SCNR, or Subtractive Chromatic Nose Reduction, is a process that intelligently removes color noise of a specific color. At its default settings, it removes the green cast in your astronomical images.

SCNR can be used at almost any point in your workflow – your image can be linear or nonlinear. Ideally, your image shouldn’t have any gradients on it (resolved by using DynamicBackgroundExtraction) and should be color calibrated (by using ColorCalibration) if processing LRGB/color images (do not use ColorCalibration with narrowband images).

Using SCNR

The default settings for SCNR are applicable to a broad range of images; however there are cases when the noise reduction offered by the default settings are not enough.

In case you find SCNR’s effect is not strong enough, try to change the Protection method to Maximum mask and reduce the Amount. If you still find Maximum mask too strong, try Adaptive mask or the other options for an even gentler application of the process.

The Preserve lightness option is enabled by default and should be left enabled in most cases.

If you find your image has another color cast to it, change the Color to remove option to red or blue as required.

The effect of SCNR can range from very subtle to quite strong. If you find your image has a red hue to it after using SCNR to remove green, you may have had a gradient across your image which should be corrected using DynamicBackgroundExtraction.

Conclusion

In this article, you learned about SCNR and learned how to  use it.

More Articles In This Series

This article is part of a whole series of articles about processing images using PixInsight: