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ESOcast 34: How To Stop a Star's Twinkle - The astronomy podcast exploring the cosmic frontier with Dr J

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The Allgäu public observatory lies amidst the picturesque landscape of southern Germany. As night falls, a team of scientists and engineers prepares to field test a very cool piece of technology: a laser guide star unit, which will soon be on its way to ESO’s Paranal Observatory. This is the ESOcast! Cutting-edge science and life behind the scenes at ESO, the European Southern Observatory. Exploring the ultimate frontier with our host Dr J, a.k.a. Dr Joe Liske. Hello and welcome to the ESOcast. Today we’re at the Allgäu public observatory in southern Germany because this is where a team of scientists and engineers from ESO is testing a brand new laser guide star unit. ‘What’s that?’ you ask. Let me explain. Now, we have all looked at the sky at night and seen the stars twinkling. Now, the stars themselves, of course, don’t do any twinkling. The twinkling is caused by turbulence in the Earth’s atmosphere. As the starlight crosses the atmosphere it encounters different pockets of air with different temperature and pressure which bend the light in different ways, thus causing distortions. In fact you can see this effect often in broad daylight, whenever you look towards a distant object on the horizon on a hot day. Now the twinkling is all very pretty and even romantic, but for us astronomers it’s actually a real problem because it means that our images are blurred and less sharp than they could be if it wasn’t for the atmosphere. So, what do we do about it? Essentially we need a method to cancel out the distortions, in effect, to “un-twinkle” the stars. The way to do it is to bounce the starlight off a mirror that is slightly deformed in exactly the right manner to cancel out the distortions. But how do you know how to deform your mirror? As ESO’s Very Large Telescope observes the sky, a specialised computer can pick a bright star and constantly monitor how it twinkles — deducing the atmospheric conditions above the telescope many hundreds of times a second. The computer then sends commands to a series of devices attached to a mirror in the telescope, bending and flexing it precisely in time with the atmospheric turbulence, cancelling out the distortion in the images. So, for this correction process to work you need a really bright star in the field of view of your telescope. But bright stars are very few and far between, and remember that the VLT was designed to image only a very small part of the sky at any given time. So for most observations there just won’t be a bright star in the field of view of the VLT. So what do we do now? Well, we make our own. 90 kilometres above our heads, in the upper atmosphere, is a relatively thin layer of sodium. If you fire a powerful laser beam into the sky you can make these sodium atoms glow, thereby effectively creating an artificial star for the computer to lock on to. In 2006, ESO installed the Southern Hemisphere’s first laser guide star on the VLT. This system greatly improves the telescope’s power, meaning the VLT can even make sharper images than Hubble for certain types of observation. But this existing system has limitations. It can only create one artificial star at once meaning it can only correct the telescope’s vision for a small part of the sky at any one time. It’s also very bulky – the equipment has to be kept in a separate laboratory and the laser beam fed along an optical fibre to the telescope. Based on the experience obtained with its first system, ESO engineers have been working to build a much improved, new laser guide star unit. So, Domenicos, this is it — this is the laser. It’s incredibly small, it fits on the back of this small telescope, that’s amazing. Yes. So this is what we’ve been working on for the past five years, to make a 20-watt laser, very compact and lightweight so that it can be mounted directly on the back of the telescope. So we had to develop fibre lasers first and then developed these kinds of laser heads. So, you’ve just said it, it’s a 20-watt laser. That’s quite a bit of power isn’t it? Yes. This is the power we’ll need, actually, for the next generation of laser guide star systems. And right now, for example, at Paranal we have about 5-watt in the sky, so this is quite a jump in power. Is the laser beam that comes out of the end of this telescope dangerous? What happens if I put my hand into it? If you put your hand in, you’ll feel warmth. But don’t have to look into the beam. OK, so it won’t burn my hand. But what about aeroplanes, is it dangerous for them? It’s not dangerous for the equipment or for the aeroplane, it’s dangerous for the eyes of the passengers. And, this laser is above the maximum permitted exposure so we have to avoid planes crossing the beam. In fact, here where we are now we have obtained a no-fly zone above us, so we don’t risk hitting a plane. The new device is more reliable, easier to maintain, and much smaller. In fact, as we’ve just seen, the whole unit fits into one small package which is easy to mount on the launch telescope. Because it’s so much smaller, up to four of these lasers can be installed on a single telescope, correcting the VLT’s image over a much wider field of view. So what’s happening here in Germany is that our team is testing the new prototype to make sure that it works perfectly before it gets shipped to Paranal. The facilities here at the Allgäu public observatory are perfect for this — and, what’s more, they’re only a short drive from ESO Headquarters. Laser guide stars like this will be crucial for the forthcoming European Extremely Large Telescope, which will use adaptive optics routinely. The telescope will be many times the size of today’s biggest telescopes, which should mean much sharper image quality. But this great image quality will depend on how well the adaptive optics and the laser guide stars work. Pioneering new technologies like these will make a big difference to the world’s most advanced observatories of the future, especially the E-ELT. This is Dr J signing off for the ESOcast. Join me again next time for another cosmic adventure. While we were filming this episode, we got a stark reminder of why ESO’s telescopes are located on mountaintops of Northern Chile, and not here in the hills of Southern Germany. Thankfully, storms like this are not something you ever see at Paranal. ESOcast is produced by ESO, the European Southern Observatory. ESO, the European Southern Observatory, is the pre-eminent intergovernmental science and technology organisation in astronomy, designing, constructing and operating the world’s most advanced ground-based telescopes. Transcription by ESO ; translation by — Now that you've caught up with ESO, head 'out of this world' with Hubble. The Hubblecast highlights the latest discoveries of the world´s most recognized and prized space observatory, the NASA/ESA Hubble Space Telescope

Video Details

Duration: 8 minutes and 50 seconds
Country: Germany
Language: English
Producer: Lars Lindberg Christensen
Director: Herbert Zodet
Views: 277
Posted by: esoastronomy on Sep 6, 2011

We have all looked up at the sky at night and seen the stars twinkle. It may be pretty and romantic, but it is also a big problem for astronomers, as the shimmering starlight blurs observations.

In this ESOcast we visit the Allgäu public observatory amid the picturesque landscape of southern Germany. Here, as night falls, a team of scientists and engineers prepares to field test a laser guide star unit, which will soon be on its way to ESO’s Paranal Observatory. This cutting edge technology can help astronomers cancel out the effects of the turbulence in the atmosphere and obtain much sharper views of the Universe.

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