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What goes into making a picture? June 2, 2009

Posted by jcconwell in Astronomy, Observatory.
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Anyone who has ever seen the amazing pictures that are taken from the Hubble space telescope knows how beautiful the universe is. But those pretty pictures are like making sausage. The raw material may look ugly. To give you an example, let’s take a look at a picture taken by the students for the astronomical techniques course. Taken at EIU’s 16″ scope , the Whirlpool galaxy M51.

Raw 30 second picture direct from the telescope

Raw 30 second picture direct from the telescope (Click to enlarge)

If you click you see the grainy detail. Most of this grain is heat noise on the digital camera. You don’t see this in your home camera because your exposures are short. Over 30 second the noise  will build up. To get rid of this astronomers cool their cameras, for this camera, the SBIG-8, every 6 degrees Celsius I cool it, half the pixel noise goes away. This camera is cooled to 40 degrees below the outside. This image you see is a jpeg. The original file is in a format used by astronomers called FITS.  In that format and on my computer screen it’s even uglier, for some reason when it’s on wordpress it looks better.

Now I’m old enough that when I was 14 and using film,  I would have sold my soul (almost), for this good of a picture. And yes, my students, I do have a soul , I just lack a heart.

But we can do better.

One of the tricks you can do with a digital camera is subtract the heat noise. You do that by taking what is called a dark field…. a picture at the same temperature for the same time 30 seconds, with the shutter closed. This is a picture of the heat noise.  Then you subtract that picture from the real one canceling (almost ) the heat noise. There are other things you can also do to correct for dust on your optics, take a picture of a blank white wall, which are called flat fields, and divide that out. But the main problem is not enough photons.

Now to get more photons you either get a bigger scope or a longer exposure. But with digital cameras you can also add many short  exposures to make a long exposure. That what we did here. By taking ten 30 second exposures and cleaning them up using our darks and flats we get:

ten exposure added together

ten exposure added together

Now in the interest of truth in advertising, there is on more enhancement that was done here. If you look at the first, raw, photo you’ll see that the center of the galaxy looks brighter. Onr problem is looking at astro-photos is you’d like to see the details in both the bright and dark regions, but the computer and eye can only handle about 256 shades of grey. If you look at dim regions in thspiral arms, the  center get burnt out and looses detail. If you dim the image to see the center the spiral arms become too dim.  But between computers and digital images we can enhance the image, to see both. That’s what was done here. We told the computer to assign more of the 256 shades of grey to the dimmer sections and less shades to the very black space  and very white center, bringing out the detail of both. But to make a picture pretty you may loose some information, like the relative brightness, so you have warn people, that may use it for scintific work, when an image is enhanced like that.

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