The Very Large Telescope is a powerhouse. Although you might be forgiven for looking
forward to the completion of the Extremely Large Telescope in 2025,
few telescopes today can compete with the sheer productive output of its predecessor, the VLT.
The number of scientific papers this telescope contributes to averages out at more than
one a day – nearly 600 in 2019, dwarfing the competition from all other land-based telescopes.
And even once newer, larger telescopes with their 30m+ mirrors start to come online, the VLT will
still have its niche. Thanks to a little trick up its sleeve, its 8 telescopes can combine into a
virtual mirror with an effective diameter of 200m, giving it punching power far above its weight.
So, what is the Very Large Telescope? What contributions has it made to science?
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we investigate how this
incredible piece of engineering works, and see its discoveries behind black holes,
neutron stars, and even how it helped prove general relativity.
The Very Large Telescope finished construction in the year 2000, although that sentence is a
little misleading on two different levels. The first is that the VLT isn’t just a single mirror
telescope . A misleading name then? Maybe. Let’s see why it’s still only considered one telescope.
Initial designs had it as a limited array of 4 behemoth telescopes, 25m tall each, with mirrors
stretching 8.2m across. These were called Unit Telescopes, and they could work independently
at studying the stars. Each one is capable of an angular resolution of about 50 milliarcseconds,
which is good enough to see details smaller than a DVD on the International Space Station.
The second level is that the VLT is constantly evolving. In some sense, it is not finished even
now. In 2004 and 2007, a total of 4 new Auxiliary Telescopes were added to the VLT’s array.
Each of these Auxiliaries had a smaller mirror, only 1.8m across. Additional instruments have been
routinely added over the years, some as recently as 2018. There are 16 currently in operation,
ranging from spectrometers to infra-red imagers, with more to come in the future, such as 4MOST,
which is an upgrade to the VISTA telescope instrument that is also part of the VLT site.
Maintaining such a massive array can be difficult. VLT mirrors are exposed to the open air each
night. The Atacama Desert in Northern Chile where it’s situated may be ideally suited
for observatories – it hasn’t rained there since records began – but dust can sometimes blow in.
Every 18 months, the mirrors must be delicately washed with chemicals designed to remove their
aluminium coating, and then a new one is applied. This process does not need very
much aluminium – only about 12g is used to coat an entire 8m mirror; less than the amount used
in a drinks can. This is because the aluminium is coated on the mirror about 80 nanometres thick.
Each Unit Telescope is a pristine feat of engineering. The 430-ton structures are each about
the weight of a jumbo jet, and yet they’re so perfectly balanced on their hydrostatic oil-film
bearings that they can be rotated by hand. The mirrors are so precise, if you scaled them up to
the size of the whole Earth, the imperfections on them would be smaller than a pebble.
But the truly impressive feats are still to come. As powerful as each Unit Telescope is on its own,
it is when they work together that they can act as “one telescope”.
Let’s talk about the telescope’s technique knowns as Interferometry.
Light travels in waves, and when you combine two waves just right, their peaks can amplify
each other. Underneath the VLT there are a series of tunnels lined with mirrors in
complex configurations. Light from the various telescopes is fed into them. They are placed
with accuracy down to a fraction of a wavelength to ensure that the light enters in just the right
phase so that everything lines up. In this way, a far greater resolution of detail can be gained.
An individual Unit Telescope can see objects that are 4 billion times fainter than what you can see
with the naked eye. When interferometry is brought to bear, this becomes 25 times more effective.
It should be noted that the amplified waves of light do have some drawbacks.
Light, as it bounces around all these mirrors, can get lost. The process is not very efficient.
As a result, the object the VLT is looking at needs to be quite bright to begin with. Secondly,
interferometry does not naturally create full images, instead merely picking out certain details
and features of what is visible in what are called “fringes”. However, this is often enough
for astronomers’ purposes. And if a more detailed image is required, the VLT’s observational data
can be processed into crisper full images – it just takes more time and processing power.
The VLT’s effective 200m diameter virtual mirror provides excellent resolution,
but that’s not to say that it’s able to pick up the same quantity of light as the 30m mirrors.
Obviously, there is a lot of empty space between the units that is not covered by any mirrors,
that are not picking up any light. But what it does perceive is revolutionary.
Ranging from the beautiful, like this eye-catching look at Galaxy NGC 1398, to the ground-breaking,
like this first-ever image taken of an exoplanet, VLT is often at the forefront of discovery.
Exoplanets are normally only seen through the absorption of light travelling through
their atmospheres, or the slight dimming of any stars they pass in front of. It’s rare to see an
actual photograph of one. And yet in 2004 the VLT captured this image of 2M1207 b, a Jupiter-like
planet that is located 230 light years away from us in the Hydra constellation. This image was
taken with the aid of NACO, the Nasmyth Adaptive Optics System, which has since been replaced by
the more advanced adaptive optics system NAOMI, the New Adaptive Optics Module for Interferometry;
which, according to the ESO website, is putting the VLT’s Interferometer “on steroids”.
