Tutorial:
Gimp for astrophotography

Content:

First steps:
• 0.1: Introduction: why use Gimp ?
• 0.2: How to load 32-bit r,g,b images in Gimp
• 0.3: Set levels of rgb image
• 0.4: Align the red, green, blue layers

Background correction:
• 1.1: Background subtraction, easy case
• 1.2: Background subtraction, difficult case, during work
• 1.3: Background subtraction, comet & bright stars

Vignetting correction:
• 2.1: Correct image for vignetting
• 2.2: Correct for really bad vignetting

Separation of stars and DSO:
• 3.1: Separation of Stars and DSO
• 3.2: Star select
• 3.3: Star and DSO mask
• 3.4: Star image and DSO image

Alternative methods for selecting stars:
• 4.1: Selecting by color
• 4.2: Selecting by threshold
• 4.2: Selecting by difference
• 4.3: Selecting by wavelet (coming soon)

Note:
I take no responsibility or liability for what are written here, you use the information at your own risk!

3.4: Star image and DSO image

Star flat:

We want the holes in the DSO image to be filled in. I make a copy of the Star mask and rename it to Star flat.

Erode:

With the Erode filter I can flatten out the stars, it replace the brighter pixels with the darkest. If not enough, run it one more time, I used it twice, but three or four times had been better. Ctrl+F repeat the commando.

Merge DSO mask Star flat:

I merge the two active layers, DSO mask and Star flat. The brighter stars are not perfect, has to use another parameters for the Gaussian blur and Feather and more of Erode. The stars with low brightness was not removed at all from the beginning, I didn't set the parameters for that to happen. This image can now be processed with high contrast algorithm without blowing out the stars' color information.

The stars can be made of Star mask minus Star flat merged together to make a star image. The stars image can be processed with a gentle gamma function to preserve the colors.

The last step is to add together the DSO image with the star image, a simple add function.

Later I will take a look if it's possible to setup a scrip that do this, at least semi automatic.

Too complicated ? Use the StarNET function in Siril instead.

I must find out a more efficient procedure that doesn't consume that much memory, or I have to buy a more power full computer.