There's an asterisk
in the title of this video. Of course there's an asterisk. There has to be.
And I'll be honest... I had a whole script
ready to go here as the sun sets over the
Atacama Desert in Chile. But, I've found so much
more than I expected over the last day and a half.
So many stories. I've been exhausted
both from the altitude – we're three kilometres
up here – and from dashing around
with handheld cameras trying to capture everything. So, what's going to
happen now is we're going to cut to
a version of me in a couple weeks' time
who is coherent and who's had time
to process all this. [Caption+ by JS*
caption.plus | @caption_plus] <i>Here's the story.</i> I got an invitation to visit
the Extremely Large Telescope. Scientists love giving
telescopes names like that. It's a telescope.
It's extremely large. The invite was from the UK Science and
Technology Facilities Council and the European
Southern Observatory. They arranged everything, <i>but they have
no editorial control</i> <i>over what I'm saying.</i> <i>And I paid for my own travel.</i> Those flights were expensive. So there'll be an advert
for NordVPN later on, because it was actually useful
to me while I was in Chile. Anyway, the
Atacama Desert is <i>the most desolate place
I've ever been to.</i> <i>Rolling hills
of stone and rock</i> <i>for hour after hour
of driving.</i> <i>I'm sure there's life
out there somewhere,</i> <i>but we didn't see any of it.</i> Eventually, we arrive at
the base camp at Paranal. <i>Let me give you the
lay of the land.</i> <i>At the bottom of the hill
is the Residence,</i> <i>where visiting scientists
and non-scientists stay.</i> <i>Not a hotel.
Very definitely not a hotel.</i> <i>You can't pay for a
room there if you try.</i> <i>It's more like university dorms,</i> <i>if university dorms ever
won architectural awards.</i> <i>That's at the
bottom of the hill,</i> <i>along with all the support
buildings and structures</i> <i>that are needed to
support life out there.</i> <i>Up at the top of the hill
is the Very Large Telescope,</i> <i>built about 20 years ago.</i> <i>And a few kilometres away,
a little dot</i> <i>on a flattened-off hill
on the horizon:</i> <i>that's the Extremely Large
Telescope, the new one.</i> <i>And that's likely going to be</i> the largest optical telescope
that will ever be built. <i>But to understand why that is,</i> <i>I need to show you around.</i> <i>Our first stop is at</i> the Very Large Telescope
– the VLT – up on the hill about 2km
from base camp. <i>In theory, you can walk it.</i> <i>There's a path called
the Star Track, but...</i> <i>I didn't feel like
getting altitude sickness.</i> <i>The VLT has been operational
for more than 20 years now.</i> <i>Results from it have
won Nobel Prizes.</i> <i>It was the first telescope
to take an actual picture</i> <i>of a planet around
another star.</i> It tested Einstein's
general relativity <i>by tracking a star around</i> <i>the supermassive black hole
at the centre of the galaxy.</i> <i>And it's actually
several telescopes.</i> <i>The four big ones are called
the Unit Telescopes, or UTs.</i> <i>And we were visiting UT4.</i> Are we good to
just go in? Okay. Oh, wow! <i>(laughs)</i> <font color="#FFFF00">So, at Paranal, we
have four UT telescopes.</font> <font color="#FFFF00">These are the large
telescopes we have here.</font> <font color="#FFFF00">Where the primary mirror,</font> <font color="#FFFF00">this thing that you see here,
underneath us here,</font> <font color="#FFFF00">is 8.2 metres in diameter.</font> And that is a mirror... very, very scientifically precise, but the same kind
of optical mirror that you would have just to
do your makeup in the morning or something like that. <font color="#FFFF00">Um, yes, yes.
<i>(laughs)</i></font> Lesson one:
Do not compare the extremely expensive
scientific instrument to a makeup mirror. The important thing is that the telescopes here
are optical telescopes. That's the first reason
for the asterisk. There are already
bigger radio telescopes. <i>I was lucky enough to visit
Arecibo before the collapse,</i> <i>and I recently visited
Parkes in Australia.</i> <i>They're all much bigger.</i> But they collect
radio waves, not light. They pick up very
different frequencies, and they're useful for
observing different objects and doing different science. There's stuff
you can do with radio that you can't do with
optical, and vice versa. Radio telescopes have
to be much larger because the wavelengths
of the microwaves and radio waves
they pick up range from millimetres
to tens of metres. But they don't have
to be as precise. As long as you build
a big bowl <i>that's the right shape,
you don't need to make</i> <i>a perfect optical mirror finish.</i> It will still work.
