Sunday, March 31, 2013

Amazing Engineering - Episode 1

As an experimental physicist, I spent more time engineering than doing science. As a result, I've come to greatly appreciate sophisticated or clever works of engineering. Sometimes you read about accomplishments that don't seem possible. I'm starting a series on these sorts of feats.

For the first episode, I'd like to start with something I first learned about as an undergrad. See if you can guess what this is:

If you guessed "sea creature's eye", you would be wrong. If you guessed anything biological, you would be wrong. It's actually an extremely high-resolution image of a sunspot. (By the way, the image was taken from this article) Here's what's unique:

Astronomical observations almost always involve focusing light of some sort onto a sensor. But after traveling all the way here in a more-or-less straight line through mostly nothing, light from the stars has to pass through our atmosphere. X-rays get scattered like light in a dusty room, while visible and infrared light get jumbled up from atmospheric turbulence. Look above a hot car in the summer to see a strong example of this.

One way to make up for that is to stick your telescope in space, à la Hubble.

The other, more radical way is to correct for the jumbling. That's right. Make a shape-shifting mirror that changes fast and accurately enough to compensate for the atmosphere's messiness, and do it with enough resolution to matter. It's the optical equivalent of noise-canceling headphones. And it's significantly harder. Also, much more precise. It's called Adaptive Optics. And, remarkably, it's been in use since the late 90s.

The technique involves using a point source in the sky, like a star, as a reference. The active mirror element continuously changes so as to get as tight an image of this reference as possible. This improves the angular resolution of a telescope by a factor of about twenty or so. Which means what used to be one blob can be resolved into a few hundred smaller blobs, if they were ever arranged like that.

If there is no star that's bright enough to use, then you can make one. Lasers are used to illuminate a spot higher up in the atmosphere to use for reference. Artificial stars!

If you want to look at the sun, though, things are harder for three reasons. First, heating of the air at ground level worsens the jumbling. Second, visible light is more susceptible to atmospheric turbulence than the infrared light that's often more interesting for night-time observations. Third, there's no point-like reference star to use, and it's too bright out to make one.

So that image above captures quite a lot of engineering magic. It seems like a given that we'd have clear images of our closest star, but it takes something as crazy as adaptive optics to actually achieve it. Oh, and by the way, this technology has led directly to medical applications (like improved laser eye surgery), so it's another case of pure science leading to unexpected benefits elsewhere in society.

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