I have been going through my images taken over the years and have I been reworking many of them with image processing techniques that I have gradually learned. I found myself going back to images that I thought were "final and done" by my previous standards only to discover that I could make further significant enhancements. I use my images to study star charts and zoom them to 100% to see detail. Often however, I did not find the image quality satisfactory at 100% zoom even after stacking dozens or hundreds of images, so I sought to rectify that. Here's a technique that I learned in GIMP for noise reduction. The before and after difference in image quality is like night and day.
Make use of the Gaussian blur feature. Here's generally how to do it:
(1) If your image has layers, flatten the image. This will be your Background layer.
(2) Duplicate the Background layer and apply a modest blur (Size X, Size Y ~0.75). Call this layer "Star Blur" and place it above the Background layer. Set the opacity to 100% and keep the blending mode as "Normal."
(3) Duplicate the Background layer again and set it as the top layer. This time, apply a very strong blur (Size X, Size Y ~255). Call this layer "Sky Glow Blur") and set the opacity to 50% and keep the blending mode as "Normal."
(4) Flatten the image and apply your brightness, contrast, etc. adjustments.
Also save every version after each step as a new file so that if you make a mistake, you can go back without necessarily having to start over. I export my files in JPG, PNG, TIF and PSD formats in addition to saving the XCF files. That way, if I decide to switch software, I can switch over easily.
This example is a 100% zoom of this image, versions 1 and 3:
Cassiopeia I also applied the technique to the following images:
Altair 
Deneb 
The Andromeda Galaxy 
Sagittarius I have many more images in my library that I can improve this way.
@Steven Fanutti
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