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
• 3.2: Star select
• 3.3: Star and DSO mask
• 3.4: Star image and DSO image

Separation of stars and DSO, alternative 2:
• 4.1: Separation of stars and DSO
• 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 with high contrast and still the stars not blown out, you want to keep the colors of the stars. 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 difficult to do with my earlier software but easy with Photoshop or some similar software. Here I try to do it with Gimp, it take a long time for me to find correct method, but almost there.

Much easier than what I explain here is to use the StarNET plugin filter, it can be used in Siril. One drawback is that the file must be reduced from Fits 32 bit floating point to TIFF 16 bit.

Upload the Fits file/s:

The test image I use is the NGC 7000 nebula, California Nebula. Camera is a Nikon D800 DSLR and the lens Pentax 645 FA 300 mm f/4 ED lens. Exposure 24x240 seconds at iso 400. Photo taken in a Borlte Class 4 area. It's in 32 bit floating point format. Flat and photometric color calibrated. The edges has been cropped. In Gimp I have increased the saturation, crank up the gamma and set its base level, that's all.

Working copies:

I made two extra copies of the nebula image fits file and have giving them different names for each layer.

Stars select:

First I build mask to filter out the stars. I have the Stars select layer active and visible. The NGC 7000 copy layer is also visible. Set the Stars select layer to Difference mode. The image windows is now black because both images are the same and there are no difference between them.

Gaussian blur:

To Stars select layer I set some Gaussian blur. I have the window in split mode, then I can easy see how much I have to set the size X/Y blur filter. In my case it looks optimal with a pixel size of about 6. The stars has come out from the darkness and not much background is disturbing.

Merge layers:

The two difference layers that are visible I merge to one layer. Click on the Merger icon at bottom to merge them.

The merged layer got one of the old layers name. It's a different layer now and must be given a new name, or it will be confusing.

Give the name "Stars select" that one of the layers had earlier. It tell what this layer do when finished.

One more working copy:

A new working copy is needed. Activate the org layer and use the icon at bottom to copy it.