Turbulent Moon

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Popped out to watch B.I.K.E. at the Bicycle Film Festival this evening, which was ridiculous and self-absorbed, but featured much tallbike-jousting, a working jetbike, and brief appearances by a handful of Chicago bike freaks I've met, so it gets a pass in my book. The preceeding short was more vicerally entertaining, though, consisting mainly of a death-defying bicycle messenger race across Manhattan from a helmetcam's perspective.

Now, a little more astronomy. Despite dramatically improving the signal-to-noise ratio of my Jupiter pictures, the composite photo I posted yesterday is still dramatically more blurry than one might expect from a high quality 10-inch instrument. This brings us to a concept called "seeing". As the animation below demonstrates, from one instant to the next an image will distort in numerous different ways. This is caused by turbulence in the atmosphere, and is similar to the phenomenon that causes stars to twinkle. However, most of this turbulence is much closer to us than that high-altitude effect; the ripples visible here are primarily the result of hot air rising off the building's roof and mixing with the air streaming through the observatory dome opening. In spring or autumn the seeing from the 10-inch improves considerably, as the days are cooler and the sun sets earlier, and thus the temperature gradients involved are less extreme. In any research-grade observatory, steps would be taken to ensure that the temperature inside the telescope enclosure is equalized with the outside before observations begin.

There's no getting rid of the higher-level turbulence, though, which is why the Hubble telescope produces such wonderfully crisp images using a comparatively puny aperture by current standards. That's changing, though. Since these are quick snapshots it's easy to see that each one is a reasonably crisp image and basically identical with some random pattern of squashing and stretching applied; it's only when they are combined together that you'd get a blurrier image (and for that matter, if I had a more light-sensitive camera I could get even sharper images by taking even shorter exposures). But you could imagine, say, choosing fixed landmarks on the Moon and designing an algorithm that would distort each image until the landmarks lined up every time, essentially un-doing the distortion. Most astronomy, however, is concerned with details much smaller than the distance between landmarks on the Moon. It turns out you can do something similar, though, by distorting an image until a star appears as close as possibe to a round point. This is called adaptive optics, because it isn't done with images after the fact, but in real time using a computer to distort a flexible mirror in just the right way to un-blur the image.

Now if only they could get it to work with visible light.

jul05_moonseeing.gif
Sequence of six 1/15 second exposures of the northern Lunar highlands, combined as an animation to highlight varying atmospheric distortion. The images are roughly centered on the broad Clavius Crater; the famous young Tycho Crater is the smaller, deep crater with prominent central massif to the lower right. 2006:07:05 21:37:35

2 Comments

Cool. I feel like I'm looking at a puddle reflection in a CGI rendering of a Sci-Fi novel.

Life imitating art imitating art imitating life, eh?

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This page contains a single entry by Milligan published on July 8, 2006 1:13 AM.

Image Combining: Jupiter was the previous entry in this blog.

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