It’s worth noting, Adaptive Optics is a vital part of ground-based telescopes. Movement in
the atmosphere can cause distortions in the light arriving at the telescope from distant stars. This
can make it much harder to make out detail, and is the main advantage of space-based telescopes
over ground-based ones. In space, you don’t have to compete with the atmosphere between you and
the object you’re observing. However, the VLT’s adaptive optics showcases an ingenious solution
to this problem. By using guide stars, or lasers if no suitable star can be found,
the VLT can track the distortion taking place as a result of the atmosphere by seeing how
the light from these guides differs from what it should be. Then, using actuators under the
mirrors of the Unit Telescopes themselves, the VLT causes tiny distortions in real-time
in the mirror to counteract the distortion in the air. This can negate much of the negative
effects of the atmosphere, granting the VLT a much better look at the universe.
Let’s talk more about the VLT’s discoveries! With so much data to choose from, it’s really about
picking favourites. The VLT has looked at the aftermath of colliding neutron stars, detecting
for the first time the freshly made heavy element Strontium – an element used in fireworks.
It has measured the temperature of the distant universe by detecting Carbon Monoxide atoms,
confirming that 11 billion years ago space was slightly warmer than it is now (-264°C vs -270°C,
so admittedly still quite chilly). But perhaps one of the most exciting discoveries of the
VLT was its role in helping prove the theory of General Relativity, which earned Reinhard Genzel
the Nobel Prize for physics in 2020, as well as confirming the existence of the monstrous black
hole at the centre of our solar system. Einstein’s theory of General Relativity had long predicted
the existence of black holes, as shown by Karl Schwarzschild in 1916. For decades, though,
scientists wondered whether these universe-warping objects actually existed. A mathematical physicist
named Roger Penrose went on to prove in his 1965 paper “Gravitational Collapse and Space-Time
Singularities” that Black holes were not just mathematical artefacts in Einstein’s theory,
but concrete certainties. It was inevitable that they could and would form. But still, it
took until the turn of the millennium before the first black hole was confirmed. During this time,
scientists began to suspect that there was one such black hole at the centre of our own galaxy.
Radio signals had been detected coming from the centre of Sagittarius, 25,640 light years away, as
far back as 1931. In 1971 two radio astronomers – Bruce Balick and Robert Brown – noticed a bright,
compact object at our galaxy’s centre that gained the name Sagittarius A*. But telescopes
were not powerful enough to truly peer into the heart of Sagittarius A* to confirm what it was.
This changed with Andrea Ghez and Reinhard Genzel in the 1990s. Through detailed observations that
spanned nearly 30 years, each astronomer began to map out the motion of stars in the centre of
Sagittarius, to see how they interacted with Sagittarius A*. General relativity
predicted rapid movement of any star moving past a black hole. However, it also predicted
precession – a change in the orbital path over time in the direction of the star’s motion.
Rather than looking like a circle or an ellipse, a star showing such precession would travel the
path of a rosette through space: But this would require detailed evaluation of the stars around
Sagittarius A* for multiple orbits, and each orbit could last up to 16 years.
So decades of observations were needed to confirm Einstein’s theory. And fortunately, telescopes
like the VLT were by this point available for use. Genzel’s team made use of the VLT to watch
the motion of the stars around Sagittarius A*, while Ghez made use of the Keck Observatory in
Hawaii to do the same. Together, they were able to confirm this precession was indeed taking place.
As further evidence, in 2018 the VLT observed star S2 travelling around Sagittarius A* at 7650km/s,
or 2.55% the speed of light; exactly in line with the speeds Einstein had predicted. This
finally confirmed Sagittarius A*’s identity as a Supermassive black hole. Thus, in 2020 Ghex and
Genzel both received the nobel prize of physics for “the discovery of a Supermassive compact
object at the centre of our galaxy,” and Roger Penrose, who had first proved mathematically that
black holes inevitably arose out of Einstein’s theories, received a Nobel prize for the same.
So there you have it. The VLT has been massively influential up until this point, providing data
for countless scientific papers and placing itself as one of the most productive telescopes mankind
has yet produced. But going forward, in an era where Extremely Large Telescopes begin to be more
common, and as telescopes like the JWST orbit in our skies, the VLT will need to up its game
to stay relevant. It will need to evolve, with its instruments continuing to be updated and modified.
The VLT still has the potential for years more of valuable contribution to the scientific community.
Astronomers will need to think carefully about how this is best achieved. But it should not be too
difficult. The VLT is a powerhouse. I’m certain it will continue to find its niche for years to come.
So, were you already familiar with the VLT? Would
you like to hear more about the VLT and its discoveries? Let me know in the comments.