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Language: en

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I am taking this month off,
and while I'm away,

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some other folks are
taking over my channel.

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We start this week with a couple of
astronomy and physics majors who,

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along with Professor Chris Impey
and their team at the University of Arizona,

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make a channel called
Active Galactic Videos.

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Now the video they've sent is
a little rough around the edges in places

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but the idea is so good
that I am genuinely annoyed

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that I couldn't go out
and film this myself.

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So Jenny, Melissa, over to you.

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- Hi, I'm Jenny.

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- And I'm Melissa, and we're here

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underneath the University of
Arizona's football stadium

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in the Richard F. Caris Mirror Lab.

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Behind me is the world's
largest spinning furnace

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filled with 38,000 pounds of molten glass.

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- Over the next couple of months,

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the glass will slowly cool
and start to form a mirror,

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one of seven that'll be part of
the Giant Magellan Telescope.

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Once completed, the GMT will
be the largest telescope in the world.

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The GMT is being built in Chile

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at the Las Campanas Observatory
in the Atacama Desert,

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and it's scheduled to
see first light in 2022.

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This massive telescope will have ten times
the resolution of the Hubble Space Telescope,

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and more than seven times
the light-gathering power

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of the largest telescopes today.

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The GMT is made of seven
eight-and-a-half-meter mirrors

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for a total diameter of 25 meters,

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and it will be used to study
galaxies, stars, and exoplanets.

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- The GMT-5, which is the
mirror being cast right now

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is actually the 20th mirror
made at this facility.

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Other mirrors have been
made for other telescopes

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like the MMT, the Large
Synoptic Survey Telescope,

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the Twin Magellan telescopes,
and the Large Binocular Telescope.

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It was built under the stadium
to take advantage of the large columns,

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which act as load-bearing structures
for all of the heavy equipment needed.

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It's also nice and quiet when
there aren't football games going on.

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It's a great way to make football
benefit both the university and science.

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- To start off, we need
38,540 pounds of glass.

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Sand is shipped from all over the world

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and brought to Japan, to
the Ohara glass factory.

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They're the only factory in the world 
that can make the E6 low-expansion glass

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that's needed for the GMT mirrors.

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The glass is made in 2,000 pound batches

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made exclusively for the
University of Arizona.

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It takes around two and a half years
to make enough glass for a single mirror,

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so, you know, plan ahead.

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- The glass is then
shipped here to Tucson.

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Once the mirror lab gets it,
they have to
clean it, inspect it, and weigh it.

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If there are any stresses or imperfections
in the glass, then the piece is rejected

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and made into paperweights.

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- Each individual piece of glass is placed
carefully into the mold and inside the oven.

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The 40-foot diameter oven
is then gradually heated

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to a temperature of
1,350 degrees Fahrenheit

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over the course of a week.

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There are cameras in the
oven to monitor the process

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but once it begins,
there's no turning back.

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At this point, the massive oven
is filled with 19 tonnes of glass

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and starts to spin five
revolutions per minute.

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Once the oven reaches
2,129 degrees Fahrenheit,

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or 1,165 degrees Celsius,
the consistency of the glass

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is about the same as honey and
starts to flow into the mold.

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- And it might make sense
why we need an oven to melt the glass,

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but you might be wondering, 
"why does it spin?"

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- It spins to make the shape
we need for a mirror,

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a parabola.

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Modern telescopes use parabolic mirrors
to collect and concentrate light.

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They use the parabolic shape to
get the light into a central focus point,

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and the bigger the mirror,
the more light is collected.

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- But what does have to do with spinning
besides making us dizzy?

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Well, let's take a look.

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So it turns out that spinning
a liquid actually causes it

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to automatically become
this parabolic shape.

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So whenever you spin the glass,

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it creates the perfect
shape needed for a mirror.

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This is a massive improvement
over how mirrors used to be made

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and it "grinds" years off the process.

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The mirror spends three months
cooling and annealing in the oven.

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So now it's the right shape
but it isn't quite done yet.

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When light hits the mirror,
the surface has to be smoother

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than a fraction of the
wavelength of light.

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Otherwise the image is distorted.

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In order to get it this smooth,

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the mirror lab spends two
years polishing the glass.

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It has to be smooth to the point
where no imperfection on the surface

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is larger than a virus.

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After all of that, after melting the glass
in this giant oven, and polishing it for two years,

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all you have is a huge,
really smooth piece of glass.

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- So the mirror lab actually
doesn't make any mirrors.

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- The final step in turning
this giant piece of glass

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into a mirror is actually done
on-site at the observatory.

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A layer of aluminum
about 100 nanometers thick

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is deposited onto the surface.

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To cover a mirror of this size,

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it actually only takes a
soda can's worth of metal.

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- Unlike your mirrors at
home, the reflective coating

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is only on the front surface,
so it's very delicate

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and needs to be re-aluminized
every so many years.

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- Thank you to everyone here
at the Richard F. Caris Mirror Lab

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and the Steward Observatory.

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Tours are open to the public
every day of the week.

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- Go subscribe to Active Galactic
Videos for more like that.

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I would recommend starting with the video

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where they get to walk on
the dish of the telescope

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and yes, I am extremely envious
that they got to do that.

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Next week, a language video that you
are probably going to need subtitles for.