While the James Webb Space Telescope is still capturing headlines in media for being the
most exciting future telescope project, there’s another ground-based telescope that hasn’t
had as much attention that’s under construction right now, and it will make the JWST look
like a minnow in comparison. Introducing ESO’s Extremely Large Telescope, or the ELT. So,
what is it? What does it hope to achieve? And how does it compare to the many other
telescope missions out there? I’m Alex McColgan, and you’re watching Astrum, and together
we will explore the ELT, and see why it may be the most exciting telescope project of
the decade. The ELT is a project of ESO, or the European Southern Observatory. I must
say, ESO is an amazing organisation, and it is a prolific gatherer of data, due to its
many telescopes and instruments. Although it is a European project, the observatories
themselves are based in the Atacama Desert in Chile. This region was picked due to the
elevation of the sites, meaning there is less atmosphere between the telescope and space,
and also because some sections of this desert haven’t seen rainfall since records began.
As the sky is generally clear, it makes this region perfect for astronomical observations.
While all the individual telescopes of ESO are too numerous to mention, I’ll mention
a couple of notable projects. One is ALMA, which is the best observatory for looking
at the cold universe, like molecular clouds and dust. It consists of 66 antennas, all
working in tandem together. Another is the VLT, or the Very Large Telescope, which unsurprisingly
will end up being the ELT’s predecessor. It comprises of four 8.2 metre telescopes,
and four additional 1.8 metre auxiliary telescopes. They can all work in tandem or separately,
depending on the requirements. When they work together, they can achieve an exceptionally
high angular resolution, or in other words, they can see details 25 times fainter compared
to operating separately. These telescopes are currently the flagships of ESO, and they
are the most advanced optical instruments in the world. Now, I show you these other
projects, because to truly appreciate how ambitious the ELT is, you need to compare
it to the next best projects we have at the moment. Let’s start with its size. Remembering
that the VLT has 8.2-metre-wide mirrors, the ELT’s primary mirror is going to be an incredible
39.3 metres across, with a surface area of 978m². A telescope of this size means the
main mirror is going to be segmented, with 798 segments in all. With the main mirror
being segmented, it means individual segments can be replaced and cleaned. Together, the
ELT’s main mirror will collect 100 million times more light than the human eye. And around
26 times more than a VLT unit telescope. Due to the size of the main mirror, several other
mirrors are necessary to condense all that light into the science instrument. That means
there will be a secondary, tertiary, fourth and fifth mirror before the light even hits
the instrument. These smaller mirrors will also be special. The secondary mirror will
be the largest convex mirror ever produced. The fourth mirror will be the largest adaptive
mirror ever produced, with 8000 actuators, so that the surface can be adjusted at very
high time frequencies. It will be adjustable to counter the distortions in the image caused
by the Earth’s atmosphere, and also from wind vibrating the main mirror, all in real
time. Now, unsurprisingly, with all the glass and the need for sturdy supports, this structure
is going to be really heavy, 3700 tonnes, meaning it is going to need a behemoth of
a dome to support it and rotate it. Combine this with the fact that the site is based
in a desert that is prone to earthquakes, and these required specifications make the
dome almost as ambitious as the telescope itself. Once completed, it will end up being
80 metres high and 88 metres in diameter. To give you some perspective, here it is compared
to the Colosseum in Rome, or Manchester City’s Etihad Stadium. To protect the mirrors during
the day, these doors can close, and there are wind shields which reduce the amount of
wind and sand reaching the mirrors. The whole design was tested in a wind tunnel to ensure
as much wind as possible is directed around the structure. While a lot has been done in
the design to reduce the weight as much as possible, the total weight of the dome and
telescope still comes in at 6100 tonnes. The dome itself is also protected against earthquakes,
as it is built on top of massive shock absorbers, which will protect the sensitive equipment
from the regular, strong earthquakes Chile experiences. Now, if you have ever tried photographing
the night sky before, you will know that unless you have an automatic tracker, you are going
to get some blurry images if you do a long exposure, and the ELT is no different. One
of the big design challenges of the ELT is not only being able to rotate the telescope
around, but also up and down, with as few vibrations as possible, to allow it to get
as long of an exposure as it needs. To achieve this, the moving part of the dome will rest
on 36 stationary trolleys with wheels on the top. The upper dome will have a track where
the wheels fit into. These trolleys and wheels have been specially designed to produce the
minimum of vibrations possible. Next is the alt-az mount, the machinery that controls
the altitude, or the up and down, and the azimuth, or the rotation of the telescope.
