First, I want you to watch the video below. I want you to fullscreen it and select the 720p option. It’s eight minutes of pure astrological eye candy with a very nice soundtrack to compliment things.
Did you see those lasers shooting up into the sky from the telecsopes? Well about those…
Of course you’re viewing our galaxy from the side since we are inside the Milky Way, on one of its outer bands. Were you to go outside tonight and look up at the sky you would see it were it not for all the lights from you, your neighbors, and the town center nearby. But if everyone turned off their lights, you’d see something just as spectacular. This light-pollution problem is also why you don’t see stars in photographs taken on the moon; the light reflecting off the moon (and Earth!) is just too bright. Take some vacation photos from the dark side of the moon and the view would be spectacular.
The telescope in this film is the Very Large Telescope located high up on a mountain in Chile. Telescopes are especially effective the higher up they are because there’s less atmosphere to get in the way. But there’s still quite a lot of atmosphere above even at the heights of the VLT and this can be a problem.
Ever stare down a long road on a hot day and see what might look like a pool of water or some weird wavy distortion of far away cars driving down the road? That’s a mirage and it’s caused by the difference in temperature between the air near the hot asphalt and the cool air above it. The difference in temperature means a difference in density and light that travels between different densities will be refracted. The same thing happens to light as it travels through our atmosphere, although the effect is much smaller.
When looking at relatively large areas of the sky (like something the size of a pea held out at arm’s length) this refraction isn’t an issue; it’s too small to be noticeable. But if you want to focus on an extremely small area of the sky (think the diameter of a single hair held out at arm’s length) this refraction becomes a very real problem.
Very large telescopes, like the Very Large Telescope in Chile, tend to look at things very deep in space, focusing on a very, very tiny point in the sky. If they’re to get a clear picture then they need to deal with these distortions in the light caused by the different layers (i.e. densities) of the atmosphere. And so we turn to the relatively new field of adaptive optics. The idea is simple in theory, you look at something in the sky that has a known property (color of light, brightness, shape, etc.) that’s close to the object you want to observe. Based on how the known object has been distorted by the atmosphere you correct the image and, in the process, correct the image for the object you’re observing. In practice this is really difficult to do and why only within the last 20 years, with the advance of computers, has this sort of thing been used in practice.
But there’s a catch.
The object being used as a guide to adjust the image (almost always a star) needs to be very close to the object you’re trying to observe. The greater the distance between the two the lower the quality of the image correction. And not all known stars can be used as references, they need to be of sufficient brightness to be of any use. This limitation means a majority of the sky cannot be viewed using this technique. If only there was a way to create an artificial star.
Turns out there’s a layer of sodium gas in the mesosphere and if you tune your laser to 589.2 nanometers you’re going to piss off those sodium atoms enough that they start glowing. And that gives you and artificial star (called a Laser Guide Star) with known properties to tell you how the atmosphere is distorting the image. And since you can make one of these artificial stars anywhere in the sky, there’s no limitation on what area of the sky you can view.
You can start to get an idea of the difference adaptive optics makes in this article that includes examples images with and without adaptive optics.
So now go back and watch that video again and when you see that yellow laser (hey, look at that, 589.2 nanometers is within the wavelength boundaries for the color yellow in the visible spectrum) you know that at that moment there are people in white coats looking at something especially far away from us.
By the way, it’s okay to feel really fucking small as you contemplate how fucking big that universe thing is. Humility is the most important life lesson there is.
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