When to apply noise reduction before stacking (preprocessing)

When to apply noise reduction before stacking (preprocessing)

In Nebulb you have two options to reduce noise in images before stacking:
  1. Luminance noise reduction
  2. Color noise reduction


But why would it be necessary to reduce noise before stacking if stacking is the best technique to reduce noise?

Well, actually this question would not be quite well formulated, since, one would have to change the "why...?" to a "when...?", i.e.:

When may it be of interest to reduce noise before stacking?

And the answer is that, depending on the type of noise in the images and the type of stacking being performed, there will be some situations where the noise cannot be removed during stacking.

For example:
  1. If all images have hot-pixels (static and invariant noise), stacking will not remove them from the ground part of the image, since those pixels have always the same value (they do not change). However, those same hot-pixels will be removed in the sky part, since Nebulb (before stacking) performs an alignment of the stars and, therefore, the hot-pixels that are in the sky area are displaced (change their position) and then they can be perfectly removed from the sky during stacking.
  2. And in the case of a stacking for star trails, the hot-pixels present in the images are not removed during stacking either, since it is a stacking based on maintaining the maximum levels of the images and, therefore, the hot-pixels will still be present in the final image.

Therefore, as a general recommendation, it could be interesting to apply a noise reduction before stacking (preprocessing) if:
  1. The images have a lot of hot-pixels (static noise) and, in addition, they have a ground part (it is not all sky).
  2. The images have a lot of hot-pixels and you are going to stack star trails, but you don't want the hot-pixels to appear together with the star trails.
  3. You are going to stack very few images and/or they have a lot of noise.
  4. You want to refine the stacking (for a specific time of night) and you want the ground to come out as clean as possible from noise.
In short, these would be the main scenarios in which it is usually advisable to select one of the noise reduction options that Nebulb has. 

But why (if it improves the final result) should noise reduction not always be applied?

When may noise reduction not be of interest?

Noise reduction, although a priori it may seem that it is something that always adds (improves the image), it may not really be so.

Thus, it is not interesting to apply a noise reduction if:
  1. The image noise is random ( there are no or very few hot-pixels) and you don't want the stacking process to take longer. Obviously, pre-stack noise reduction increases the total stacking time a bit (at least 1 second per image and, depending on computer performance, maybe more).
  2. The whole image is sky (no ground at all) and you have not used a star tracker for the capture.
  3. The noise that appears on the ground part is mostly random.
  4. After processing in Nebulb, you are going to apply some other specific software for noise reduction.
  5. You want the star trails to be as colorful as possible.
  6. The images you have imported into Nebulb already have a previous noise reduction done in another software.

ATTENTION 1: Noise reduction (especially color noise reduction) reduces the color intensity of the stars, so applying both noise reductions (luminance and color) on a stack of star trails can significantly reduce the color of the star trails (making them paler).

ATTENTION 2: Applying a noise reduction in another software (prior to processing in Nebulb) could affect the good information contained in the images (especially if the level of light pollution is high). Because of this, it is possible that Nebulb may then extract less information when applying light pollution reduction.




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