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the james webb space telescope is what's

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known as a reflector telescope two

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mirrors one primary and one secondary

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used to collect and focus light coming

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from far away objects they're looking at

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the primary mirror on james webb is so

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huge it needs to be assembled from 18

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separate mirrors to find out more about

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these mirrors and how nasa is making

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sure they work as well in space as they

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do on the ground we've come to the

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marshall space flight center in

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huntsville alabama we're here with jeff

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cagley and he's the director of the

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x-ray and cryogenic facility here at

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marshall space flight center so jeff

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tell us a little bit about where we are

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and what are you about to see

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mary we are in the entry area for our

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large clean room

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these james webb space telescopes

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mirrors have to stay in a very clean

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environment and we have that type of

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environment here so we're going to see

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if we can get a better look at those

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mirrors and to do that we're going to

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have to

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get dressed up a little bit so that only

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we're just about to pull the entire test

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stand assembly into the vacuum chamber

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with the three james webb mirrors aboard

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i noticed there's just three mirrors why

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is that i think james webb has 18 right

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right the james webb telescope is made

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up of 18 mirrors we're going to have

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each one of those mirrors come through

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here twice

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the first time through we actually

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measure the deformation as a function of

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temperature as we transition down to

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minus 400 degrees fahrenheit

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the second time through we verify that

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the mirrors actually perform correctly

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at that temperature

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and you said it's about to go into the

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chamber can we go inside the chamber

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just to see what it's like and maybe

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tell us a little bit about the chamber

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yeah we'll try to run in there and see

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the mirrors behind us will be pulled in

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and we'll will actually reside just up

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in front of us here inside the chamber

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where they can be cooled to minus 400

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degrees why a vacuum chamber

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well we have to simulate not only the

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thermal environment but also the vacuum

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environment of space you pull these in

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and what kind of things are you looking

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for in the mirror well the the ball

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aerospace team is actually responsible

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for the testing and they will actually

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be looking at these

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mirrors as they transition to cryogenic

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temperature through a window up in the

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front of our vacuum chamber and they'll

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be looking at these mirrors with an

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instrument called an interferometer

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and that's going to give them a very

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detailed surface map

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of what that mirror looks like and how

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it deforms

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as a function of temperature and my last

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question is why is it so important to be

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so precise with these mirrors

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well the primary mirror for a telescope

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is the

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part of the telescope that really

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matters it's gathering all the light

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that ultimately gets focused on the

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instrument

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and the larger and the better the

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these mirrors will be in the vacuum

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chamber for about 15 weeks going through

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seven cycles of temperature changes from

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room temperature to the cold extreme of

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space