Arecibo's surface was just <i>kind of rough mesh,
but that still worked.</i> Optical astronomy looks for
visible light or infrared. The stuff we can see
with the naked eye, or close to that. The wavelengths
are from about 100 micrometres to
100 nanometres. <i>And so the optics have
to be so, so precise,</i> <i>just like any other camera lens.</i> <i>Well actually, more precise,</i> <i>because mirrors this big
have other problems.</i> <font color="#FFFF00">When you get to
this large telescope,</font> <font color="#FFFF00">it's actually extremely difficult</font> <font color="#FFFF00">to keep the mirror stable
and in the same position.</font> <font color="#FFFF00">It has 150 actuators
underneath it.</font> It's like poking the
mirror ever so slightly. That must be microns. That must be a tiny,
tiny amount it's moving. <font color="#FFFF00">Yeah, and this is basically</font> <font color="#FFFF00">to take into account
gravity of the mirror</font> <font color="#FFFF00">and the temperature difference
we have within here,</font> <font color="#FFFF00">because when
you move the mirror,</font> <font color="#FFFF00">then from gravity, then
the shape of the mirror</font> <font color="#FFFF00">is also changing as well.</font> <i>That's called
"active optics",</i> <i>deforming the mirror
ever so slightly</i> <i>to deal with the effects
of gravity as you tilt it,</i> <i>and from thermal expansion
as the temperature changes.</i> That's how accurate
this has to be. We'll come back to that later,
'cause I got to see the mirror and those actuators
close up, but first... We've got to explain what
that mirror actually does. They were getting it
ready for the night, so they had to test it. Because the whole thing moves. Oh, oh!
<font color="#FFFF00">We're moving.</font> That's way faster than
I thought it was gonna be. <font color="#FFFF00">We basically
test the rotation.</font> <font color="#FFFF00">We test all of the
movements of the telescopes.</font> <font color="#FFFF00">We check for the
safety of the system,</font> <font color="#FFFF00">before we're going to start
the nighttime operations.</font> <font color="#FFFF00">You can't hear the
movement, right?</font> This is so fast! Oh, we're going
back the other way. I'm just giggling. <font color="#FFFF00">This is several hundred
tons of material.</font> <font color="#FFFF00">It's moving like this,
and basically</font> <font color="#FFFF00">it's kind of floating
on top of</font> <font color="#FFFF00">this very small amount
of oil film</font> <font color="#FFFF00">that's underneath here.
Because during the night,</font> <font color="#FFFF00">when we're doing the
scientific observations,</font> <font color="#FFFF00">we may be staying on one target.</font> <font color="#FFFF00">So observing one galaxy
for several hours.</font> <font color="#FFFF00">Because the Earth is rotating,</font> <font color="#FFFF00">so therefore we need
to move the telescope</font> <font color="#FFFF00">so that we keep pointing at
exactly the same position.</font> <font color="#FFFF00">So this movement needs
to be extremely smooth,</font> <font color="#FFFF00">because we don't want to add</font> <font color="#FFFF00">any additional movement
onto the observations</font> <font color="#FFFF00">when we're observing
our science targets.</font> <i>Rotation test complete,
now the tilt.</i> <font color="#FFFF00">We're going to
open the dome...</font> <font color="#FFFF00">and we want to put the
mirror in such a way</font> <font color="#FFFF00">that if anything falls
down from the dome,</font> <font color="#FFFF00">that the probability of that</font> <font color="#FFFF00">actually falling on the
mirror is extremely low.</font> <font color="#FFFF00">In the middle of the desert,
constantly you're having dust</font> <font color="#FFFF00">that's accumulating
onto the mirror.</font> <font color="#FFFF00">I mean, large things
falling onto it</font> <font color="#FFFF00">is extremely rare.
<i>(chuckles)</i></font> I just looked over there.
Oh, wow! So the light would
come in from a star. <font color="#FFFF00">Yes.
</font><font color="#EEEEEE">Hit the main mirror,</font> which is the big
reflective thing there... back to M2 just here. <font color="#FFFF00">Exactly.</font> And then down
to M3 in the middle. <font color="#FFFF00">It can rotate to send it
to any two at the instrument,</font> <font color="#FFFF00">or it opens to send it to
the instrument that's below the M1 mirror.</font> Right. There's a lot of very bad
animations out there, which show
satellite transmissions and stuff like that beaming not into the
bowl of the dish, <i>but to the detector,
the thing that sticks out.</i> <i>And that is the wrong way round.
