror is composed of 18 hexagonal-shaped mirror segments, with each one of them being 1.32 meters in diameter and weighing approximately 20 kilograms. In addition to this, the convex secondary mirror is about 0.74 meters in diameter.
Each of the telescope’s mirrors has been carefully covered in an extremely uniform and microscopically thin layer of gold. The gold coating optimizes the segments specifically for reflecting infrared light, which is the primary wavelength in which the James Webb Space Telescope will decode the cosmos.
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Why exactly 18 hexagonal mirrors?
The power of a telescope directly depends upon how big its primary mirror is. The larger the primary mirror, the greater the amount of light collected from the distant objects, and the better the observations. Looking at JWST’s bucket list of future observations, it needed a monstrous primary mirror. But there was a problem! Such a mirror could not fit inside any of the existing rockets as one single, large mirror. So what now? This was when the team came up with a revolutionary idea to dissociate the primary mirror into 18 small segments that can fold to fit inside the rocket fairing.
Now you might wonder why they chose to go with hexagonal geometry and not a square, triangle, or other shapes. There’s a logic behind it.
A roughly circular overall mirror shape is desired to focus the light into the most compact region on the detectors. Any other geometry doesn’t give the required results. For example, an oval mirror produces elongated images in one direction, while a square mirror sends a lot of the light out of the central region. So overall, an approximately circular shape was needed to make the most of the James Webb Space Telescope.
The hexagonal shape has a six-fold symmetry that gives it a high filling factor, which further aids the segments to fit together without a gap. So, a honeycomb-like pattern allows each mirror to perfectly fit together with the least amount of dead space in between each other. This effectively creates a singular and massively powerful unit which also makes the desired approximately circular shape of the primary mirror.
Managing the 18 mirror segments
Okay, so now we have 18 mirror segments with us, but how to align and focus them perfectly as a single flawless mirror? This is where actuators come to work. Actuators are tiny mechanical motors that beautifully aid in achieving a single perfect focus. In the case of the James Webb Space Telescope, the primary mirror segments and the secondary mirror are moved by six actuators attached to each mirror piece’s back.
Moreover, the primary mirror segments also have an additional actuator at their center that helps in adjusting their curvature. All the actuators have been designed to make adjustments precisely without disturbing the other segments, which will further allow Webb’s mirrors and other instruments to work in perfect coordination to create compelling and error-free observations.
Speaking about the fine engineering involved in the construction of Webb’s mirrors, Lee Feinberg, who is the Webb Optical Telescope Element Manager at NASA Goddard, once said that “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. What’s even more amazing is that the engineers and scientists working on the Webb telescope had to invent how to do this.“
The James Webb Space Telescope is primarily going to work in the range of infrared wavelengths. Infrared radiations are nothing but an object’s signature in the form of heat. This means that every object at a non-zero temperature would emit some infrared radiation. So if Webb’s primary mirror is kept at the same temperature as the Hubble’s, then the faint infrared light from distant galaxies would get lost in between the infrared glow of the mirror itself.
So, to get rid of this interference, Webb’s mirror will be kept at around -220 degrees C (-364 degrees F), and to achieve this temperature, the James Webb Space Telescope is going to have a five-layer sun shield that is the size of a tennis court and will eventually weaken up the heat from the Sun by more than a million times.
In total, Webb’s primary mirror is nearly seven times the size of the Hubble’s and many times more advanced and powerful than all the space telescopes launched so far. So with all of its advanced features and especially its unique primary mirror, Webb will curate a new story of cosmic observations. Undoubtedly, the James Webb Space Telescope is about to bring a revolution.
We are excited! Are you?
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