Gold Plating That Touches the Stars

NASA's James Webb Space Telescope (JWST), launched into space on 25 December 2021 aboard an Arianespace Ariane 5 rocket, is an infrared space observatory. Capable of examining every stage of cosmic history from the first bright glimmers after the Big Bang through the formation of galaxies, stars and planets to the evolution of our own solar system, JWST will play a crucial role in our quest to understand the universe and our origins. The James Webb Space Telescope's first full-color images and data collection were released on 12 July 2022, following six months of deployment, instrument testing and subsequent data gathering. This collection, demonstrating that the telescope was functioning properly, contains the highest-resolution images of the universe captured to date. Along with these remarkable visuals, JWST also captured the distinct signature of water, along with evidence of clouds and haze in the atmosphere surrounding a hot, puffy gas giant planet orbiting a distant Sun-like star. This observation, which reveals the presence of specific gas molecules based on small decreases in the brightness of particular light colors, is the most detailed example conducted in this field to date and demonstrates JWST's unprecedented ability to analyze atmospheres hundreds of light-years away. Although the Hubble Space Telescope has analyzed numerous exoplanet atmospheres over the past twenty years and captured the first clear detection of water in 2013, JWST's immediate and more detailed observations represent a giant leap forward in the search to characterize potentially habitable planets beyond Earth. Development of JWST began in 1996, and the telescope was initially scheduled to become operational in 2007, but the process was prolonged due to numerous setbacks and problems, requiring the redesign of many components. However, scientists never abandoned the project and, despite the COVID-19 pandemic, continued their work, achieving success by the end of 2021 when JWST began its mission.

Primary Mirror System

Now let us examine the coating technologies that made this technological marvel possible. JWST's primary mirror system consists of 21 mirrors with 18 mirror segments, each 1.32 metres (4.3 feet) in diameter, working together as a single mirror of 21.3 feet (6.5 metres) in size. A secondary mirror is mounted on the end of long booms that fold during launch and later unfold. The mirrors themselves are made of beryllium and were shaped by Axsys Technologies (Cullman, Ala.) and polished by SSG/Tinsley (Richmond, California). [caption id="attachment_144131" align="aligncenter"] Flight mirrors for the James Webb Space Telescope undergoing cryogenic tests at NASA Marshall. Image source: Ball Aerospace[/caption] The decision to use beryllium for JWST telescope mirrors was based on data obtained from Spitzer (another space telescope) mirrors. Spitzer's mirrors are also made of beryllium. Beryllium was selected for its hardness and its ability to provide the required stability at cryogenic temperatures, approximately -220°C (-364°F). This is important because JWST needs to remain cold. As objects heat up, they emit infrared light, which covers the same spectrum that the telescope is trying to capture. If the mirrors heat up, it would prevent the telescope from capturing light from distant galaxies. In addition to maintaining its shape at various temperatures, beryllium has additional properties that helped determine its selection for the project. It is lightweight, which is important for any space launch, with each mirror segment weighing only 20 kilograms (46 pounds). Beryllium is also a good electrical conductor. However, beryllium does not reflect infrared light very well. For this reason, JWST's primary mirror system is coated with gold. [caption id="attachment_144132" align="aligncenter"] Gold Coated Engineering Design Unit (EDU) Primary Mirror Segment This is a photograph of one of the primary mirror segments of the James Webb Space Telescope coated with gold by Quantum Coating Incorporated. It is an engineering design unit, not a flight segment. The photograph was taken by Drew Noel at BATC. Image source: Photograph - Drew Noel[/caption]

Gold Coated Mirrors

JWST's primary mirror, with its reflective gold hexagonal component mirrors, is instantly recognizable and extremely impressive. Each mirror is coated with approximately 0.12 ounces of gold to optimize the reflection of infrared light. The microscopically thin layer of gold was applied by Quantum Coating Inc. (QCI, Moorestown, N.J.) through vacuum vapor deposition. The company invested in a two-year study before coating the first flight mirror. QCI built a coating facility and clean room to coat the mirror segments and developed the coating to meet performance properties in areas such as homogeneity, reflectance, durability, stress and cryogenic conditions. QCI's Coating Director Ian Stevenson noted that they encountered many technical challenges in the JWST mirror coating program, with one of the most daunting being the requirement that all flight hardware runs be conducted without a single failure. The gold coating must be thick enough to completely cover the mirror, yet thin enough not to cause any expansion or contraction in the mirrors due to temperature changes. The coating is applied by placing the mirrors inside a vacuum chamber. Complex masking protects areas that will remain uncoated. Gold is heated to its boiling point (2,500°F - 1,371°C), vaporized and injected into the chamber. The vaporized gold is deposited on the optical surface of the mirror, resulting in a coating 120 nanometres thick (approximately 200 times thinner than human hair). While apparently impressive, the actual amount of gold used for the entire telescope is not large. Only 2.5 cubic centimetres – approximately 48 grams – of gold was required to coat the entire mirror array of JWST.

