Reflecting on the Webb telescope mirrors
From ore to space, an incredible 24-year project takes flight Metal Tech News – December 29, 2021
Last updated 1/14/2022 at 1pm
Although delayed by a few extra days, a historic event occurred this Christmas with the launch of what is set to become mankind's most powerful "eye" into the unknown, the James Webb Space Telescope.
Composed of 18 4.3 feet diameter hexagonal-shaped mirror segments, with a central primary mirror of 21.4 feet, years of innovation, ingenious problem-solving, and sheer determination have set the stage for a view into the cosmos we may never have even conceived of before.
Compared to the famous Hubble, with only a 7.9-foot mirror, not only did NASA scientists have to devise a way to safely launch such precious cargo but also had to develop an entirely new mirror, as the nearly eight-foot Hubble mirror would have been too heavy to launch into orbit had the same design been used.
The Webb team had to find something that would not only be light enough – roughly one-tenth the mass of Hubble's mirror per unit area – but also strong enough to withstand any contingencies the researchers could dream up.
Therefore, the team decided to make a segmented design composed of beryllium, which is both strong and light, ultimately leading to each segment weighing approximately 46 pounds.
Simple solutions to advanced problems had to be formulated, leading to the unique hexagonal shape that can fold like the "leaves of a drop-leaf table" to fit into a rocket. Afterward, each mirror will then unfold before finding its place in orbit to observe beyond.
So why the hexagonal shape?
This allows for a roughly circular, segmented mirror with "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.
Symmetry is good because there need only be three different optical prescriptions for 18 segments, six of each. Finally, a roughly circular overall mirror shape is desired because that focuses the light into the most compact region on the detectors. An oval mirror, for example, would give images that are elongated in one direction, while a square mirror would send a lot of light out of the central region.
Besides its apparent design challenges, other logistics had to be factored in, as once in space, getting the mirrors to focus correctly on faraway galaxies was yet another Herculean task.
So, the Webb team designed special actuators, or tiny mechanical motors, to provide the answer to achieving a single perfect focus. The primary mirror segments and secondary mirror are moved by these actuators that are attached to the back of each piece. While the primary mirror segments also have an additional actuator at their center to adjust the curvature, the telescope's tertiary mirror remains stationary.
Shape and positioning, what else did NASA have to develop for outer space? The cold.
However, not in the way one might think. The true challenge was to keep the mirror cold. To see the stars and galaxies in the early universe, astronomers have to observe the infrared light given off by them and use a telescope and instruments optimized for this light.
Because warm objects give off infrared light or heat, if Webb's mirror was the same temperature as the Hubble Space Telescope's, the faint infrared light from distant galaxies would be lost in the infrared glow of the mirror. Thus, Webb needs to be very cold – cryogenic – with its mirrors at around -364 degrees Fahrenheit. The mirror as a whole must be able to withstand very cold temperatures, stay very cold, and hold its shape.
The solution was to build shades for the mirrors! Being sent into deep space, far from the Earth, sun shields were made to prevent the sun's heat from warming them up and potentially distorting any pictures we get of distant celestial phenomena.
Finally, all those logistical issues and concerns handled, the fine-tuning on the materials sciences for each component, the computational aspects refined and calculated, the Webb team could then direct its attention to something NASA is quite skilled at, shooting stuff into space.
With the telescope soon to be in orbit, engineers on Earth will need to make corrections to the positioning of the Webb telescope's primary mirror segments to align them – ensuring they will produce sharp, focused images.
As of the exact moment of this passage, the telescope is still hurtling through space at approximately 281,369 miles from Earth, roughly 30% of its projected journey, with over 600,000 miles to go.
Its destination, L2 or second Lagrange point, to orbit the sun 1 million miles away.
Lagrange points – so named after Joseph-Louis Lagrange, an 18th-century mathematician who determined a solution to what is called the "three-body problem." That is, is there any stable configuration in which three bodies could orbit each other yet stay in the same position relative to each other? As it turns out, there are five solutions to this problem, and they are now called the five Lagrange points.
