Focal Pointe Observatory
Astrophotography by Bob Franke

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Correcting Galaxy Color


 

The above image, of NGC 6946, shows a typical color shift to red. Because NGC 6946 is at low galactic latitude, we view it through a lot of dust and various nebulae. This causes the light to become reddish, just as the Sun is red at sunset. The effect is called Galactic Extinction.

Using G2V stars, or in this case eXcalibrator, the color was calibrated using the foreground stars. These nearby stars are not affected by extinction. This results in an image with correct star color and the galaxy shown in its apparent color.

There is nothing wrong with this image, as it shows the galaxy approximately as viewed from Earth. However, we may prefer to represent the galaxy with its intrinsic color. The following tutorial shows two ways to correct the color. The first uses galactic extinction data, from the NASA/IPAC Extragalactic Database (NED). The second method corrects the color by balancing the histograms for the red, green and blue channels.
 

The NED Correction Method

The below table shows the NED foreground extinction data for NGC 6946. We can see that the blue is reduced by magnitude 0.342 and the red increased by 0.219, relative to the green.

Bandpass

B

V

R
Wavelength  [um] 0.440 0.540 0.650
A_lambda [mag] 1.475 1.133 0.914

 

This makes it easy to calculate the red and blue correction factors.

Red = 2.512(0.914 - 1.133) = 0.817

Blue = 2.512(1.475 - 1.133) = 1.370

 

Correcting the color with PhotoShop

  1. Copy the image to a new layer. All modifications are made on this layer.

  2. Use your favorite tool to select the stars. I use Noel Carboni's PhotoShop actions or Russ Croman's method.

  3. Now cut out the stars. This preserves the star colors in the base layer.

  4. With the lasso tool, set the feather to 50 or 75 and select just the galaxy.

  5. Use the Channel Mixer to set the red channel to 82% and the blue to 137%.

  6. Finally deselect all and flatten the image.

Here is the result. 

 

In this case, the NED correction has produced an image that is too blue. This may be for one or two reasons. First, the NED data is not correct... sometimes it is not. Secondly, the original image is incorrect.

 

Correcting The Color With Balanced Histograms

 

This method works well with nearby spiral galaxies. This type of galaxy generally contains a variety of stellar types. Therefore, it is a reasonable assumption that the sum of all the light will be white. This is the basis for correcting the color by balancing the black and white points for the three color channels. Here's how to it.

 

First, repeat the above steps one through four.

Here's how the histograms look for the three color channels in the original image. We can see that the blue and green already match each other quite well. All we have to do is adjust the red.

Applying a single point curve, using an input of 255 and an output of 205, brings the red histogram in line with the blue and green.

Here's the result of the above curve.

Unfortunately, severely reducing the white point of a channel will leave the image darker and lifeless.

This curve was used to stretch the three histograms into a profile similar to the original image. This restores the image's brightness and contrast.

The same result may be possible by simply adjusting the brightness and contrast.

Here is the profile of the final image.

With this image, only the application of a single curve aligned the three histograms. Usually the process requires using both curves and levels. With a little creative practice, it doesn't take long to match up the three channels.
 

 

Here is the final flattened image.