If you shoot a lot of astrophotos, you know all about gradients. They are almost always present in your images, and they have to be removed to smooth out the sky background and make your images look their best.
Gradients are caused by light pollution, a bad flat frame that hasn't cleaned up your optical system, vignetting, and even natural brightness gradients appearing in the night sky.
There have been some good software-based solutions to getting rid of gradients, such as the DBE and ABE tools in PixInsight, a paid image editor. There are also some excellent plug-ins for Photoshop and Affinity Photo from RC Astro, which are excellent. I use them all the time.
Now, many nighttime imagers are praising this free open source software called GraXpert. From my evaluation, it works as well as the paid options, and as a bonus, gives you the option to save an auto-stretched version, which will save imagers some time and effort.
Here's an example of the galaxies M65 and M66. I've loaded the file, which is in FITS format, but you can load TIFF or even JPEG files. You're better off with the best image formats for dynamic range, which would make JPEGs not as desirable.
Here's the image before any gradient work:
You can see the uneven reddish coloring of the background to the left, and it's even more obvious to the right of the image.
You hit the "Create Grid" button on the app and this appears:
The software will try to put markers or what the software calls sample points where the background sky is. It avoids the galaxies and stars. It's just looking for background sky. Those markers can be removed with a click or dragged to another position. I noticed the software had missed some areas to the upper right and needed some more at the extreme left edge, so I added them. If the sample point winds up in part of a nebula or part of a galaxy, just move it or delete it.
Then, when you are happy, you press the "Calculate Background" button, and your gradients should be nicely reduced.
It did a really nice job on this image, although there's still a trace of red tint to the left.
I've tried GraXpert on several images and gotten good results every time. I'm also happy to see the software evolving. Now, you have the option to save the image that GraXpert created in a stretched format. I still had some work to do in Photoshop to get the image just right, but the auto stretch is a useful and time-saving feature.
Here's one I processed of the Veil Nebula. GraXpert reduced some nasty gradients in the original, then I did some star reduction and color balancing in Photoshop.
As I said above, GraXpert is not the only software that can clean up astrophotographs, but it's free and works quite well. It works on Windows, Mac, and Linux. It's not a plugin, but a standalone app. There's a video tutorial and a PDF manual, and the software is easy to use.
It's not a complete editing solution. Once you clean up the gradients, you'll want to be in another editor for color balancing, noise reduction, sharpening, or whatever your image needs.
It's available on the GraXpert website, and I'd love to know your impressions.