00:00En el desierto de Atacama, en Chile, a más de 2.600 metros de altura, se encuentra el Very Large
00:06Telescope.
00:08Construido en la década de 1990, está formado por cuatro telescopios de 8.2 metros que pueden operar juntos mediante
00:16interferometría,
00:18logrando la resolución de un telescopio de más de 100 metros.
00:22En funcionamiento desde finales del siglo XX, es una de las herramientas más avanzadas para explorar el universo.
00:30We've been working towards this for decades, and now the time has finally come.
00:36ESO's Very Large Telescope, high up on Cerro Paranal in the Chilean Atacama Desert, is host to a spectacle unlike
00:43any other.
00:44For the first time ever, all four of its giant telescopes are firing powerful lasers into the sky, all in
00:52the name of science, of course.
00:57Optical telescopes have come a long way since the time of Kepler and Galileo.
01:02When ESO first started its journey building and operating ground-based telescopes in the 1960s and 70s,
01:09these already had main mirror diameters in the 1-4 metre range, specialised instrumentation and access to some of the
01:16clearest skies on Earth.
01:18But in the 1980s, ESO had an even bigger ambition, to build the largest, most powerful optical telescope the world
01:27had ever seen.
01:29And what's more, this Very Large Telescope wouldn't just be one single telescope.
01:35Instead, it would consist of four unit telescopes, each with a main mirror diameter of 8 metres.
01:41These so-called UTs could be used individually or combined into one huge unified telescope, the Very Large Telescope Interferometer,
01:51or VLTI for short.
01:53The big perk of interferometry is that it makes use of the wave characteristic of light to yield images sharper
02:00than would be possible with a single telescope.
02:03It's commonly used at longer wavelengths, such as the radio or the millimetre.
02:07An example of an interferometer that we've talked about quite a bit on this channel is ALMA, in which ESO
02:12is a partner.
02:13However, interferometry becomes more difficult the shorter the wavelength.
02:18At the infrared wavelengths the VLTI operates at, extremely complex processes are required to make it work.
02:25If you'd like to know more about this, please let us know in the comments and we'll dedicate an entire
02:30episode to it.
02:32Optical and infrared interferometry is so challenging, in fact, that the VLTI is one of only a few optical interferometers
02:39in the whole world.
02:41When it saw first light in 2001, it was possible to combine only two of the unit telescopes, and it
02:47took nearly a decade for all four of them to be linked together.
02:51Today, the VLTI can combine the light from all four unit telescopes, or it can use four of the smaller
02:59auxiliary telescopes.
03:01These are smaller telescopes with a main mirror diameter of 1.8 metres that can be moved around and were
03:07added to the observatory for more flexibility.
03:10The result is a huge virtual telescope that gives images as sharp as a telescope with a diameter of 130
03:18metres when using the unit telescopes,
03:20and a diameter of a staggering 200 metres when using the auxiliary telescopes.
03:26At near-infrared wavelengths, that would be good enough to see me on the Moon.
03:37Well, in theory, ground-based telescopes have one major adversary, Earth's atmosphere.
03:44It constantly shifts, swirls, and distorts the incoming light, making it extremely hard to observe faint, distant objects.
03:52To get around this, we use a method called adaptive optics, a technique where we measure the atmospheric turbulence in
03:59the line of sight
03:59using either a real star or an artificial one created with a laser.
04:04We then correct for the turbulence using deformable mirrors.
04:08If you want to know more about how adaptive optics works, check out this video we made.
04:13For the VLTI, adaptive optics is crucial.
04:17In order for the light from the individual telescopes to be combined, it first needs to enter a tiny fibre,
04:23which is only about as wide as a human hair.
04:25If the light's too blurry due to atmospheric turbulence, this simply won't work.
04:31Up until now, the VLTI has been operating with a basic adaptive optics system
04:36and entirely relies on bright, natural reference stars.
04:40But in order to live up to its full potential, it needs something more sophisticated.
04:46And this is where the new lasers come in.
04:49They are part of the Gravity Plus upgrade, which enhances the existing VLTI instrument gravity.
04:55Gravity was designed to, among other things, probe the galactic centre and the region around Sag A star,
05:03the supermassive black hole lurking there.
05:07But Gravity Plus is more than just another instrument upgrade.
05:10It transforms the VLTI's infrastructure itself,
05:15gradually implementing an extreme adaptive optics system into the interferometer.
05:20And now, it finally brings a laser to each of the UTs.
05:25By equipping all four telescopes with laser guide stars,
05:28the VLTI no longer depends on bright reference stars to correct for atmospheric turbulence.
05:34So, essentially, this upgrade opens up the whole southern sky to the VLTI,
05:39allowing us to observe many more objects in much greater detail than before.
05:46Just like its predecessor, Gravity Plus will peer deep into the central regions of the Milky Way.
05:52But it will go further, uncovering even fainter objects orbiting around the supermassive black hole Sag A star.
05:59These will allow astronomers to better understand the black hole and measure its spin.
06:05But it won't stop there.
06:07Gravity Plus will extend our view of the universe to include galaxies and their black holes billions of light years
06:14away.
06:14It will also unveil hidden worlds,
06:18allowing us to peek into the atmospheres of exoplanets and search for signs of life.
06:24Gravity Plus and its lasers will open a whole new window to the vast volume of space,
06:28unlocking the full potential of the VLTI,
06:32just like astronomers and engineers dreamed of over 40 years ago.
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