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
• 4.3: Selecting by difference
• 4.4: Selecting by wavelet (coming soon)

Processing a nebula with masks:
• 5.1: Setup masks
• 5.2: Editing nebula
• 5.3: Editing nebula with holes filled (coming soon)

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

3.3: Star and DSO mask

Star mask:

I start to make the Star mask. Right click on the NGC 7000 copy layer and choose "Add a Mask to the layer". Set Selection and then click Add.

The mask has now selected only stars, not all but the brighter ones. I renamed the layer to Star mask. When I tried to get more of the weaker stars the computer got over loaded. The camera has 36 million pixels and that consume the RAM memory. I have 16 GB RAM and 256GB extension memory in the form of a SSD. Once a lot of memory but today on the lower limit.

DSO mask:

Now we want another mask to the DSO, the nebula. Make a new working copy of the NGC 7000 original file.

Right click on the layer and do the same as with the Star mask, but one difference, set Invert mask.

Now it's the surrounding to the stars that is masked out.