That's really important.</i> Light bounces off
M1, the bowl, <i>and gets focused
onto the focal point.</i> <i>And then here it gets
bounced down again,</i> <i>either to the
equipment at the sides</i> or down into the basement. Light's going to
come in and hit M1. <font color="#FFFF00">Yes.
</font><font color="#EEEEEE">But isn't M2</font> in the way of what
you're trying to observe? <font color="#FFFF00">No, it doesn't, because
you have parallel light beams</font> <font color="#FFFF00">that are arriving to the M1.</font> It's not going to come
across on camera, I don't think... But there is quite a little
white mark just there, and a lot of dust
on the mirror. How much is that going to
affect the observations? <font color="#FFFF00">For any given object
of a given brightness,</font> <font color="#FFFF00">as the mirror gets dirtier,</font> <font color="#FFFF00">then you'll have to
observe for longer</font> <font color="#FFFF00">to get the same amount of
light on your detector.</font> <font color="#FFFF00">We recoat the mirror once
every two or three years,</font> <font color="#FFFF00">maintain the reflectivity.</font> <i>They do use dry ice</i> to dust the mirrors
every few weeks, but that can't get everything
that builds up over time. Dust on the mirror
doesn't change the optics. It doesn't spoil the image. It just means that some light
isn't reflected at all. So over time, the image
gets less and less bright. I did get to see the
mirror recoating lab later. <i>They'd just started to clean</i> <i>one of the other
telescope mirrors,</i> <i>the one from UT1.</i> <i>And when I arrived
at the lab,</i> <i>the first thing I saw was
the structure they used</i> <i>to transport the whole
thing down the hill.</i> How on earth did
you get that thing all the way down
the hill from UT1? <font color="#00FFFF">First, we have to
disconnect the cell</font> <font color="#00FFFF">from the main structure
of the telescope.</font> <font color="#00FFFF">And then we put this structure</font> <font color="#00FFFF">on the yellow part
you can see,</font> <font color="#00FFFF">which is the carriage.</font> <font color="#00FFFF">And this carriage is put
on top of a truck.</font> <font color="#00FFFF">And we drive
three kilometres.</font> <font color="#00FFFF">This system is
over air cushions,</font> <font color="#00FFFF">and we can move with
the air cushions.</font> Wait, it's a hovercraft? <font color="#00FFFF">Yeah. Exactly.
</font><font color="#EEEEEE">It's basically a hovercraft!</font> <font color="#00FFFF">It is exactly
the same principle.</font> <font color="#00FFFF">Because the mirror is
not here right now,</font> <font color="#00FFFF">maybe we can go up to it.</font> Yeah, yeah. Let's go up.
<font color="#00FFFF">Do you want to go up?</font> Yeah, absolutely. <i>So what you can see
from up top</i> <i>is the telescope without
the mirror on it.</i> The mirror, I thought, was kind of a bowl shape. But this looks flat to me. <font color="#00FFFF">The sag is not so high.</font> <font color="#00FFFF">It's 30 cm.</font> <font color="#00FFFF">So it looks flat,
but it's a sphere.</font> Okay, it's just a
section of a very big sphere. <font color="#00FFFF">Exactly.
