Behind the Webb: Webb’s First Eye Exam (Episode 30)

At the Johnson Space Center in Houston, Texas…
you can walk along a Saturn V rocket that was designed to help humans get to the moon.
But now, this center is helping NASA prepare for our next great adventure in astronomy:
a telescope what will look way past the moon out to the universe’s farthest horizons. Here is the vacuum chamber that was used to
test spacecraft during the Apollo era, so Lee, I understand it’s being used for the
James Webb Space Telescope, too? Lee Feinberg/JWST Optical Telescope Element
Manager: Yes, this is where we will be doing the full-scale test of the James Webb Space
Telescope, including the telescope and the instruments, and we’re going to be doing
optical testing and thermal testing of those components. We’ve seen tests before but this is like
the largest scale that we’ve done so far on Webb, right? Yeah, in fact, it’s probably the largest
optical test that NASA has ever done and that’s because Webb is the largest space telescope
we’ve ever built. So we’re going to be testing the full primary mirror, which is
6 ½ meters in diameter … And we’re going to be testing the whole telescope and the
instruments with it. So before we get in there, give us a preview
of what’s been done to prepare for this test right now. Well, first of all, this is the first test
where we actually have flight hardware. It’s the Aft Optics System which is part of the
telescope itself. We’ve put the Aft Optics System onto the Pathfinder telescope, which
is a model of the telescope and includes primary mirror segments on it. We then put that Pathfinder
telescope with the Aft Optics system onto a large structure and a rail system that allowed
us to put it in the chamber. So, what’s the real purpose of the test? We’re going to be cooling the telescope
down and we’re going to be making sure that all of the mirrors are aligned the way they’re
supposed to be and also that the system as a telescope really works the way we expect
at the very cold temperatures that it’ll be at in space. It looks like they’re all
set up for the tests, so let’s get out of the chamber now. Now they’re going to be closing the chamber
door. The liquid nitrogen is used to cool the inside of the vacuum shell to about 70
degrees above absolute zero. But inside of that is a helium shell. The helium shell is
what cools us to even colder temperatures to the 30 to 50 degrees above absolute zero. We’re now well into the optical testing. So what do you mean by optical testing? We actually put light through the entire telescope
chain, including the tertiary mirror and the fine steering mirror. Do you want to see some of the images from
light that’s going through the entire telescope? Oh yeah! Let me show you that. We actually simulate what looks like little
stars running through the entire end-to-end telescope and um, by looking at how these stars
go through focus, we can understand what the telescope is doing. So that’s how you test the end-to-end telescope
through the full light path. How about the primary mirror alone? We use special test equipment that lets us
test the primary mirror just by itself and by doing that, we can determine the alignment
of each of the mirror segments individually, but also how they relate to each other and
make sure that they are aligned properly. When we have the full telescope, we’ll have
18 of these hexagons. The pathfinder only has two mirror segments but it’s enough
to really let us practice all the things we need to practice in terms of the testing and
the alignment. The images look like they have fingerprints
on them? Yeah, those fingerprints are really just contour
maps … kind of like taking slices through a mountain. As we tilt the mirrors around,
the contour maps tell us how much the mirrors are tilted and we use that information as
we align the primary mirror. What’s the deal with this dark line right
here, through one of the primary mirror segments? Yes, so that is just a shadow from the big
strut that holds the secondary mirror. Remember the secondary mirror’s on a tripod and so
the light that is hitting the mirrors is actually going and it’s casting a shadow from that
strut. And what we’re trying to do is align the mirrors to create essentially a single
mirror. Ok, so we’re finishing up the test. And
what we’re doing is sort of reversing the process of getting it in. We have to be very
careful with all of the sensitive hardware, including flight hardware, that we’re removing. Well, thanks, Lee, for your time and letting
us see what the first large-scale test on James Webb has been like. You’re very welcome. The Aft Optics System has been removed from
the test setup and will be joining other flight hardware for more testing at NASA’s Goddard
Space Flight Center in Greenbelt, MD. Thanks for joining us for this edition of Behind
the Webb.


  1. In every video you seem like a 5 year old that cant keep their hands off things.When the first image comes back with a big greasy finger print on it, we know who it was..

  2. …Let me get this straight.After 20 years, and after 8 B….B…..Billion dollars spent, the JW Telescope failed all it's functional tests over at Goddard……One of those gold-plated mirrors just won't line up just right, and now NASA is fumbling how to fix it ; short of sending the thing back to incompetent Lockheed Martin who built it…..Maybe it's time to put an end to this zombie money suck program, and cut our losses…

  3. Could this test have been performed on Hubble prior to its launch? If so, would it have found the defects that caused Hubble's imaging problems when its mission began?

  4. Thanks for making America Great again! This should be what we're about; exploration and discovery. I've joke I would be happy to be a janitor down there in Houston, it seems more worthy then my job. Does a BSEET qualify me?? Ha! Getting tired of fixing Semiconductor Implant equipment. Toured and seen that vacuum chamber. So impressive in person. Beats our itty bitty ion beam path chambers. Looking forward to another great chapter.

  5. launch it already!! NASA delayed this since 2014. Next it will move the launch window to 2020. Are the stretching the time so that they can collect more work hours?

  6. we are getting one step closer to get rid of the Damn Religions..i hope all goes as planned, our key to explore the truth.

  7. How can James Webb Space Telescope once at its destination in space avoid being hit by the incoming dusts, asteroids, or comets?

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