Once we stretched the image, we often want to further reduce the green and magenta tint, especially prevalent with narrowband SHO images.
This is where SCNR comes in. It allows us to remove a color without impacting the brightness. If you tried to remove the green tint with other tools like Curves Transformation or Histogram Transformation, removing the green channel will also reduce the overall brightness of the image. With SCNR, we can remove the green tint while maintaining the lightness.
The SCNR is the Subtractive Chromatic Noise Reduction. Basically, it will remove a color from our image without reducing the overall brightness.
Note: See the Tips section below for information on the amount of SCNR to apply to the image.
This makes a huge difference in the image, but I need to take are of the magenta stars as this is an uncommon color for astro photos. What is wonderful about magenta is that it is opposite from green on the color wheel. This means we can invert our image and run SCNR again
When to do SCNR
One of the things I’ve seen is that many people recommend doing SCNR in the linear state. However, I’ve found I get better star colors when SCNR is done after the image has been stretched.
Which one is better? I prefer the SCNR done with a non-linear image. I believe the star color is richer.
Leaving green is often overpowering due to the strength of the hydrogen-alpha channel. However, removing 100% of the green channel will also leave less color variation in your final image. Green helps create a nice transition between blue and yellow. Without a green tint, the transition becomes much more harsh in the final image.
It might be useful to lower the percentage SCNR uses on the green channel. This will leave some green within the image. Take the following example of M16 – The Eagle Nebula.
This becomes a personal preference, but for me, I prefer the 50% SCNR application. The colors can be further optimized further along the processing flow.
It often helps to see what the image will look like in a future stage. To help, I’ve gone ahead and done just that. Here are three images further processed.
I personally prefer the image with SCNR at 50%. I feel like the green adds a nice transition between the oranges, yellows and blues. But because these are not the real colors, you are free to follow your own path.
Color correction made a big improvement to the image
With the green and magenta subtracted, the next step is focus our attention on a separate luminance channel to help bring out the brightness and detail of our image. The first step is to stretch luminance.