You can imagine that</font> <font color="#00FFFF">if we make the full
bowl out of this mirror,</font> <font color="#00FFFF">it will be a bowl
of 60 m diameter.</font> <i>So let's go see the
mirror being cleaned.</i> <i>They'd moved it into the
clean room next door.</i> <i>And to start with,
it was just being washed.</i> <i>Also, I realise I look stupid</i> <i>with the hoodie on
under the clean room gown.</i> <i>I mean, first,
it's not a medical</i> <i>or semiconductor-grade
clean room.</i> <i>They assured me it's fine.</i> <i>And second, the reason
I'm always in the hoodie</i> is that we were always
going out and in, so the temperature was
always switching from cold, air conditioned interior
to brutally hot sun that I needed to
cover myself from. <i>So it just seemed like the
best uniform for the job.</i> Oh, that is the mirror. <font color="#00FFFF">Yeah, so we have a
small window so you can...</font> Oh, I'm scared
to breathe now. <font color="#00FFFF">What you can see
is the washing unit.</font> <font color="#00FFFF">Basically, it's the
place where we do</font> <font color="#00FFFF">the stripping of the
coating and the washing</font> <font color="#00FFFF">before entering
the vacuum chamber.</font> <font color="#00FFFF">Right now, we are in the
first step of the process,</font> <font color="#00FFFF">which is a first cleaning
to remove all the dust,</font> <font color="#00FFFF">also all the stains
that was collected</font> <font color="#00FFFF">during two years by the mirror.</font> <font color="#00FFFF">And then, we can start to do</font> <font color="#00FFFF">the stripping or
etching of the coating,</font> <font color="#00FFFF">using this rotating arm
to pour the acid</font> <font color="#00FFFF">and to have the layer going out.</font> <i>So, wash the mirror
to remove the contaminants,</i> <i>then acid etch the
old coating away,</i> <i>and then...</i> <font color="#00FFFF">When we have
cleaned the mirror,</font> <font color="#00FFFF">and we will close
this big chamber,</font> <font color="#00FFFF">do the vacuum, and
deposit the aluminium.</font> <font color="#00FFFF">You have the
heart of the machine.</font> <font color="#00FFFF">This is what we
call the magnetron,</font> <font color="#00FFFF">and it's where we have
the target of aluminium.</font> <font color="#00FFFF">99.9% of aluminum, very pure.</font> That's way beyond
what you'd normally get for industrial use. <font color="#00FFFF">And by the way,
there are some shutters.</font> <font color="#00FFFF">It is closed, so we
cannot see the target.</font> <font color="#00FFFF">The rainbow colours are some
deposition during the process.</font> <font color="#00FFFF">During years you have
these thin layers,</font> <font color="#00FFFF">then it's nice.</font> How much is being added here? <font color="#00FFFF">The thickness of the coating
is around 0.1 micron,</font> <font color="#00FFFF">which is very, very small.</font> And a micron is
1/1,000th of a millimetre. <font color="#00FFFF">Exactly.</font> 1/10,000th of a millimetre. You can basically count
the number of atoms there. <font color="#00FFFF">It's a 1,000 atoms' layer.</font> <font color="#00FFFF">One thousand.
<i>(heavy exhale)</i></font> <font color="#00FFFF">The weight is around
seven grams.</font> Over the entire
eight metre mirror? <i>They atomise aluminium to
create the mirror layer.</i> <i>And when I say atomise,</i> I don't mean like
perfume sprayer atomiser. I mean literally,
they use plasma <i>to make individual
atoms of aluminium</i> <i>float around in a vacuum,
and then they just let it</i> <i>softly rain down
onto the mirror</i> <i>until they have a coating
about 1,000 atoms thick.</i> <i>Anyway, up at UT4,
the dome was opening.</i> So this is going to
point at the sky. <font color="#FFFF00">Yes.</font> That's obviously
a lot more light than it would normally get? <font color="#FFFF00">You have the shutter in place.</font> <font color="#FFFF00">So this, you know,
black curtain</font> <font color="#FFFF00">that you have here is there</font> <font color="#FFFF00">so no light's going
to get through there.</font> Oh, it's just a
physical blackout curtain. Okay, yeah.
I can see that just there. And I just noticed
the guide star lasers. I know they've been
there all the time. They've been really obvious. But now it's pointed this way, I can see one, two, three,
four giant laser emitters! Those are enormous! It's giant laser time! But we can only see
that after the sun sets. <i>That's not the ocean,
by the way.</i> <i>Those are clouds.
