Bad Astronomy

The one-dimensional volcano

Think this is just another devastatingly gorgeous picture of a volcano from NASA?

Well, you’re right. Kinda.

First, the image is from NASA’s Earth Observatory-1, which – surprise! – observes the Earth. The volcano in question is Volcán Villarrica, a 2850 meter (9300 foot) snow-capped stratovolcano at the southern tip of Chile. It’s a fairly active mountain, frequently ejecting ash and airborne rocks called pyroclasts, and causing lahars (mud flows). You can see the mess it’s made to the east (right), and to the west there is a vast network of grooves caused by flowing mud and lava.

So, cool picture, right?

The thing is, this isn’t a picture. At least, not really! You’d expect that EO-1 is equipped with a camera much like a digital camera you can get in a store (though probably a tad more pricey). And in fact, most cameras on board satellites are like that: a two-dimensional array (or grid) of light-sensitive diodes. When exposed to light, they create electrons which fill each pixel like water fills a bucket. Electronics then read out the electrons and count ‘em up. Brighter spots have more electrons, dimmer spots have fewer. Tadaa! Picture.

But that’s not how the Advanced Land Imager on EO-1 works. The following description simplifies things a bit (go to their page for more details), but essentially the imager has a row of pixels instead of a grid. When sitting at the back on of the telescope facing downward, each pixel sees a square of the Earth about 10 meters (33 feet) on a side. Imagine the satellite were standing still, and took a picture. It would see a long thin rectangular region of the Earth, 10 meters wide and some kilometers long. It takes some tiny fraction of a second to take that picture and to read out the row of pixels – that is, get the electrons out of each pixel and record how many there were (that’s why your camera pauses for a moment after you take a picture).

But the satellite is moving, orbiting the Earth. So now imagine that the exposure and readout time of the row of pixels is exactly the same as the time it takes for the view of the camera to move by 10 meters. In that case, just as the camera is ready to take another picture, the pixels have moved (or the Earth has slid underneath by) exactly their own field of view. When it takes the second shot, it’s seeing the very next strip of land adjacent to the first shot. This happens continuously, so the camera is basically taking a picture of strip after strip of the Earth. Once all those rows are beamed down to Earth, software can be used to stitch the image together, turning a pile of one-dimensional images of the Earth into a glorious two-dimensional picture like of the volcano, above.

In other words, that picture of the volcano wasn’t taken all at once: it was taken row by row, each one individually, and then stitched together after the fact. Cool, huh?

And maybe it sounds familiar: I bet you’ve used this method yourself. Ever had a scene you couldn’t fit on one picture, so you took two? And then later, using some software like Photoshop, you stitched the two pictures together (creating what’s called a panographic picture). Well, that’s what the imager on EO-1 does, but instead of turning the camera to get the second shot, the satellite allows its orbital motion to naturally put the next shot into frame.

Also, this is how scanners work! They don’t take a two-dimensional image of what you’re scanning; they have a single row of pixels that moves across the document, continuously reading out what it sees. Once it’s done, all those individual rows can be stuck together to make an actual picture.

For an Earth-orbiting satellite this is a pretty clever move. Having only a single row of pixels saves weight, space, and power. Since the satellite orbits the Earth you just use its motion to make the panning movement for you. Some astronomical observatories do this as well, like WISE, which spins around the Earth and takes huge scans of the sky as it does. Other big ‘scopes like Hubble point at their targets and sit there, letting the picture build up, but that’s not always the best way to get your data. It just depends on what you’re trying to do.

The cool things about all this for me is just how many ways we can observe the Universe (and our home in it!) and the fact that smart people have figured this out! I’ve said it before, and no doubt I’ll say it again: I’m glad smart people are around. They make life so much more interesting for the rest of us!

Image credit: Jesse Allen and Robert Simmon, using EO-1 ALI data provided courtesy of the NASA EO-1 team