00:00En la astronomía moderna, los láseres no sólo iluminan el cielo, lo transforman.
00:06Gracias a la óptica adaptativa, los telescopios corrigen en tiempo real las distorsiones de la atmósfera,
00:12utilizando estrellas artificiales creadas con haces láser.
00:16Esta tecnología, cuyas bases se desarrollaron en el siglo XX y se consolidaron con avances en la computación,
00:23ha permitido obtener imágenes del universo con una precisión sin precedentes.
00:30Lasers, they're cool.
00:32They invoke futuristic spaceships and epic space battles.
00:37But if we're not under attack from aliens, then why in the universe are ESOS telescopes firing giant laser beams
00:44into the sky?
00:49For years, astronomers have been fighting an enemy up in the skies.
00:54But it's not aliens. It's our own atmosphere.
00:57And lasers are proving to be a game changer.
01:01Now, don't get me wrong. As a human, I love our atmosphere.
01:04It contains the oxygen we need to breathe and also prevents us from being grilled by the sun's radiation.
01:11But as an astronomer, I have a big problem with it.
01:15See the way the stars twinkle?
01:18That is caused by turbulence in the atmosphere.
01:21And while it makes for a beautiful view, it leads to blurry images and distorted observations.
01:27Luckily, there is a way to magically remove much of the effect of our atmosphere without us suffocating in the
01:35process.
01:35Adaptive optics.
01:38Adaptive optics uses cool things such as lasers and shape-shifting mirrors to correct for the distortions.
01:44It works a bit like noise-canceling headphones, but for light rather than sound waves.
01:49So, how does it work?
01:52First, you need to figure out exactly how much the atmosphere is distorting the light from the astronomical image that
02:00you want to observe.
02:01For this, you need a benchmark.
02:03A bright and stable star close to your object.
02:07Its light is captured by a wavefront sensor, which is an array of tiny lenses and a detector divided into
02:15small areas.
02:15Without any turbulence, each tiny lens creates an image of the star in the center of the respective detector area.
02:24So, we end up with lots of dots perfectly aligned with each other.
02:28But when turbulence distorts the light, these dots are offset from their central positions.
02:35These offsets are measured and sent to a computer that performs some super-fast calculations to determine exactly how the
02:44signal needs to be corrected.
02:46These instructions are then fed to a thin deformable mirror, which reflects the light of the astronomical object before it
02:54is recorded.
02:55Like a lake reflecting the mountains surrounding it, the exact shape of the mirror influences how much the image that
03:03you get is distorted.
03:04So, by changing the shape of the mirror in exactly the right way, we can effectively cancel out the turbulence.
03:13But the atmosphere is a tricky opponent.
03:16Turbulence changes very rapidly, on the order of a thousand times per second.
03:21That means that our mirror needs to change its shape just as quickly.
03:25The secret to this lies underneath its thin surface.
03:29Adaptive mirrors are only a couple of millimeters thick and rest on hundreds of computer-controlled actuators,
03:36which can make the mirror move and change its shape very quickly.
03:41All of this requires state-of-the-art technology and is a major effort on the computing and engineering side.
03:49But it's worth it.
03:51Using adaptive optics, we can study fainter images and make out finer details in the images,
03:58almost as if we were in space.
04:00Instead of this, we get this.
04:04But what does all of this have to do with lasers?
04:08Well, remember that for adaptive optics to work, we need a bright star as a benchmark.
04:14The problem is, there may not be one close enough in the sky to the astronomical object that we want
04:19to observe.
04:20So, we need to make our own by shooting a laser up into the sky.
04:25The laser excites sodium atoms in a layer of the atmosphere about 90 kilometers up, which is nearly in space.
04:33The sodium atoms start to glow, creating a temporary artificial star.
04:40And, hey presto, the adaptive optics system can perform its magic and remove the blurring no matter where we point
04:47the telescope.
04:48But, of course, a laser like this just isn't going to cut it and get all the way up to
04:5490 kilometers.
04:54The lasers used on ESO's telescopes are about 4,000 times stronger and bright enough to create a point-like
05:02star high up in the atmosphere.
05:04As a pilot, I must admit that that's slightly worrying.
05:08Much less powerful lasers have been used to blind people flying airplanes.
05:12This is why ESO's VLT has an aircraft avoidance system, which shuts down the laser automatically if an aircraft gets
05:20too close to the beam.
05:21And, of course, we always make sure to point one telescope's laser in such a way that it doesn't interfere
05:28with the observations of another telescope.
05:31I hope I've managed to convince you that a telescope launching a laser is a useful thing.
05:36But why does the VLT need fall?
05:41Well, turbulence varies even over small patches of the sky.
05:45So the more lasers and the more artificial stars you have, the larger the patch of sky you can unblur,
05:51and the better your image.
05:52The VLT is set to soon gain some more lasers, so stay tuned.
05:57ESO's Extremely Large Telescope, which is currently under construction just a few kilometers from the VLT,
06:03will have a whopping six lasers and the most sophisticated adaptive optics system ever used on a telescope,
06:09including several types of wavefront-sensing cameras and the largest shape-shifting mirror ever built,
06:17more than twice the diameter of current adaptive optics mirrors.
06:21All of this means that once it comes online later this decade,
06:25the ELT will make images six times sharper than the James Webb Space Telescope,
06:30despite having to battle the atmosphere.
06:32So, in the epic battle of the skies, the final score reads,
06:37Astronomers won, atmosphere zero.
