Astronomical Engines, Big Complicated Machines

Making the James Webb Space Telescope’s Mirrors

The back side of a JWST mirror segment

Good morning, everyone.

Today I’m going to talk more about the mirrors on the James Webb Space Telescope (JWST) and what went into making them. As you remember from the article on the journey that the mirror segments take during manufacturing (you all did read that article, correct?), the segments make 14 stops during the process. I’m not going write up each stage of the fabrication, otherwise we’d be here all day and I have other things that need to be done, plus my foot hurts.

There are a series of videos entitled “Behind the Webb” produced by the Space Telescope Science Institute (STScI) in Baltimore, Md. that show a lot of information as to how the mirrors are made and installed in the JWST. They are all short, about 3 minutes per video, so they won’t eat up a bunch of time at the office. The presenter is a twit, but you’ll just have to put up with her. It will also save wear and tear on my now heavily bandaged hands, the end result of hand-to hand combat with the sheet metal on two dust collectors and hose connections.

For starters, let’s take a closer look at what the mirrors are made of: beryllium.

Beryllium:  symbol – Be, atomic number – 4, atomic weight – 9.012182 blah blah blah, poisonous, a dark gray alkaline metal in the same group as magnesium and our old friend radium. It’s used in applications where light weight, structural stiffness and thermal stability are required, so you find it a lot in aerospace applications. Because it is transparent to x-rays, it’s also used in the medical industry and the physical sciences for x-ray windows. It’s also expensive, the price of highly refined Be is somewhere north of $600-$800 per pound, and very difficult to machine.

One interesting fact about Be that I discovered on one of the NASA pages, is that the beryllium on Earth didn’t come from stars. From NASA’s “Recipe for the Perfect James Webb Space Telescope Mirror”:

What’s also interesting is that beryllium is a relatively rare element in both the Earth and the universe, because stable forms of beryllium are not formed either in the atomic reactions inside stars or in the Big Bang. Instead, when carbon and oxygen atoms in the gas between the stars collide with each other or are struck by other particles, the nucleus of the atoms will occasionally break into up into the lighter elements lithium, beryllium and boron.

Here is a video about how BE is mined and refined:


Next, is a Northrop-Grumman video on the manufacturing process:


Grinding and polishing.



Inspecting a mirror segment ("is that a booger?")

Cryogenic testing and surface inspection. The presenter is a twit.



The cryo chamber at the Marshall Space Flight Center

Installing the actuators:


The mirror mounting framework:


And finally, an overview of the mirrors:

[youtube] [/youtube]


Now you know how to make a proper beryllium mirror. I’m thinking of conducting a lab on this, so be prepared.



NASA’s The “Not So Heavy Metal Video”: James Webb Space Telescope’s Beryllium Mirrors

NASA’s Recipe for the Perfect James Webb Space Telescope Mirror

Understanding Beryllium (Thanks mdharrell!)


  • I haven't watched the videos yet (although I most certainly will), so perhaps this is addressed, but my initial reaction to the "Inspecting a mirror segment" figure is that doing so by holding an unsecured Maglite above the surface, even slightly off to the side, is just asking for a very, very expensive accident.

    I'm glad the Be machining link came in handy.

    • The Professor

      You noticed that too? Usually there are quality assurance inspectors wandering around, looking for stuff like that. Probably gone due to austerity cuts.

    • pj134

      It isn't so bad compared to the guy in the background using the equipment as a stair master.