I am taking this month off, and while I'm away,
some other folks are taking over my channel.
We start this week with a couple of astronomy and physics majors who,
along with Professor Chris Impey and their team at the University of Arizona,
make a channel called Active Galactic Videos.
Now the video they've sent is a little rough around the edges in places
but the idea is so good that I am genuinely annoyed
that I couldn't go out and film this myself.
So Jenny, Melissa, over to you.
- Hi, I'm Jenny.
- And I'm Melissa, and we're here
underneath the University of Arizona's football stadium
in the Richard F. Caris Mirror Lab.
Behind me is the world's largest spinning furnace
filled with 38,000 pounds of molten glass.
- Over the next couple of months,
the glass will slowly cool and start to form a mirror,
one of seven that'll be part of the Giant Magellan Telescope.
Once completed, the GMT will be the largest telescope in the world.
The GMT is being built in Chile
at the Las Campanas Observatory in the Atacama Desert,
and it's scheduled to see first light in 2022.
This massive telescope will have ten times the resolution of the Hubble Space Telescope,
and more than seven times the light-gathering power
of the largest telescopes today.
The GMT is made of seven eight-and-a-half-meter mirrors
for a total diameter of 25 meters,
and it will be used to study galaxies, stars, and exoplanets.
- The GMT-5, which is the mirror being cast right now
is actually the 20th mirror made at this facility.
Other mirrors have been made for other telescopes
like the MMT, the Large Synoptic Survey Telescope,
the Twin Magellan telescopes, and the Large Binocular Telescope.
It was built under the stadium to take advantage of the large columns,
which act as load-bearing structures for all of the heavy equipment needed.
It's also nice and quiet when there aren't football games going on.
It's a great way to make football benefit both the university and science.
- To start off, we need 38,540 pounds of glass.
Sand is shipped from all over the world
and brought to Japan, to the Ohara glass factory.
They're the only factory in the world that can make the E6 low-expansion glass
that's needed for the GMT mirrors.
The glass is made in 2,000 pound batches
made exclusively for the University of Arizona.
It takes around two and a half years to make enough glass for a single mirror,
so, you know, plan ahead.
- The glass is then shipped here to Tucson.
Once the mirror lab gets it, they have to clean it, inspect it, and weigh it.
If there are any stresses or imperfections in the glass, then the piece is rejected
and made into paperweights.
- Each individual piece of glass is placed carefully into the mold and inside the oven.
The 40-foot diameter oven is then gradually heated
to a temperature of 1,350 degrees Fahrenheit
over the course of a week.
There are cameras in the oven to monitor the process
but once it begins, there's no turning back.
At this point, the massive oven is filled with 19 tonnes of glass
and starts to spin five revolutions per minute.
Once the oven reaches 2,129 degrees Fahrenheit,
or 1,165 degrees Celsius, the consistency of the glass
is about the same as honey and starts to flow into the mold.
- And it might make sense why we need an oven to melt the glass,
but you might be wondering, "why does it spin?"
- It spins to make the shape we need for a mirror,
a parabola.
Modern telescopes use parabolic mirrors to collect and concentrate light.
They use the parabolic shape to get the light into a central focus point,
and the bigger the mirror, the more light is collected.
- But what does have to do with spinning besides making us dizzy?
Well, let's take a look.
So it turns out that spinning a liquid actually causes it
to automatically become this parabolic shape.
So whenever you spin the glass,
it creates the perfect shape needed for a mirror.
This is a massive improvement over how mirrors used to be made
and it "grinds" years off the process.
The mirror spends three months cooling and annealing in the oven.
So now it's the right shape but it isn't quite done yet.
When light hits the mirror, the surface has to be smoother
than a fraction of the wavelength of light.
Otherwise the image is distorted.
In order to get it this smooth,
the mirror lab spends two years polishing the glass.
It has to be smooth to the point where no imperfection on the surface
is larger than a virus.
After all of that, after melting the glass in this giant oven, and polishing it for two years,
all you have is a huge, really smooth piece of glass.
- So the mirror lab actually doesn't make any mirrors.
- The final step in turning this giant piece of glass
into a mirror is actually done on-site at the observatory.
A layer of aluminum about 100 nanometers thick
is deposited onto the surface.
To cover a mirror of this size,
it actually only takes a soda can's worth of metal.
- Unlike your mirrors at home, the reflective coating
is only on the front surface, so it's very delicate
and needs to be re-aluminized every so many years.
- Thank you to everyone here at the Richard F. Caris Mirror Lab
and the Steward Observatory.
Tours are open to the public every day of the week.
- Go subscribe to Active Galactic Videos for more like that.
I would recommend starting with the video
where they get to walk on the dish of the telescope
and yes, I am extremely envious that they got to do that.
Next week, a language video that you are probably going to need subtitles for.
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