Keeping 3700 tonnes steady and without buckling means that this has to be an impressive mount
indeed. The whole lower pier rests on three circular tracks of hydrostatic oil bearings,
so accurately constructed that their positioning will be correct aligned to a precision of
just a few tenths of a millimetre over their full diameter. The altitude control section
will rest on top of the pier, and is the structure holding the mirror itself. There are two semi-circular
plates on either side of this structure, resting on top of a cradle of hydrostatic pads. Hydrostatic
bearings and pads basically mean that the structure is resting on a thick oil layer,
which produces very low friction and also can provide a vibration absorbing effect,
helpful to further reduce vibrations caused by the equipment and earthquakes. The design
of this telescope gets even more impressive when you see how they deal with the distortions
caused by the earth’s atmosphere, and also the variations caused by wind and temperature
changes. The design problem with ground-based telescopes is that you have an atmosphere
above you that moves and wobbles incoming light photons, so to speak. This puts a hard
limit on how much detail you can see in space. Not to mention the vibrations on the mirrors
caused by the slightest wind, or the tiny expansion or contraction of the structure
caused by temperature changes, meaning the segmented mirrors go out of alignment ever
so slightly. As I mentioned, engineers have tried to reduce these problems as much as
possible. By building it in the Andes, at an altitude of 3000m, there is less atmosphere
between the telescope and space, compared to sea level. Also, with the wind breaker
and powerful ventilation and air conditioning, wind and temperature changes should be kept
to a minimum. But this isn’t enough to get the quality of images ESO are hoping for.
This hasn’t deterred the brightest minds in astronomy and engineering, as they have
come up with a process called adaptive optics. As I mentioned, the fourth mirror on the telescope
will be a hugely impressive adaptive mirror. A computer will keep track of turbulence in
the atmosphere by observing the movement of guide stars, or even artificial stars created
by these powerful lasers, and send corrections to the fourth mirror’s 8000 actuators to
distort the mirror so that light reflecting off it hits the fifth mirror almost as if
the atmosphere was not there. If you look up at stars and notice that they twinkle,
this is the atmosphere causing the starlight to wobble. Twinkling happens rapidly, and
the distorted mirror has to keep up with those changes in real time. The computer will also
keep track of the primary mirror segments, to make sure they stay aligned too. Adaptive
optics will mean images taken by the ELT can be sharper than space telescopes, due to its
much bigger mirror. While adaptive optics don’t completely eliminate atmospheric distortions,
the fact its mirror is so big means it will easily keep up with or even exceed the James
Webb Space Telescope in some observations. So, where does this light end up? Well, into
a variety of instruments, all designed to do something slightly different. Some of the
highlights include three powerful spectrographs, and two cameras that will put the one on the
James Webb Space Telescope to shame. Here you can see an expected spatial resolution
difference between the Hubble Space Telescope, the James Webb Space Telescope, and the ELT.
These cameras will work effectively from the visible light spectrum to the mid-infrared.
Both cameras will be housed in cryostats to keep them cool, so the infrared light won’t
be affected by the heat coming off the camera itself. Because James Webb will be in space,
it can better detect the dimmest of objects emitting infrared light, like galaxies billions
of light years away, however the ELT will still be useful for looking at nearer objects,
especially objects obscured by dust. Cool dust becomes more or less invisible in infrared
frequencies of light, meaning the ELT will have the best capabilities for seeing through
these shrouds in our galaxy. And what science do they hope to collect with these instruments?
