Pix Insight
A New User's Quick Start Experience
Applying Dynamic Background Extraction

Usually I'll use the "Dynamic Background Extraction" process on galaxy images just due to some stray light that is present around the observatory. Short of a 22LR I'm afraid that's the best I can do. I find that large Nebula images are large and bright enough not to need this process but you mileage may vary and if gradients are an issue, give the process a try. The settings will generally be dependant on your setup. I find that 30 samples on my bin 1x1 and 20 0n bin 2x2 seem to work well.

You will need to experiment to see what works for your system. You want to be sure that the samples don't fall on a questionable boarder area so the image should be cropped well and you want to be sure that the samples aren't on any stars. You can move the sample by dragging the sample to another area. You can also click to add additional sample points. When you are sure you have a good sample of the background, make sure the "Target Image Correction" is set for Subtraction and hit the check mark icon to execute. If you find problems with the resulting image you can use the "Undo" button on the tool bar to undo your change and try again. With a little practice you'll find this an extremely powerful tool. I always use the default setting except for the "Samples per Row" and setting the correction to subtraction. Placement of the samples may need re-arranging but that's pretty much it for me. Save the image such as NGC2359-11_avg_c_HST_DBE. See Harry's video for more.