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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!


Introduction Gimp:

If you have followed me and what I do in astronomy you have seen how I work with my astro images. I do the calibration and pre editing work in AstroImageJ software. AstroImageJ is a serious software that work with 32 bit floating point figures. A lot of software only has 16 bit integers which is not good enough in most cases. I don't want any round off errors or clipping in the image because of to limited dynamic range. The last thing I do in AstroImageJ before hand over the files to next software is to stack the aligned R, G, B files.

There is one exception, sometimes I want highest possible resolution. Then I do a Drizzling stack in some other software.

You can read here how to use AstroImageJ:

In the next step I have used the software Fitswork, with that I combine the Red, Green and Blue images in 32 bit floating point format. In this software I can also correct the colors, background, add some advanced filters and more, but it's not a photo editing software. I have not done the last fine adjustment of my images earlier, it has not been worth it because the heavy light polluted area I live in.

One problem with Fitswork is that is not developed anymore and it's very old. I have tried a lot of other software including older versions of Gimp to do the last work on the image. But have not been satisfied with any of the free software. Gimp has been promising with it's 32 float bit support, but in reality I got problem with it.

But now it has changed, the latest version of Gimp, ver 2.10 looks to be very good and I have started to use it. Very different how to work with it compare to the more specialized astro editing software. I have never used Photoshop either so it will take some time for me to learn how to use its fully potential.

Fitswork as many other software depend on DCRaw. DCRaw is used to read cameras raw files, but the development of DCRaw is shut down. No support for new cameras.


Gimp:

Gimp is free to use and download, it's a multi platform software if you don't have a Windows system. There are also plugins to it and now when it works so well with astrophotos I'm sure it will come a lot of specialized astro plugins to it.

I will still continue to use the AstroImageJ for the pre process of the images, but there is possible to load DSLR camera's raw format files in Gimp and process them directly. But if you have as I do fifty or hundred images to align and stack I don't think it's very practical.

As always you need a power full computer if you work with large image files. I have a Windows10 64 bit system with 16 GB ram memory and double SSD disc, it's not very high end today.

I have not done very much in Gimp yet, but now I understand how to load the images and do some simpler editing. Below is my first processed image in Gimp. It's the M45 open cluster, much more can be done later when I learned how to use Gimp.

M45 with Gimp software

If you search internet and Youtube you will find a few tips how to process astrophotos with Gimp, for Photoshop there are a lot more. This image I took two years ago with a Sigma APO 150 mm f/2.8 set to f/4, the camera is a Canon 6D full frame. Here are more information about M45 images taken with different lenses and locations.

My plan is now to replace Fitswork with Gimp. What I can see I can do all the things I do in Fitswork, and it's much more user friendly when doing advanced image editing. One thing that I really missed earlier was the function to separate (mask) the stars when editing the background and weak nebulas. That is easy to do in Gimp.

On the following page I describe how to load these three separate 32 bits R, G, B grey astro images to Gimp which is not very difficult (nowadays).

Note: I'm new to Gimp and its workflow, when I find better mtehods I update the page or do complement.

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