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Tutorial:
Gimp for astrophotography


Content:

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


Separation of stars and DSO, part 1.1:

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.

Separation of stars and Deep Sky Objects, Gimp

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:

Separation of stars and Deep Sky Objects, Gimp

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:

Separation of stars and Deep Sky Objects, Gimp

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:

Separation of stars and Deep Sky Objects, Gimp

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


Offset layer:

Separation of stars and Deep Sky Objects, Gimp

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:

Separation of stars and Deep Sky Objects, Gimp

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


Preparing for the mask:

Separation of stars and Deep Sky Objects, Gimp

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:

Separation of stars and Deep Sky Objects, Gimp

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') !

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Next: Separation of stars and DSO, part 1.2

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