We're above the clouds.</i> <i>Anyway, sun sets,
stars come out.</i> <i>The Milky Way is just
stretched out above Paranal,</i> <i>and it is so beautiful!</i> <i>I've never seen
the stars this clearly!</i> <i>And they don't seem to twinkle,</i> <i>'cause there's less
atmosphere up here</i> <i>to cause that twinkling,
that distortion.</i> The atmosphere isn't
one consistent thing. There are different temperatures
and pressures of air, and the light gets refracted. It's not like wind
blowing balloons around. The photons of light
aren't being blown away. But it's the same reason that an object in a swimming pool
looks distorted when you look down at it
from above the surface. That's a difference
in the medium that the light is
travelling through. The different pockets of air that are constantly moving
about in the upper atmosphere cause the same sort of effect. <i>That's why the stars twinkle.</i> <i>The light is slightly
bending its path</i> <i>as the pockets of air
move around.</i> <i>That's the reason that
the telescope is built</i> <i>high up on a desert mountain.</i> <i>There is less atmosphere
to get in the way.</i> <i>But there is still
some twinkling,</i> even if I can't spot it myself. Enough that it'd still cause
problems with observations. <i>And that's what
the lasers are for.</i> <i>The lasers didn't look
quite that bright in person.</i> <i>Those are long exposure photos,</i> <i>but they looked cool enough.</i> <i>And down in the control room,</i> <i>the scientists there explained
what they used them for.</i> This is the laser control? <font color="#00FF00">In some way, it's
like a status monitor,</font> <font color="#00FF00">where we can see the status
of the laser beacons,</font> <font color="#00FF00">which are these four
images we see up here,</font> <font color="#00FF00">and also what commands</font> <font color="#00FF00">are being sent to
the deformable mirror</font> <font color="#00FF00">to correct the wavefront.</font> Four lasers beamed out, camera scanning exactly
what's happening to the beams in terms of just position? Is it just finding the
brightest dot in the sky? <font color="#00FF00">The lasers generate
a point source</font> <font color="#00FF00">that we can measure with
the wavefront sensor.</font> <font color="#00FF00">And the idea behind
the adaptive optics</font> <font color="#00FF00">is to make that point source</font> <font color="#00FF00">as tight and as
compact as possible.</font> <font color="#00FF00">So we generate</font> <font color="#00FF00">these artificial stars,
the laser beacons.</font> <font color="#00FF00">We measure them,</font> <font color="#00FF00">we apply correction to
the deformable mirror,</font> <font color="#00FF00">and that hopefully improves the
quality of the laser beacon,</font> <font color="#00FF00">and thus improving the
quality of the science image.</font> <font color="#00FF00">Every millisecond,
we're making</font> <font color="#00FF00">a measurement
of the wavefront,</font> <font color="#00FF00">and it's being
commanded to the DSM.</font> What's the lag on that? <font color="#00FF00">It's a few milliseconds.</font> <font color="#00FF00">But the DSM is wibbling
like this really quickly.</font> Yeah!
<font color="#00FF00">Every millisecond.</font> That's absolutely amazing. <font color="#00FF00">Yeah.</font> Why four lasers? <font color="#00FF00">So, multiple lasers allows us
to correct the wavefront</font> <font color="#00FF00">over a large area on the sky,</font> <font color="#00FF00">providing uniform
image quality</font> <font color="#00FF00">across the science observation.</font> <font color="#00FF00">If you only had one laser,</font> <font color="#00FF00">you would get good
correction in the middle.</font> <font color="#00FF00">But then it would degrade
as you go further out.</font> And I figure they're
at a specific frequency that you can then notch out in the rest of the observations?
<font color="#00FF00">Exactly, yes.</font> <font color="#00FF00">This is the sodium wavelength.</font> <font color="#00FF00">On the instrument,
we have a filter</font> <font color="#00FF00">to filter out that light.
Also on the telescopes.</font> Yeah.
<font color="#00FF00">On the guide star.</font> <i>So, in short,
they shoot the laser up,</i> <i>see how it wobbles,</i> <i>and then do maths</i> to subtract that wobble
from the observation. Look, I'm gonna say
"they do maths" at a few points
during this video, because it's the
sort of calculation that people spend
years learning about. Suffice it to say,
they do maths, work out how the laser
guide stars are moving, and then physically move
and distort the M2 mirror to subtract that
atmospheric distortion from the actual stuff
they're looking at. So, now you know <i>how a modern
optical telescope works,</i> <i>and how precise and
difficult the work is.</i> <i>That's the first thing
you need to know</i> to understand why the Extremely Large
Telescope, the new one, is probably the biggest
that will ever be built. <i>The next day,
we're able to go</i> down into the basement underneath the four telescopes. Because when I said
that the VLT was four Unit Telescopes... <i>Yes it is, but they can
all act as one,</i> <i>called the VLTI:</i> <i>Very Large Telescope
Interferometer.</i> There is a lot of competition for 'biggest telescope
in the world'. There's things like the
Event Horizon Telescope, which is a network
of radio telescopes all around the globe.
