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: http://www.badastronomy.com/bitesize/vlt.html
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Taking out the TwinkleWeek of August 30, 1999 Last week I talked about why stars twinkle. Briefly, it's because little packets of air bend the light from stars in random ways, making the image dance. It's very pretty, and many poems have been written about it. Unfortunately, science is often not so poetic. While pretty, this twinkling (what astronomers call ``seeing'', an old and confusing term that has unfortunately stuck) plays havoc with astronomical observations. Since telescopes tend to take images that have long exposure times, the seeing smears out the object in the image. Seeing is measured by how much the light from a star is smeared out, and the usual unit is an arcsecond (one arcsecond is very small; it's 1/3600 of a degree). This is then a fundamental limit to how small an object we can see. Imagine two objects, one half the sized of the other, but both smaller than the seeing. Because of seeing, they will both get blurred out to the same size, meaning we cannot tell which object is larger! Even worse, two objects that are close together get blurred together, and we cannot distinguish them. That really puts the brakes on how small an object we can detect. Luckily, there are ways around seeing, but they cost. One solution is to get above the atmosphere. That's what Hubble does. It is not limited by atmospheric seeing, and so it can see objects with very high resolution. This is a nice solution, and it works very well indeed for space telescopes, but it's expensive. Another method is to take very fast exposures of an object; in effect taking images faster than the seeing can blur the image. This only works with really bright objects, if you think about it. You can't see faint ones in such short exposures. Still, this method has been used to actually detect the size of the star Antares, a red giant that's so big it actually has a measurable (but very tiny) disk this way.
Either way, the control over the mirror is amazing. The distortions must be compensated on timescales of fractions of a second! Some amazing computer and engineering wizardry is needed for this. But the results are astounding. By distorting the mirror in a known way, one of the VLT 'scopes was able to write its name on the sky! Those letters in the image are actually stars, wiggled around by the actuators in the VLT mirror. You can read more about how they made that image on their adaptive optics press release page. It's not all fun and games. The images taken by the VLT array are astonishing and beautiful, some of the most dramatic and gorgeous astronomical images I have ever seen. Take a look for yourself! You can find them at their image index page. You can even order posters of some of the better images too. You can bet that at least one of these will be hanging in my house soon!
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