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In
image one, let's say the white 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.
The top
color bar shows what happens to the white light when the galactic
extinction for IC 342 is added. The most effected, blue light, is reduced
by magnitude 2.4 or down to 11%. The extinction of the RGB colors
changes the white core to a dark brown. |
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The
below table shows the NED foreground extinction data for IC 342.
We can see that the blue is reduced by magnitude 2.4, the green 1.85
and the red by 1.49.
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Bandpass
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B |
V |
R |
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Wavelength [um] |
0.440 |
0.540 |
0.650 |
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A_lambda [mag] |
2.40 |
1.85 |
1.49 |
This makes it easy to calculate the RGB correction factors.
Red = 2.512(-1.49)
= 0.253
Green = 2.512(-1.85)
= 0.182
Blue = 2.512(-2.4) =
0.110 |
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Obviously, first color bar does not reflect what we see in the sky. If a
magnitude zero star is reduced to 2.4, it is still bright and easy to
see. However, the RGB intensity shifts show what happens in image
processing.
As the image is stretched, the color of IC 342's core changes to
the color second bar and finally the third. Galaxy cores are often stretched
until one of the RGB colors nears saturation. In the third color bar,
this is true with the red channel. The resulting color is very
similar to what we often see in images of IC 342.
The color change, in bars two and three, is the result of adjusting the
brightness, with PhotoShop. It is interesting to note that the three
color channels were not increased evenly. In the third bar, the red was
increased by a factor of 3.85, the green by 5.06 and the blue, 6.39. |
Image 2
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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 correct star colors and the galaxy shown
in its apparent color, as modified by the galactic extinction.
The color of the galaxy core is similar to the third bar in image
1. |
Image 3
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In
image 3, the eXcalibrator RGB factors were adjusted for removing the
galactic extinction. The color of the outer arms is obviously too blue
and, of course, the foreground stars have the same problem.
However, the galaxy core has returned to a shade of pure
white. |
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