They all point at the same thing
at the same time, and because
they are so far apart on other sides of the planet, the astronomers can record
the data that's received, analyse it with signal
processing algorithms and supercomputers,
and do maths, and create a virtual telescope
that is the size of the planet. <i>That's called interferometry,</i> <i>because the maths is all about</i> <i>how those signals
interfere with each other.</i> <font color="#FF99FF">We basically
completely change</font> <font color="#FF99FF">the way that
we handle the data</font> <font color="#FF99FF">the minute we start
combining these beams.</font> <font color="#FF99FF">We're thinking about
interfering waves of light</font> <font color="#FF99FF">rather than an image.</font> <font color="#FF99FF">And the pattern we get from
the interference of that light</font> <font color="#FF99FF">is where we extract all of
the information that we want.</font> <i>Radio astronomy can do that
by recording the radio waves,</i> and then processing
it all afterwards. But remember how I said the wavelength
of radio is longer, and the telescopes don't
have to be so precise? Well, turns out
a lot of other things don't have to be so
precise for that either. <font color="#FF99FF">Atmospheric variations are</font> <font color="#FF99FF">a huge limitation
for us in the optical</font> <font color="#FF99FF">because, you know, the
atmosphere is full of water.</font> <font color="#FF99FF">It's full of things
that vary very quickly</font> <font color="#FF99FF">at optical and
infrared wavelengths.</font> <font color="#FF99FF">The atmosphere is not
varying so crazily</font> <font color="#FF99FF">at radio and
submillimetre wavelengths.</font> <font color="#FF99FF">So you have—
you're able to maintain</font> <font color="#FF99FF">all that phase information.</font> <font color="#FF99FF">It doesn't get
completely smeared out</font> <font color="#FF99FF">by the atmospheric variations.</font> <i>You know if there's a party</i> with a really loud
sound system a way away? Then you won't hear
the treble, you won't hear the
tss-tss-tss-tss. But you will hear the thump-thump-thump
of the bass? Radio waves, and... Apologies to physicists
watching this, but radio waves are the bass of the
electromagnetic spectrum. They travel just fine
through the atmosphere with minimal distortion. That's why nobody
bothers sending radio telescopes into space. We don't need to. <i>But at optical frequencies,
it's different.</i> <i>All the information that you
need to do interferometry,</i> <i>that information gets
smudged much more easily</i> <i>as it passes through
the atmosphere.</i> <i>And that's not even
the worst problem.</i> For light, there is not a computer in
the world fast enough to deal with
all that information. And there is not a clock in
the world precise enough. Even radio interferometry requires very precise
atomic clocks synchronised across the globe. But there just isn't
a clock in the world accurate enough to do that for the micrometre wavelengths
or nanometre wavelengths, and terahertz
frequencies of light. A radio wave... You can just record it. Even before digital
computers were a thing, you could just record
a radio wave onto magnetic tape.
Every single detail of it. Every frequency, every
little nuance, everything. Heck, a boombox from
the '80s can do that. When people used to tape
songs off the radio, that is a very rough
and imprecise version of radio astronomy. <i>But for light, there is
not a computer in the world</i> <i>fast enough
to record that data.</i> <i>Not with the accuracy
required for interferometry.</i> <i>So instead...</i> <i>they do it physically
in real time.</i> <i>Remember when I said</i> <i>that the light from
the Unit Telescopes</i> <i>could be sent off to the
equipment at the side,</i> <i>or down into the basement?</i> <i>Let's go see the basement.</i> So where are we headed now? <font color="#FFCC88">We are going inside
what we call the coudé room.</font> Coudé? <font color="#FFCC88">Yeah, this is
the room containing</font> <font color="#FFCC88">the adaptive optics system
for the interferometer.</font> And coudé means... <font color="#FFCC88">Elbow.
</font><font color="#EEEEEE">Elbow, in French.</font> <font color="#FFCC88">In French.
</font><font color="#EEEEEE">Okay. <i>(laughs)</i></font> So I'm guessing
from the name that this is a movable joint? A hinge that can change
the direction of the light? <font color="#FFCC88">Yes. Then this,
the coudé,</font> <font color="#FFCC88">for us, the coudé path,</font> <font color="#FFCC88">is what we call
the nine mirrors.</font> That's a serious
warning sign, that is. <i>(chuckles nervously)</i>
<font color="#FFCC88">Yeah.</font> Oh! <font color="#FFCC88">Okay, we are
under the telescope,</font> <font color="#FFCC88">and we are under what
we call the coudé path.</font> <font color="#FFCC88">And this is really where
we receive the light,</font> <font color="#FFCC88">which will be,
let's call it organised,</font> <font color="#FFCC88">in order to bring it
towards the VLTI.</font> <font color="#FF99FF">The light comes into
each of the telescopes,</font> <font color="#FF99FF">it gets bounced around the
mirrors of the delay tunnel,</font> <font color="#FF99FF">gets funnelled into
the VLTI lab,</font> <font color="#FF99FF">gets funnelled
into the instrument,</font> <font color="#FF99FF">and that's where
it gets combined.</font> <i>So the light beam
isn't recorded anywhere.</i> <i>It's just bounced into tiny
tunnels from each telescope.</i> <i>And its next stop is
a 150 metre long</i> <i>human accessible tunnel</i> to synchronise
all the beams. <i>The delay lines.</i> So these tunnels
run all the way from all the telescopes
to a central point? Oh?
