Thread by @fox_ori

Bored while waiting for #jwst's mirrors to open tomorrow? Let's have a small technical chat about the optical design in the mean time. 1/n 🧵
webb.nasa.gov/content/observ…

If you didn't know, #jwst is a three mirror anastigmat telescope that works slightly off-axis. A couple very interesting notes:
1) It has an effective vocal length of 131 meters!
2) The primary mirror is concave and the secondary mirror is convex.

2/n

What drives this design? It's all about the individual pixels on the sensors. These are finite in size (~18 microns) for both scientific and technological reasons. We'll learn more about detectors later. 3/n

If you place an individual pixel behind a telescope with an effective focal length of f=131 meters, the angular size of each pixel is 0.028" which...wait for it...is roughly the same as the diffraction limit of a 6.5 meter telescope at 1 micron. This is the key to everything! 4/n

But how can you make a 131 meter focal length on a telescope that isn't 131 meters? That's where the concave secondary mirror comes in. It magnifies the image so that you have an *effective* focal length of 131 meters. This is similar to way in which a telephoto lens works. 5/n

Don't forget, focal length isn't everything. You still need a big aperture (6.5-m) to collect enough light. So that sets our primary size. But if you made something this big from glass (like #hst), it would be too heavy to even launch off the ground. 6/n

Typo: meant convex secondary. 7/n

https://twitter.com/fox_ori/status/1479097940388024322?s=20

Anyway, to get around that, engineers tested and tested...and competed...in making a lightweight design. The project ultimately went with Beryllium (Be) which should probably be everyone's favorite element at the moment. Just don't eat it. 8/n

More trending gear you should own. 9/n CC: @startorialist

Anyway, each mirror criss-crossed the country something like 12 times during its production lifetime. 10/n

You can interactively explore the map of the mirror journey here. It's a great chance to thank all the sub-contractors and...you guessed it...technicians who make these mirrors possible. 11/n

webb.nasa.gov/content/featur…

When completed, the mirrors still only looked like THIS. 12/n

Everyone knew that gold bling would help @NASAWebb's twitter account. Plus, gold happens to improve the mirror's reflection of infrared light. So, BONUS. If you want to learn how the gold was applied, check this video out on vacuum vapor deposition. 13/n

And now you know everything you need to know...or that I know...about JWST optics. Enjoy the show! 14/14