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How Galactic Extinction Effects The Color Of IC 342

Image 1


In image one, let's say the bright gray background represents the bright core of a nearby spiral galaxy, located far from our galactic plane. In this position, our view is basically unobstructed and there is virtually no color shift.

Example A shows what happens to the white light when the galactic extinction for IC 342 is added. The most changed, blue light, is reduced to 16% of its original value. The RGB values are reduced to 65, 48 and 32.


Example B shows the result from typical image processing.  With PhotoShop, the gray background was used for the luminance and example A for the color layer.


Curiously, the brightness was generally increased with the value for the red channel at 213. However, this generally shows how galactic extinction can add a red shift our images.


The below table shows the NED foreground extinction data, for IC 342, used in the above examples. We see that the blue is reduced by magnitude 2.02, the green 1.53 and the red by 1.21.





Wavelength  [um] 0.440 0.540 0.650
A_lambda [mag] 2.02 1.53 1.21


This makes it easy to calculate the intensity reduction for each color channel..

Red    = 2.512(-1.21) = 0.33

Green = 2.512(-1.53) = 0.24

Blue   = 2.512(-2.02) =  0.16


Image 2


Using G2V stars, or in this case eXcalibrator, the color in image 2 was adjusted using white foreground stars. These nearby stars are not affected by extinction. This results in an image with good star colors and the galaxy shown in its apparent color, as modified by the galactic extinction.


Image 3


In image 3, the eXcalibrator RGB factors were adjusted, relative to the blue, for removing the galactic extinction. 

The modifications were applied to the color layer.  Color bias was then removed, to flatten the background to a neutral gray.

Red    = 2.512(1.21 -2.02) = 0.47

Green = 2.512(1.53 -2.02) = 0.64

Blue   = 1.00