<font color="#FFFF00">Sorry.</font> <font color="#FFFF00">Cannot cross to
this side of the room.</font> Okay, my entire
body needs to stay... <font color="#FFFF00">This is the boundary.</font> Alright, thank you.
<i>(chuckles)</i> Sorry. Okay, so I'm not going to
go near their equipment if I'm told not to.
But in my head, I'm like, that's a little strange. <i>That looks like fairly
heavy infrastructure.</i> <i>Surely I can't do any damage
if I accidentally just...</i> bump up against it? Well, the light arrives
in the delay lines <i>in that tunnel from the side.</i> <i>It hits a series of mirrors</i> <i>designed to synchronise
all the beams,</i> <i>and each beam is
bounced up the tunnel</i> <i>to one of the carriages
and reflected back.</i> So how precise does
that have to be? <font color="#88BBFF">I can show you.
</font><font color="#EEEEEE">Okay.</font> <font color="#88BBFF">We use the delay lines
for observation,</font> <font color="#88BBFF">and you can see here
the error.</font> 136 nanometres. I assumed that the carriage
would be accurate to maybe a millimetre? And then there'd be some
electronics and mirrors in there that would do the
nanometre correction. But is that carriage
accurate to 100 nanometres
in its position? <font color="#88BBFF">The error that
I showed you there</font> <font color="#88BBFF">is the precision that
the carriage has.</font> I understand why
I got told not to even get close to that line.
That's incredible! The carriages
can be moved with 100 nanometres of precision.
100 nanometres! <i>That's why they didn't
want me near them.</i> <i>They'll have to recalibrate
each night anyway, of course,</i> <i>and it'll get cleaned
from time to time,</i> <i>but they don't want some idiot</i> <i>potentially bumping into things</i> <i>that have to be 100
nanometres accurate.</i> <i>Because that's how
they delay the light</i> <i>and do optical interferometry.</i> <i>That's how everything
is synchronised.</i> <i>No recording, no playback,</i> <i>no computers analysing things.</i> <i>It's analogue. It's physical.</i> <i>Explaining what happens in
the science part of that,</i> <i>inside the freezing
cold vacuum of...</i> <i>science stuff,
in the rest of the lab,</i> <i>is way beyond me.</i> <i>That's the stuff astronomy
PhDs are made of.</i> <i>And it's still a lot of work</i> <i>to record the
interference patterns</i> <i>and work back from that.</i> <i>But, if all those beams
are synchronised enough...</i> it means they can
approximate a telescope the size of the
entire VLT platform, <i>all four Unit Telescopes,</i> <i>instead of just the
individual telescope mirrors.</i> <i>But all those
telescopes, for optical,</i> have to be
physically connected. So, we've covered
interferometry. We've covered the
mirrors and resurfacing. And we've covered
the sheer scale of these telescopes. You now know everything
you need to know to work out why the ELT – the Extremely Large
Telescope – will probably be the
largest optical telescope <i>ever constructed.
And that ELT...</i> <i>is our final stop.</i> If you're wondering
why I'm rocking the extremely fashionable
combination of awful raggedy old baseball cap that I found in my
checked baggage, and hoodie up... The altitude is about
three kilometres here. The UV index is just "yes" 'cause that's one of the
reasons they built here. We are as close to the
top of the atmosphere as you can reasonably get. <font color="#00FFFF">Let's go, guys.</font> Oh, we've actually got named
safety gear in our size. Okay, there we go. Required protective
equipment up here: Helmet, gloves, high vis, steel toe cap boots,
and factor 50 sunblock. <i>The construction site
is 20 kilometres away.</i> <i>Down the hill that the VLT
is on, up the next one.</i> <i>It is colossal, and it looks
so much closer than it is.</i> <i>My brain refused to admit
that a telescope dome</i> <i>could be that big.</i> <i>The construction
is 74 metres high,</i> <i>86 metres in diameter.</i> And that... That is an extremely
large telescope. <font color="#00FF00">The main mirror, the M1,</font> <font color="#00FF00">39.2 metres in diameter,</font> <font color="#00FF00">comprised of
798 hexagonal panels.</font> <font color="#00FF00">We will make the biggest
optical mirror ever built.</font> That is... colossal! <font color="#00FF00">We are expecting images</font> <font color="#00FF00">five times sharper than
the James Webb Telescope</font> <font color="#00FF00">that is in outer space.</font> Sorry, five times sharper
than the James Webb? The one that's out there <font color="#00FF00">Yeah.
