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, alternative 1:
• 3.1: Separation of stars and DSO 1
• 3.2: Separation of stars and DSO 2
• 3.3: Separation of stars and DSO 3
• 3.4: Separation of stars and DSO 4
• 3.5: Separation of stars and DSO 5

Separation of stars and DSO, alternative 2:
• 4.1: Separation of stars and DSO 1 (during work)
• 4.2: Coming Soon

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

3.4, Separation of stars and DSO:

Test of background mask:

Set 'DSO mask' and 'Stars + DSO copy' visible. The mask has made 'holes' were the stars are. They look a bit big, but first do a test before go back and do changes.

Later I shall fill the 'holes' with the color from the surrounding edge.

Add mask to 'Star mask':

Change the visibility to the 'Star mask' layers group. Right click on 'Star mask', choose selection AND invert the selected area.

Test of 'Star mask':

The mask work correct, but look a bit big as mentioned earlier.

Overview of the mask selection area:

Zoom out, the area around the Open Cluster M45 is very cluttered of stars. I have to do trail and error method to find the correct size of the mask 'holes'. It vary a bit for every image dependent of camera, telescope and object.

Note:
What I do here is very rough, a lot of tweaks to get it perfect. Just the basic so you can start with Gimp.

Star Mask copy:

I make a copy of the 'Star mask' too have as a fill area for the DSO layer.

Erode:

These areas are to use to fill the holes in the DSO mask later. I want them as a flat area. With the Erode tool I expand the blackest area in each star hole. Menu: Filters>Generic>Erode. Note: I have the layer 'Stars + DSO copy' active !

Erode:

This is the result of one erode, the stars is still visible, run Erode once more. You can use 'Ctrl' + 'F' to repeat the command. Totally I did two Erode.

DSO with filled holes:

The result, with both Star and DSO mask activated. The bright stars are gone.