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Color Balance
Color is a touchy subject in astroimaging. Suffice to say for our
purposes here we just want a pretty picture. Color depends greatly on the
sensitivity to each wavelength of the CCD camera used, as well as the
transmission properties of the color filters. Color Balance is
probably best done after the Levels adjustment is made as you will now have a
better idea what the final image is going to look like. See the
Imaging Theory section for a discussion on color
in astronomical images.
Below is the Whirlpool Galaxy after Levels adjustments have been made.
Everyone's eyes and monitors will be slightly different, but to your humble
author this image looks way too green.
Select Image > Adjust > Color Balance (or press Ctrl-B) to open the Color Balance window. In this window you can
control the amount of Red, Green, or Blue present in different parts of the
image, either the Shadows, Midtones, or Highlights. In most images
adjustments will need to be made in each segment of the picture.
Sometimes, however, each portion needs different adjustments. For example,
light pollution can make the background sky too orange in color and only the
Shadows need to be adjusted.
For the example above, only the midtones were adjusted. If the
background seemed off in color, you might adjust only the Shadows. For
this image, the background seems okay, but the galaxy itself is off, so the
Midtones are adjusted. The image seemed too green so the Green slider was moved left 10%
to decrease the amount of green in the picture. Also, since galaxies tend
to be bluish overall, the Blue slider was nudged right 10%, increasing the
amount of blue.
The result is shown below.
Above: M51 after color balance adjustments.
Saturation
Subtlety is a virtue. Many celestial objects have pale colors and
increasing the saturation can enhance the image and make it really "pop" on
screen. However, some objects already have intense colors (especially
nebulae) and overdoing the saturation can make an image seem fake (of course,
everyone has their own preferences).
Select Image > Adjust > Hue/Saturation (or click Ctrl-U)
to open the Hue/Saturation window.
Note: Hue alters the overall
color of the entire image and is not usually applicable to astronomical images.
Run the saturation slider to the right to increase the saturation. More
than 20% is usually excessive, but it depends on the image and your personal
tastes and the saturation level of the original image. Some images can
really benefit from this adjustment.
Below is the result on the Whirlpool Galaxy image:
Another example is shown below. NGC253 is a colorful galaxy, but the
original RGB lacks some punch that can be remedied with a little extra
saturation.
Above: Original RGB with low saturation
Above: Enhanced image, with saturation increased 30%
Next, Curves...
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