# Adaptive Optics in Plain English

Back in 2012, there was an xkcd comic that attempted to explain the Saturn V rocket using only the top “ten hundred” most frequently used words in the English language. (“Thousand” didn’t make the list.) From that, someone made a text editor that would automatically flag words not in the top ten hundred. I recently came across an email with my attempt to explain a research project using only the top ten hundred words in English, and thought it would be fun to try with my more recent work on adaptive optics.

# “Taking Clear Pictures of Stars From the Ground”

## explained with only the top ten hundred words

To see stars, you have to either look up from the ground, or take pictures in space. When you take your pictures from space, there is no air between you and the stars, and the pictures are very clear with many stars. When you look from the ground, though, there is a lot of air in the way. This air is always moving around, even though you can’t see it. That is why, when you take pictures from the ground, you have problems. The pictures are not very clear, and sometimes two stars look like one! This is not good for people who study stars, because they want to know how many stars they are looking at, and how bright they are.

If we want to look at stars, but we can’t go to space, how can we get very clear pictures? It turns out that if we know how the air is moving, we can make the pictures more clear even though we are still looking from the ground. Pictures are made by taking light from the sky, and we know that light is a wave. When the light goes through still air, it is just like going through space, and you can take a clear picture. We know the air is not still, which means there is more air packed into some parts of the sky than others. When the light wave goes through this packed air, we say it gets “behind” the rest of the light that went through less packed air. If we find out how far ahead or behind the light waves are when they come in, we can use a mirror to bring the different light waves in line with each other.

Once we have done this and fixed the light, we take our picture. There is still a problem, though! We can see how the air is more packed in some places and less packed in others, but everything is moving. This means we have to look again and again and again to make sure our mirror is always set up right for the light coming in at that moment. By checking over and over, we can keep our star pictures clear, which is very good for people who look at stars.

It is hard to make a computer check light waves and move mirrors for you, but it is also very hard to take pictures in space. That is why people who look at stars are interested in this kind of thing.

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Background image of the Carina nebula by NASA, ESA, N. Smith (University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA)