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The Gold Plating on the James Webb Space Telescope is Much Thinner than Human Hair

The newest telescope launched into space is as flashy as it is powerful

The date is December 25th, 2021. You may have been ripping open gifts, putting the turkey in the oven, or spending time with your family. But in Kourou, French Guiana, the James Webb Space Telescope was officially launched. This project – a telescope designed to observe and analyze astronomical phenomena using infrared radiation – was first pitched in 1996. Construction, a collaborative effort between NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA) began in 2004. Completion was a long time coming. In fact, the project is almost as old as me (I’m 18)!

The structure aims to capture images of the “hidden universe”. Our eyes are only able to see visible light, which has wavelengths between 380-700 nanometres while the JWST or Webb will detect wavelengths of infrared light ranging from about 600-28,500 nanometres. The hope is that it will reveal what we cannot see: births of planets and stars covered by clouds of dust, signs of life hinted at by water in other atmospheres, and the history of the universe told by light originating from soon after the Big Bang. As light travels across the universe it gets stretched by the expansion of space. So ancient visible and UV light is transformed into light of longer wavelengths – infrared – which Webb will be able to capture.

Webb is a reflecting telescope that uses 3 different-sized mirrors to allow for a wide field of view. The primary mirror is made up of 18 smaller hexagonal mirrors, which unfolded after launch to form a smooth concave structure. The secondary mirror faces the first. It is marginally off-axis and convex, like the back of a spoon. The fixed tertiary mirror is found in the middle of the primary mirror; it corrects astigmatism from the secondary mirror and directs light to a fine steering mirror, which finally sends the light to the infrared detector.

The mirrors are all made of beryllium, which is strong yet light, and can hold its shape at cryogenic (very low) temperatures. Every mirror is plated with an incredibly thin layer of gold. At about 100 nanometer thickness, it is roughly one ten-thousandth the thickness of a human hair. After shaping, correcting, and polishing of the beryllium, the gold was applied through vacuum vapour deposition. The mirrors were placed in vacuum chambers where the gold was quite literally vaporized and deposited on the surfaces. A layer of glass was then placed on top of the gold to protect it.

Why gold? This metal is extremely reflective of both visible light and other forms of radiation, particularly in the infrared range. The gold coating optimizes the function of these mirrors. Our basic silver and aluminum mirrors here on Earth reflect 85-95% of infrared light, whereas gold reflects 99%. Gold is also relatively un-reactive, so it won’t tarnish easily.

Although the Webb mirrors are rather large (the primary mirror has a total diameter of approximately 6.5 m), the thin gold layer weighs in at a mere 48.25 g. If you brought this amount of gold into a pawn shop, you’d be walking out with around $3,600 -- a very small fraction of the total cost of this project which is around $10 billion USD.

What does Webb have to show for this large outlay? Check out these images (produced this week) for yourself.


Haleh Cohn just finished her first year at McGill University and is interested in the health sciences.

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