After learning some tricks and a bit more about image processing in general, I decided to take a stab at post-processing the Pacman nebula again. I attempted some better stacking techniques, and was able to reduce noise a bit from the start. The background extraction also went much better in a newer version of Siril. I then finished the image in the Gimp. Compare the before and after:
Here are the steps I took for the redo:
- Subtract a background color from the base image.
- Decompose the image into seven wavelet layers.
- Apply selective Gaussian blurring to a couple of the scale layers that contained most of the color and contrast noise.
- Fiddle with the levels to get some better contrast.
- Change the color temperature to bring out the blue and green a bit more.
- I duplicated the result so far and applied a layer mask to the upper layer. The layer mask was inverted and leveled a bit to separate the stars from the nebula and background. This allowed me to apply a sharpening to the image that would bring back some of the detail without blowing out all of the stars.
- The heal tool was used to remove some stubborn dust motes.
- Finally, the color balance was adjusted to again bring out more of the blue-green in the center of the nebula to improve the contrast on the smaller details.
I am not an expert at image processing at all, I barely know what I am doing. But I am learning a little here and a little there, and I wanted to try to improve a few of my older images. Although removing the noise also removes some of the very, very small details, I think the compromise is worth it. The image is much more pleasing to look at overall without the distracting noise across the entire image.
My next step is to try dithering while imaging to see if I can reduce the larger splotches of color noise that persist across the entire stacked image. I’m hoping that is due to the inherent camera noise, and can be mitigated with a bit of randomisation in the imaging process. We shall see!