At these points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. As a result, the L1, L2, and L3 points are all in line with each other, while L4 and L5 are at the ends of equilateral triangles.
Once there, humanity will begin to see through time and possibly observe the era of our universe's history when galaxies began to form.
Light and mighty beryllium
Pound-for-pound, beryllium is six times stiffer than steel and one-third lighter than aluminum. Coupled with the ability to maintain its shape across a wide range of temperatures, this critical metal has a number of aerospace and military applications, especially where light weight and precision are a must.
Additionally, this metal is almost transparent to x-rays, making it an even more ideal material for the mirror aboard the JWST.
In the case of the James Webb Space Telescope's 18 special lightweight beryllium mirrors, they had to make 14 stops to 11 different places around the United States before fully completing their manufacturing, a journey that spanned nearly a decade.
First, they came to life at a beryllium mine in Utah called Spor Mountain. According to some estimates, Spor yields about 85% of total beryllium produced globally and is currently the largest source in the U.S.
After being mined, the material then moved across the country for processing and polishing. In fact, the mirrors made stops in eight states along the way, even visiting some states more than once, before they journeyed to South America for lift-off and the beginning of their final trip into space.
Purified at Brush Wellman in Ohio, the particular type of beryllium used in the Webb mirrors is called O-30 and comes in a fine powder form. The powder was then placed into a stainless-steel canister and pressed into a flat shape.
Once the steel canister was removed, the resultant chunk of beryllium was cut in half to make two mirror blanks about 4 feet across. Each blank was then used to make one mirror segment.
After the mirror blanks passed inspection, they were sent to Axsys Technologies in Cullman, Alabama, with the first two mirror blanks being completed in March 2004.
Axsys shaped the mirror blanks into their final shape, which started with cutting away most of the backside of the beryllium mirror blank, leaving just a thin "rib" structure – the ribs are only about one-twenty fifth of an inch thick.
Although most of the metal was stripped, the ribs were enough to keep the segment's shape steady, making each extremely light, with a resulting final weight of roughly 88 lbs (including actuator).
Next was polishing, where the segments were then shipped to Richmond, California.
SSG/Tinsley was the company that ground down the surface of each mirror close to its final shape. After this was done, the mirrors were carefully smoothed out and polished. Then, the process of smoothing and polishing was repeated until each mirror segment was nearly perfect.
At that point, the segments traveled to NASA's Marshall Space Flight Center in Huntsville, Alabama, for cryogenic testing.
Because of the properties of beryllium, this proved not too difficult a test. Although many materials change shape when they change temperature, a test team from Ball Aerospace worked together with NASA engineers at Marshall's X-ray and Cryogenic Facility to cool the mirror segments down to the temperature Webb would experience in deep space, -400 degrees F.
Testing lasted for two years between 2009-2011. Once the mirror segment's final shape was corrected for any imaging effects due to cold temperatures and polishing was completed, a thin coating of gold was applied, giving the JWST its shiny futuristic look.
After the gold application, the mirrors journeyed back to Marshall Space Flight Center for final verification before being sent to NASA's Goddard Space Flight Center in Greenbelt, Maryland, where they spent nearly a decade in assembly.
It is almost heaven-sent that a material uniquely appropriate for such conditions would be found on Earth. Yet, without beryllium, the already monumental project could very well have never made it out of theory.
Development began in 1996 for a launch that was initially planned for 2007 with a $500 million budget. However, with numerous delays and cost overruns, including a major redesign in 2005, the costs swelled to $10 billion and there was much speculation that such a project would ever truly be completed.
Yet, as of Dec. 25, 2021, the mission has finally launched, and the next giant leap for mankind may be just on the horizon.
Congratulations to every individual involved in the creation of this momentous occasion! Every effort and tribulation surpassed created a legacy that will be remembered for all time.