[MUSIC PLAYING] The primary mirror segments on the James Webb Space Telescope get the most press because, if nothing else, their sheer size. But there are other mirrors just as critical to making the telescope work. To find out more about something called the tertiary mirror is Koby Smith here at Ball Aerospace in Boulder, Colorado. Thanks for having us over. 

Thanks. 

So, Koby, first of all, tell us what is a tertiary mirror? How does it kind of figure into the whole telescope operation? 

Sure. So you have the entire James Webb Space Telescope. It is comprised of a primary mirror, which is of course built up from the primary mirror segments, which is shown here. So you imagine your whole primary mirror is 18 of these segments all the way around in space. This Aft Optic Subsystem sits right in the middle of all those mirrors, all right? 

The light from-- say, the galaxy or star you're interested in reflects off the primary mirrors, off the secondary mirror, and into the aperture here of the Aft Optic Subsystem. Then it reflects off the tertiary mirror, then again off the fine steering mirror, and back to the science instruments in the back of the telescope. 

OK, so the tertiary mirror is your third stop in this optical path. 

Exactly. 

So do you have a tertiary mirror here at Ball Aerospace? 

Actually, we, just received one from coating. Let's go down on the floor. And we'll pack it and take it in optical test. So this afternoon, we're going to unpack the coated, final-polished tertiary mirror from the shipping container using the flight transport cart and soft jaws, place it on the optical test stand here. And then wheel that into the tent for optical test table. 

Where are we now? 

We're in the optical test tent for both the secondary mirror and the tertiary mirror. It's used and reconfigured for testing both those optics. Over here, we have what's called an interferometer. And that's used to measure the surface quality of these optics, all right? It sends out a wavefront of light and then compares it to a known reference. And any deviations in that surface will appear as fringes on our camera screen. 

Thanks, Kobe, for giving us a sense of what the tertiary mirror does and the kind of testing it's going through. 

Thank you. It was great. 

So after this tertiary mirror goes through this optical testing, it'll go through some vibration and thermal testing before being integrated with the rest of the telescope. Thanks for joining us for another edition of Behind the Web. 

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