The design and technology of it are impressive enough, but what will it see? Well, this is
the most exciting aspect of the ELT. One of the primary missions is to use the cameras
and spectrographs to examine known exoplanets more closely. One camera will come equipped
with a coronagraph, meaning it will be able to block light coming from a target star,
so that it can better view dim, orbiting exoplanets without the starlight washing out the image.
Don’t expect images like we have of planets in our own solar system, however, simply confirming
their existence will prove our exoplanet hunting methods are effective. Not only that, but
the spectrographs will analyse the light coming from these exoplanets to determine their composition.
Scientists will especially look out for clues that these planets can sustain life. The obvious
substance to look out for is water, but there are plenty of other markers too if that life
were to be similar to ours, like oxygen, methane, nitrous oxide, and chloromethane. Taking of
planets, ELT will also examine planets in our own solar system. Half the planets in
our solar system do not have their own dedicated mission, so the ELT will be the next best
observatory we have to keep track of them. Scientists will also observe objects normally
hidden by dust, like protostars and old stars, to give us a better understanding of how stars
form and their evolution over time. And talking of stars, it will also peer past the dust
obscuring the most fascinating part of our galaxy, the region directly around the supermassive
black hole in the galaxy’s centre. We know of 40 stars that orbit the supermassive black
hole right now, but we expect to be able to detect a lot more with the increased spatial
resolution provided by the ELT. This is a really interesting region of space for astronomers,
as the supermassive black hole warps spacetime dramatically there, and there is still a lot
we have yet to understand about an environment like this one. The ELT will also look towards
to centres of nearby galaxies to see how they compare. The last thing it will investigate
further, and I know some people hate to hear it, is the nature of dark matter and dark
energy. Under our current understanding of physics, there is a force countering gravity
and pushing everything apart, currently known as dark energy. At small distances, gravity
overcomes this expansion of the universe, but over millions of light years, the universe
is expanding rapidly. I’ve done a video about it here if you want to know more. Additionally,
galaxies have more gravity than they should, meaning they rotate differently than we would
expect. This extra gravity is blamed on dark matter. Again, check out this video I made
about that here. To quote ESO, what dark matter and dark energy suggests “is that our theories
of cosmology and particle physics are incomplete (or possibly incorrect), and that new physics
is out there, waiting to be discovered. A key task for the next generation of astrophysical
facilities is to search for, identify and ultimately characterise this new physics.”
With the ability to better examine the haloes surrounding galaxies to characterise dark
matter, and, in tandem with the James Webb Space Telescope, see the universe expand in
real time, which is something that hasn’t been able to be done before, the ELT will
“provide unique contributions towards deciding whether general relativity or a modified theory
best describes the expansion of the Universe, including testing the behaviour of gravity
in unexplored regions, as well as mapping the expansion history of the universe”.
So, data from this telescope could provide us with a new understanding of gravity and
physics as a whole. Alright! That just about wraps everything up here. Sadly, we are still
going to have to wait four years for this project to finish construction, but if you
are already excited for the James Webb Space Telescope, just wait until this is finished.
Although, with any luck, the James Webb Space Telescope will hopefully launch this year,
everything will deploy smoothly, and it will soon be sending us fantastic data that we
can enjoy in the meantime. So anyway, there we have it, almost everything you could want
to know about the most exciting ground-based telescope of perhaps the decade, the Extremely
Large Telescope. Thanks for watching! I hope I earned your subscription today. Want to
learn about another really exciting observatory, due to be completed and operational soon?
Check out my video here about the Vera C. Rubin observatory. Thanks to my members and
patrons who support the channel too. If that would be something that interests you, check
out the links in the description. All the best, and see you next time.