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3.2, Separation of stars and DSO:
There are for sure many techniques to separate stars from DSO. This is just one example.
Preparing layers to make masks:
What I do here is to work with two layers, one of them I will apply a filter to soft it. The the point object like stars will spread out and extended objects like nebulas will not change much. When subtraction the levels of the stars will change but not very much on the extended objects.
I already know the principle how to do many of the task I want to do. But I must translate it from mathematics and physics to the working flow in Gimp, that take a lot of time when not used with the software. Not much written on the internet either.
What I have done here is to copy the merged layer in two copies. One layer that I will apply a Gaussian filter to and one as reference. I have gave them new names according to that.
Activate the layer with name Stars blur. Open the Gaussian Blur from Filter menu.
Set the Gaussian Blur filter size. This will be a kind of trail and error method. I start with the size 3 pixels.
Set the top layer to difference. I merge down the two layers to only have one to work with. Bit it's not necessary to do this.
Working with the Curves to get more contrast of the stars.
With the Threshold I divide the 'Star' image in only black and white star levels.
Setup the Threshold:
Adjusting the Threshold, only levels between the two markers will be white. Don't move the right marker to much left, it will make 'holes' in the stronger stars.
Use 'Fuzzy Select' tool, click on the background, the black area.