</font><font color="#EEEEEE">beyond the atmosphere?</font> <font color="#00FF00">Yeah, exactly.</font> <i>The first reason
that there's likely</i> <i>to be nothing bigger
than this is sheer cost.</i> <font color="#00FF00">The cost line does not
follow a linear relationship,</font> <font color="#00FF00">but it goes exponential.</font> <font color="#00FF00">So maybe you will add,
you say,</font> <font color="#00FF00">a couple of metres more,
and the costs double.</font> <i>There have been plans</i> for bigger telescopes. There was something
called the Overwhelmingly Large
Telescope, the OWLT, that was briefly planned. It could happen. But it'd cost
tens of billions of euro, and frankly, the
technology's unproven. Second reason:
We're reaching <i>the limits of what is
physically possible</i> <i>with current
construction technology.</i> <i>They're engineering
something</i> <i>that is part telescope,
part skyscraper.</i> So it's lots of different
sections all working together? <font color="#00FF00">Yeah, all the sections
are working together,</font> <font color="#00FF00">and its geometry
will be readjusted</font> <font color="#00FF00">through three actuators
per segment,</font> <font color="#00FF00">for a total number
of 2,394 actuators.</font> <i>The eight metre
mirrors in the VLT,</i> <i>they sag under
their own weight.</i> <i>And granted, the ELT is
using mirror segments,</i> <i>not a single monolithic mirror,
but keeping them</i> all aligned isn't easy,
and the entire structure <i>will still stretch and warp
as it's moved around.</i> <i>And with thermal expansion
as the temperature changes.</i> <font color="#00FF00">This wall is basically
the upper foundation,</font> <font color="#00FF00">where on top of it,
the telescope will be mounted.</font> Right. <font color="#00FF00">So this is
fixed to the ground,</font> <font color="#00FF00">and on top of it, there is
a hydraulic bearing system</font> <font color="#00FF00">that will be moving the telescope.</font> How precise does
that have to be, for those movements? <font color="#00FF00">We are talking
about 100/1,000,</font> <font color="#00FF00">depending on the observation,
of arc seconds.</font> <i>Okay, so what does
a thousandth of an</i> <i>arc second
of precision mean?</i> <i>I did some maths,
and it means that</i> <i>this colossal
skyscraper telescope</i> <i>will move so smoothly
and so precisely</i> that if you told it to look at
and track a spot on the moon, it would be accurate
to within two metres! The actual imaging resolution's
not quite that small. It could image buildings
about 10 metres across at that distance,
but again... That's a distance that can be
measured in light seconds! <i>We are at the limits of
what it's possible to do</i> <i>with current technology.</i> <i>And the last reason</i> <i>– and I think the
most important one –</i> <i>is that at some point,</i> <i>we're going to solve
optical interferometry.</i> <i>People are working on it.</i> <i>Right now, it still
has to be physical.</i> <i>Computers aren't fast enough.
Clocks aren't precise enough.</i> <i>But they're only
going to get better.</i> <i>At some point in the future,</i> <i>we are probably going
to figure that out,</i> and we'll be able to do
optical interferometry the same way that
we can do radio. <i>And as soon as that happens,</i> <i>the Extremely Large
Telescope will be...</i> not obsolete. It'll still be useful for
some types of observations, because there are
advantages to the size. <i>Big telescopes let you
detect very small things</i> <i>and very faint things.</i> <i>Interferometry gets you
the small stuff,</i> <i>but not the faint stuff,</i> <i>because there's
less mirror surface</i> <i>for the light to bounce off.</i> But odds are, at some point, there'll be much cheaper
and easier virtual ways of making
really big telescopes. The ELT is still going to do
a hell of a lot of science before that can happen,
if it ever does happen. <i>But once that's done,</i> <i>maybe in a few decades,
maybe a century,</i> <i>the case for building
something this big</i> kind of goes away,
if instead, you can build a few
small telescopes and piece them all together. <i>So this, the Extremely
Large Telescope,</i> <i>will probably be the last of
the great optical telescopes.</i> I could be wrong.
Who knows? Maybe in a century's time,
this will seem ridiculous, and there'll be a
100 metre mirror sitting high on a plateau
somewhere in South America. But if I had to place a bet, I think there's a
very strong chance the ELT will be the
largest optical telescope to ever be built. And, as I said earlier, NordVPN was extremely
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