[jase (Jason)]

Dan,
You're on the right track. You've done a decent job using a PS mask to remove the radial gradient. Analysing your before and after processing shows a significant improvement, but as you indicate (and can be seen) a slight gradient still exists (see first two attachments for analysis).

Typically, radial gradients such as this are subtracted using flat calibration/reduction frames. You can still use gradient maps and post processing to remove them, but you waste a lot of processing time which could be spent on other tasks in the routine. Are you taking any flat frames?

If you *must* use processing to remove this type of gradient, I wouldn't use the blur filter, go with median when creating the layer mask to subtract. I've attached a version which I played around with. It’s not perfect (actually a little difficult due to jpg compression). The analysis show a background average pixel value count of 55 - reasonably flat. Yours averages close to 33 as the pixel value as you've made the background darker. I didn't bother with this, just performed a quick stretch with curves to see how much detail you got in the galaxy arms.

Keep at it. Nice image by the way. You may need to collect more data to better your S/N ratio. Look forward to seeing more of your work.