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At the Johnson Space
Center in Houston, Texas,

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you can walk along
a Saturn V rocket

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that was designed to help
humans get to the moon.

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But now this center
is helping NASA

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prepare for our
next great adventure

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in astronomy, a
telescope that will

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look way past the moon out
to the universe's farthest

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horizons.

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Here is the vacuum chamber that
was used to test spacecraft

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during the Apollo era.

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So Lee, I understand it's being
used for the James Webb Space

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Telescope too.

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Yes, this is where we're
going to be doing a full scale

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test of the James
Webb Space Telescope,

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including the telescope
and the instruments.

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And we're going to be
doing optical testing

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and thermal testing
of those components.

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We've seen tests before, but
this is the largest scale

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that we've done so
far on Webb, right?

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Yeah.

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In fact, it's probably
the largest optical test

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that NASA has ever done.

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And that's because Webb is
the largest Space Telescope

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we've ever built. So
we're going to be testing

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the full primary mirror which
is 6 and 1/2 meters in diameter,

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and we're going to be
testing the whole telescope

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and the instruments with it.

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OK.

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So before we get
in there, give us

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a preview of what's been
done to prepare for this test

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right now.

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Well, first of all, this is the
first test where we actually

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have flight hardware.

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It's the aft optics
system, which is

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part of the telescope itself.

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We put the aft optics system
onto the pathfinder telescope,

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which is a model
of the telescope

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and includes primary
mirror segments on it.

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We then put that
Pathfinder telescope

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with the aft optics system
onto a large structure

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and a rail system that allowed
us to put it in the chamber.

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So what's the real
purpose of the test?

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We're going to be cooling
the telescope down

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and we're going to be making
sure that all of the mirrors

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are aligned the way
they're supposed to be.

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And also that the system as a
telescope really works the way

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we expect at the very
cold temperatures

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that it'll be out in space.

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It looks like they're
all set up for the test,

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so let's get out
of the chamber now.

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Now they're going to be
closing the chamber door.

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Liquid nitrogen is used to cool
the inside of the vacuum shell

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to about 70 degrees
above absolute zero.

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But inside of that
is a helium shell.

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The helium shell
is what cools us

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to even colder temperatures
to the 30 to 50 degrees

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above absolute zero.

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We're now well into
the optical testing.

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So what do you mean
by optical testing?

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We actually put light through
the entire telescope chain,

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including the tertiary mirror
and the fine steering mirror.

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Do you want to see some
real images that's gone

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through the entire telescope?

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Oh, yeah.

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Let me show you that.

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Great.

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We actually simulate what
looks like little stars running

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through the entire
end to end telescope.

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And by looking at how these
stars go through focus,

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we can understand what
the telescope is doing.

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So that's how you test
the end-to-end telescope

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to the full light path.

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How about the
primary mirror alone?

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We use special
test equipment that

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lets us test the primary
mirror just by itself.

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And by doing that, we can
determine the alignment

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of each of the mirror
segments individually

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but also how they relate to
each other and to make sure

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that they're aligned properly.

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And we have the full telescope.

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We'll have 18 of these hexagons.

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Pathfinder only has
two mirror segments,

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but it's enough to
really let us practice

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all the things we
need to practice

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in terms of the testing
and the alignment.

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The images look like they
have fingerprints on it?

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Yeah.

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So those fingerprints
are really contour maps,

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like taking slices
through a mountain.

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As we tilt the mirrors
around, the contour maps

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tell us how much the
mirrors are tilted.

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And we use that information as
we align the primary mirror.

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What's the deal with this dark
line right here through one

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of the primary mirror segments?

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Yes, that's just a shadow
from the big strut that

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holds the secondary mirror.

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And the secondary
mirror is on a tripod.

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And so the light that
is hitting the mirrors

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is actually going and casting
a shadow from that strut.

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And what we're trying to
do is align the mirrors

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to create essentially
a single mirror.

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So we're finishing up the
test and what we're doing

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is reversing the process
of getting it in.

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We have to be very careful with
all the sensitive hardware,

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including flight hardware
that we're removing.

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Well, thanks, Lee for
your time and letting us

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see what the first large
scale test on James Webb

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has been like.

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You're very welcome.

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The aft optics system has been
removed from the test setup,

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and will be joining
other flight hardware

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for more testing at NASA's
Goddard Space Flight Center

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in Greenbelt, Maryland.

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Thanks for joining us for this
edition of Behind the Webb.

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