File:Laser-sharp vision (potw2315a).jpg

English: Don’t worry, no planets were harmed in the making of this Picture of the Week! The powerful laser beams seen here are installed in one of the four 8.2-m telescopes of ESO’s Very Large Telescope (VLT) in Chile. They allow astronomers to obtain very sharp images of the cosmos by correcting the blur caused by turbulence in the atmosphere, but how?The lasers are tuned to a very specific colour that excites sodium atoms floating 90 km above the ground, making them glow. This creates artificial “stars” high up in the sky, whose rapid twinkling is monitored in real time by special sensors. Then, instructions are sent to the telescope’s deformable secondary mirror, which quickly reshapes itself to counteract the atmospheric distortion. And all of this happens at millisecond speeds!This technique, known as adaptive optics, can also use real stars as a reference to measure the turbulence. But there isn’t always a bright enough star right next to the object one wants to observe, and this is when artificial laser stars come in handy.How do the lasers themselves work, and what other astronomical applications do they have? Find out in this ESO blog post.