Sometimes, when using filters or because of light pollution, you end up with some color cast in your astrophotography images.
A color cast is simply an unwanted tint that uniformly affects your image, and there are many ways you can remove it. Opposite Color Neutralisation is a simple technique you can use to remove the color cast.
To illustrate how the technique works, I added a greenish color cast to this wide field of a busy region in Orion Constellation: M42, the Flame and Horse Head nebulae as well as M78 are clearly visible, with lots of cosmic dust.
Overall a nice image, I’d say, if not for that color cast.
A busy star field in Orion Constellation. Olympus E-PL6 with Samyang 135 f/2 on Sky-Watcher Star Adventurer Pro.
To make the greenish color cast more visible, I boosted the saturation of the image to “insane” :)
By over saturating the image, the color cast on the dark sky is clearly visible.
So, here what we can do.
Duplicate the level with cmd+j (Mac) or ctrl+j (Windows);
Invert the new layer with cmd+i (Mac) or ctrl+i (Windows);
Go to Filter -> Blur and choose Average … to create a solid color that is the average color for the image;
Set the blending mode for this layer to Color;
Use the Opacity slider to about 50% to tweak the strength of the correction;
Boost the colours with a saturation layer;
Optional: you can create a layer mask to selectively apply the Opposite Color Neutralisation to specific areas of your image only. For this I like to create the mask using Select -> Color Range …
The workflow in pictures…
As you can see in the comparison below, the green color cast is gone.
Opposite Color Neutralisation: a simple, yet powerful technique.
As I said in the beginning, this is only one of the many ways you can use to remove unwanted color casts and even chromatic aberration around the stars.
What I like is that is easy to do, quick and with combining the use of layer masks and level opacity I can quickly and precisely tune the correction.