Secondary Mirror

"Secondary" may not sound as important as "primary," but when it comes to the next-generation James Webb Space Telescope, a secondary mirror plays a critical role in enabling the telescope to gather information from the cosmos. JWST's secondary mirror has been completed following polishing and gold coating. There are four different types of mirrors in the James Webb Space Telescope, all made of a lightweight metal called beryllium. The JWST system includes primary mirror segments (18 in total, providing a collection area of 25 square metres for the large primary mirror), a secondary mirror, a tertiary mirror and a fine guidance mirror. [caption id="attachment_144133" align="aligncenter"] James Webb Space Telescope Secondary Mirror
Image source: Ball Aerospace[/caption] Unlike the primary mirror, which is molded into a hexagonal shape, the secondary mirror is perfectly round. This convex mirror is similar to the curved mirrors that allow drivers to see out when leaving parking spaces. This mirror is coated with a microscopic layer of gold to ensure it efficiently reflects infrared light (the image seen by JWST telescope's cameras is the result of this reflection). The quality of the secondary mirror's surface is so good that the final convex surface at cold temperatures deviates from the design by no more than a few millionths of a millimetre, approximately one ten-thousandth of the diameter of human hair. Why is this mirror so critical? Because the secondary mirror captures light from the 18 primary mirror segments and transmits these distant images of the cosmos to the telescope's scientific cameras. The secondary mirror is mounted on foldable "arms" that position it in front of the 18 primary mirror segments.

More Than Just Mirrors

JWST consists of three main components: the Optical Telescope Element (OTE), the Integrated Science Instrument Module (ISIM) and the Spacecraft Element (SCE), which includes a tennis court-sized sunshield. Gold is not the only place where it is found in JWST. Gold is also used in the telescope's infrared cameras for thermal control functions. For this task, NASA turned to Epner Technology. The company's Laser Gold coating, selected as the standard for infrared reflection at 99 per cent in the mid-infrared range of 2 to 14 microns by the National Institute of Standards and Technology (NIST, Gaithersburg, Md.), is a hard, electrochemically deposited gold coating. JWST's Near-Infrared Camera (NIRCam) observes in an infrared wavelength range of 0.6 to 5 microns, while the Mid-Infrared Instrument (MIRI) covers mid-infrared wavelengths between 4.9 and 28.8 microns. The two instruments are equipped with coronagraphs, mechanisms consisting of filters that block light from brighter objects, allowing observation of dim and distant objects. Laser Gold from Epner Technology was used to plate components for the coronagraphs and allowed for thermal control of the devices. [caption id="attachment_144134" align="aligncenter"] The James Webb Space Telescope has a cold side with its back to the Sun and a hot side facing the Sun. JWST's tennis court-sized sunshield protects the telescope from external light and heat sources, allowing it to detect weak heat signals from very distant objects. It is very important that the observation side is extremely cold. The bottom of JWST's five-layer sunshield is facing the Sun. The sun panel, antennas, computers, gyroscopes and propulsion jets that do not need to be cooled are located here. JWST's science instruments are placed behind the mirror, separated from hot communication and control technology by the sunshade. Image source: NASA, ESA, CSA, Joyce Kang (STScI)[/caption] [caption id="attachment_144135" align="aligncenter"] Photograph taken while NASA's James Webb Space Telescope was being prepared for shipment to the launch site following the successful completion of final tests. Image source: NASA/Chris Gunn[/caption]

Shield for Protection from Heat

The orbit of the James Webb Space Telescope is not around Earth like the Hubble Space Telescope, but actually around the Sun, at a location called the second Lagrange point or L2, approximately 1.5 million kilometres (1 million miles) from Earth. What is special about this orbit is that it keeps the telescope aligned with Earth as it moves around the Sun. This allows JWST's large sunshield to protect the telescope from light and heat from the Sun and Earth (and the Moon). JWST uses a tennis court-sized sunshield consisting of five thin layers of Kapton E coated with aluminium and doped silicon to reflect the Sun's heat back into space. [caption id="attachment_144136" align="aligncenter"] Image source: Photo: Northrop Grumman/Alex Evers[/caption] Each layer is coated with aluminium, and the two hottest layers (referred to as Layer 1 and Layer 2) have "doped silicon" (or processed silicon) coating on the side facing the Sun to reflect the Sun's heat back into space. The sunshield is a critical component of the JWST telescope because the infrared cameras and instruments on board must be very cold to function properly and must be kept away from the Sun's heat and light. Doping is a process in which a small amount of another material is mixed during the silicon coating process to make the coating electrically conductive. The coating must be electrically conductive so that the blankets can be electrically grounded to the rest of JWST and do not build up static electrical charge across their surfaces. Silicon has a high emissivity, meaning it emits the most heat and light and prevents the Sun's heat from reaching the infrared instruments that will be placed beneath it. Highly reflective aluminium surfaces also bounce the remaining energy out through gaps at the edges of the sunshield layers. [caption id="attachment_144138" align="aligncenter"] In this photograph, NASA technicians can be seen lifting the telescope with a crane as it is transported to a clean room at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Image source: NASA/Desiree Stover[/caption] Sources •https://JWST.nasa.gov/content/features/downloads.html •https://www.pfonline.com/articles/coating-the-JWST •https://www.nasa.gov/content/james-JWST-space-telescope-latestnews •https://www.nasa.gov/topics/technology/features/JWST-secondary.html •https://www.flickr.com/photos/nasaJWSTtelescope/albums/ •https://JWST.nasa.gov/content/about/innovations/coating.html •https://www.space.com/james-JWST-space-telescope-trapeziumstars-preview •https://JWSTtelescope.org/contents/articles/JWSTsfirstimages#:~:text=After%20six%20months%20of%20unfolding,data%20on%20July%2012%2C%202022. •https://www.space.com/21925-james-JWST-space-telescope-jwst.html •https://www.nasa.gov/mission_pages/JWST/science/index.html •https://webb.nasa.gov/content/about/orbit.html Compiled and translated by: B. Serhat Cengiz