Color noise reduction
This second option that Nebulb has to reduce noise is generally used as a complement to luminance noise reduction (which is more effective and affects less the color of the stars), that is, color noise reduction is usually interesting to apply in those cases in which luminance noise reduction does not manage to eliminate completely the existing hot-pixels in the image.
Color noise reduction is effective in removing small color artifacts, but the drawback is that in many cases the stars are of a similar size to the noise (hot-pixels) and, because of that, color noise reduction also affects the color of the stars, making them lose some color and become paler.
Thus, it can be interesting to apply color noise reduction in those difficult images where there are a lot of hot-pixels and they are not completely eliminated with luminance noise reduction.
Below, you can see an example with the effect of both luminance and color noise reduction:
Cropping (at 400% zoom) of a region of an image with many hot-pixels | Result of applying only luminance noise reduction, in which some small color artifacts derived from the hot-pixels (red circles) are still visible. | Result of applying luminance noise reduction and color noise reduction. The color artifacts have disappeared, but the stars have also lost some of their color. |
IMPORTANT 1: Because color noise reduction generates a decrease in color in the stars, it is usually not recommended to apply it in star trails stacking, as the star trails become much paler. Generally, for star trails stacking it is not necessary to apply any noise reduction, but, in case the images have hot-pixels and you do not want them to appear together with the star trails, it is usually sufficient to apply luminance noise reduction.
Star trails on which no noise reduction has been applied | Star trails applying luminance noise reduction | Star trails applying luminance and color noise reduction |
IMPORTANT 2: In the case of an aligned stars stacking (for a photograph of the Milky Way, for example) the color reduction does not affect the stars as much, since in this case the stars are punctual (not as trails) and the color reduction effect is much less noticeable. Moreover, in the ground part of the image the color loss is not noticeable either, so the color noise reduction can be applied without any problem to the aligned star stacking.
IMPORTANT 3: Unlike luminance noise reduction, color noise reduction can be applied on any type of image (either on any type of RAW (Fuji included) and also on TIFF or JPEG images).
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