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Biplab Bose
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The Thing with Colors in Astronomy

I posted this a while back, but since my Astronomy collection gained a lot new followers this is one of the questions most regularly asked on astrophotography images. So in case you're still wondering, read about colors in astrophotography below.

You've seen many stunning and spectacular images of objects in space, galaxies, star clusters, supernova remnants, different nebulae and many more. Often those images are very colorful but sometimes images of the same object look very different.

But I see your true colors shining through...

Have a look at these images of the famous "Pillars of Creation" in the Eagle Nebula (M16, NGC 6611). The image on the left was taken by the Hubble Space Telescope, the image on the right by the MPG/ESO 2.2-metre telescope. As you can see, the colors are quite different. But how is that possible?

How we get color

Both images are actually composite images made from three monochromatic images taken through filters for different wavelengths. Those monochromatic images were then assigned ('mapped') to one of the three color channels in a digital image (red, green and blue) which combined result in a color image.

The images taken by those two telescopes used filters for different wavelengths, so after being combined the resulting images of course do look different.

But how would the object look like to the naked eye?

In most cases deep space objects seem very faint because of their distance and since our eyes (unlike a camera) can't change the exposure time to gather more light, colors are usually not very saturated, if visible to the naked eye at all. Some objects are even too faint to trigger the cones (https://goo.gl/49QDXu) in the eyes (the photoreceptor cells responsible for color vision) leaving us with a monochromatic view generated by the rods (https://goo.gl/987L3K) which are far more sensitive and triggered by less light.

But if we would have super-sensitive color vision, the "Pillars of Creation" would look more like the right image to our naked eyes.

And here is why:

The Hubble Space Telescope image used filters with wavelengths of 673 nm, 657 nm and 502 nm and mapped those to the red, green and blue channel. And while 673 nm is indeed a reddish color mapped to the red channel, they mapped 657 nm, another red, to the green channel and 502 nm, a greenish color to the blue channel. The resulting color image is stunning and beautiful but not like what you would see with your naked eyes.

The MPG/ESO 2.2-metre telescope image used filters with wavelengths of 651 nm, 539 nm and 451 nm and mapped those to the red, green and blue channel. As you can see those three wavelengths are within the color range of the channel they are mapped to and therefore - when combined - result in an image close to what you could see with your naked eyes. That is, if you had super-vision, sensitive enough to see colors at all.

Aren't most colors fake then?

Some people would say the colors in the Hubble image are fake, because the image doesn't reflect what it would look like to the human eye. And while it's true that it does look different, the colors aren't fake. They are not chosen by some artist to make an impressive image, no, they reflect certain chemicals within the object and therefore are based on actual data. It is just a different way to look at the object to gather more information and the mapping of different filters to the red, green and blue channels helps visualizing the distribution of certain chemicals in the nebula. So those colors are not fake, they just focus on a different part of the spectrum. An image of you in the infrared part of the spectrum is still you and "real", although it does look a lot different than a "normal" photograph which uses the visible part of the spectrum.

In the image below you see both versions of the "Pillars of Creation" and the monochromatic images used for each channel. The color of the text more or less represents the color of the light mapped to the channel.

Take a look here to learn more about colors in astrophotography:
http://hubblesite.org/gallery/behind_the_pictures/meaning_of_color/

An image of the galaxy NGC 1512 demonstrating how different the same object can look in different wavelengths:
http://www.spacetelescope.org/images/heic0106a/

More on the Pillars of Creation:
https://en.wikipedia.org/wiki/Pillars_of_Creation

The fullsize Hubble image can be found here:
http://hubblesite.org/newscenter/archive/releases/2015/01/image/c/

The fullsize MPG/ESO 2.2-metre telescope image can be found here:
http://www.eso.org/public/images/eso0926a/

Image credit: Upper left: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) http://goo.gl/213IIn / Upper right: ESO http://goo.gl/Ug9DWC CC BY 4.0 http://goo.gl/T0aSqd / Edited by +Pierre Markuse 

#science #astronomy #color #m16 #eaglenebula #ngc6611 #pillarsofcreation #astrophotography #hubble #space #hst #hubble
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