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SYFY WIRE Bad Astronomy

Stunning Picture of the Milky Way…Over Los Angeles?

By Phil Plait

Los Angeles is a fun townâas long as youâre not a) driving around in it, or 2) trying to see any stars except for the TV and movie kind.

Itâs a big city, and a lot of the light used to illuminate it goes into the sky. We call this âlight pollutionâ, because itâs wasted, and also because it can ruin the view of the sky. LA is particularly bad because itâs spread out over a huge area, and to see anything at all in the sky you have to get really, really far out of town.

So I will admit to being pretty skeptical when I first saw the picture below: it purports to show the Milky Wayâthe faint fuzzy band of light strewn across the sky from our galaxy itselfâseen over LA!

Seriously, right? Thatâs nuts.

But itâs also real. It was taken by Aaron Kiely, who works on spacecraft data at NASAâs JPL, and whoâs familiar with techniques to squeeze extra information out of them. That lends him more credence right away. He also has a more detailed explanation of how he put the image together on his Flickr page, and after reading it I was satisfied itâs legit; the techniques he used were very similar to ones I used myself back when I worked on Hubble images!

The idea is that even when you have a very bright foreground (like LA), the fainter background (like the Milky Way) is still there, itâs just that the photons from it are vastly outnumbered. But if you take lots and lots of pictures, those photons build up. Then you can add the pictures together to create one where you can see fainter objects.

The problem is the Earth spins, so the Milky Way and the stars in the sky move. Normally you could just shift all the pictures to line them up, but in this case, though, Kiely used a wide-angle 11mm lens, so the pictures are distorted. That makes a simple shift much harder to do. So instead, he used some math to make a model of how the stars moved across the frame of the picture over time. This created a series of curved lines, all different depending on where they were in the frame:

That is essentially a map, a grid, showing where a star would be given its position and the time the picture was taken. He then used that model to warp each image, placing them all on a common frame, and added them together:

Cool. The Milky Way can now be seen, but itâs still faint; the bright sky is still swamping the Milky Way light. He needed to subtract it, reduce its influence. So Kiely turned to math once again.

Imagine the sky were the same brightness everywhere. All youâd need to do is find out what that value is (using Photoshop or any number of other image manipulation packages) and subtract it. But the sky brightness changes from spot to spot. Kiely wrote some software that examined the sky brightness all over the image and made a smooth two-dimensional map of itâlike how throwing a blanket over a bunch of boxes on the floor smooths out their bumpiness. For those math nerds out there, he fit a polynomial to the background excluding stars and the landscape at the bottom, fed the coefficients into a least squares fit routine, and boom. 2D map made.

Subtracting that from his co-added picture, and voilà! You get the Milky Way hanging eerily over Lalaland.

Well, almost. Shifting and adding all the images together blurred out the hills and city at the bottom, so he took the nice, sharper shot of that from one of the single pictures and replaced the blurred portions.

Some people might think this is âcheatingâ, since so much manipulation is involved. I can understand that, but Iâm not so upset. First of all, this is art, not science. Well, it is science; science used to make art. And itâs beautiful.

But moreover, let me ask you this: What isnât cheating? A camera by its very nature shows us things our eyes cannot see. It collects light for far longer than our eyes do, it responds to color differently than our eyes do, it converts light to digital data, and it even performs a lot of mathematical manipulation of the picture before we even see it.

For some, âcheatingâ is when youâre showing something in the picture that wasnât there in the first place⦠but even then it may not be so bad; astronomers combine images from different telescopes all the time. I only get upset by that when itâs presented as an actual photo; the person doesnât let you know itâs been manipulated. Honesty is the best policy.

So to me, what Kiely did is not only legit, but also useful. He was able to tease out information that was in his pictures but far too faint to see in any one shot. And the result is amazing.

Tip o' the lens cap to FakeAstroPix on Twitter.

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