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1/ I took this picture of @ESO’s UT4 telescope back in January. You may have read that we use lasers to correct atmospheric turbulence, but how does this work exactly? And why do we need several lasers rather than just one? Thread!
2/ Ground-based observations are affected by atmospheric turbulence, which blurs the images of astronomical sources. But we can use high speed deformable mirrors to counteract this turbulence and get very sharp images, as I explained here: https://twitter.com/astro_jcm/status/1229398249699774465
3/ This technique requires taking 1000s of images per second, so your target can’t be too faint. If it is, then you need to have a bright star next to it to do the correction. But bright stars are rare, so with this technique you can only cover about 10% of the sky.
4/ What if there isn’t a bright star next to the target we want to observe? We create one! By shooting a laser tuned to a specific frequency we can excite sodium atoms high in the atmosphere. These atoms then glow, creating an artificial “star”, as I explain in this infographic:
5/ Note that this “star” is elongated because the sodium layer has a certain thickness, but when seen from the telescope it looks like a point source. We can then monitor how this "star" twinkles, derive a correction, and apply it to the target we want to observe.
6/ For instance, in this image that I took a few years ago you can see UT4 pointing its old single laser towards the center of the Milky Way. The goal was to compute the mass of the supermassive black hole by measuring how fast stars move around it.
7/ The problem with using a single laser star is that its light doesn’t pass through the same portion of the atmosphere as the light from the more distant astronomical target. We call this the cone effect, because we can’t measure the turbulence outside of this cone.
8/ Moreover, the cone effect is worse in large telescopes, because the cone of light from the artificial star misses much larger areas full of turbulence.
9/ The solution? Use more than one laser. That way we can probe the turbulence at different elevations without missing information. This provides a much better correction, and it also allows us to correct larger areas rather than the target close to the laser.
10/ This technique will become even more relevant for upcoming behemoths like the Extremely Large Telescope, the Thirty Meter Telescope, and the Giant Magellan Telescope. That’s it! I hope you learned how useful lasers are to beat atmospheric turbulence.
(All infographics in this thread created with @affinitybyserif Designer)
