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!

3.1, Separation of stars and DSO:

When working with astroimages you will find that the dynamic of them are very high. At the same time you want to have the weak nebula perfect and still the stars not blown out, if you can keep the colors of the stars it's much better. One way to do it is to separate the stars with a mask and another mask for the Deep Sky Objects. That is something that was hard to do with my earlier software but easy with Photoshop or some similar software. Here I try to do it with Gimp, it take long time for me to find correct method, but almost there.

Upload the Fits files:

Earlier I got stuck with the comet Panstarrs on an earlier page, but I found something that could be improved which I will show here.

Upload of the three gray r, g, b images and compose them to a color image. Rough adjust the level and make a working copy of it.

Despeckle filter:

After I fine adjusted the levels I made a new copy and apply the Despeckle filter on it. This is how I did it earlier too. I have not applied much gamma here, that's why the M45 center is so weak. I wait to apply a stronger gamma until I have separated the nebula from the stars.

Heal tool:

The Clone tool I used earlier is a bit rough, the Heal tool works better, especially if there is a complex light polluted background. This tool is new for me, but looks to work best if the pencil size is comparable to the area you want to paint over. Much better result now.

Heal tool:

The Heal tool take much more data from the edge of the area you paint and fill in, very smooth transitions.

Offset layer:

One thing I noticed earlier was when I subtract the background layer I got the background all the way down to zero. That's not good, I solved it simply by add a layer with a constant of 20 for each color channel, just to lift up the background a bit. In Photoshop you can already set these figures when doing the subtraction, but couldn't find it in Gimp. Before I add the offset layer I set the fore ground color to 20, 20, 20. Fill the new layer with the 'Bucket' toll whit this color.

Offset added:

The new working copy of M45. Background subtracted and an offset added and a rough level adjustment.

Preparing for the mask:

I keep the first files, good if I need to do some fine adjustment of them. I place all the files in a sub folder, 'Stars + DSO, move the layers to that folder. Make a copy of that folder. Merge the files in the new folder to get just one layer of them. Only to have a better overview of what I'm doing.

Merging of layers:

A new window pop up when merging the folders. Set it up for your needs and click 'merge'. Note: It will merge all layers that are visible, have only correct layers open (the 'eye') !