[Shiraz (Ray)]

Hi Graham. looks like the gradient is messing up an otherwise useful image. What to do depends on what software you have. I'm no expert, but suggest you could try:

as a first step, maybe just wind down the bottom end of the histogram using curves to make the background darker - could help make the gradients less objectionable.

Since you have tried all the calibration techniques as well as WIPE in StarTools, you might (in desperation) try a mask that protects the stars and galaxies and then reduce the colour saturation in the intervening sky, possibly followed by darkening of the sky. The only way I could find to generate the required mask in StarTools was using an automatic star mask plus "flood fill lighter pixels" to highlight the galaxies (or you could do it manually) - then invert the mask to protect the stars and galaxies and wind back the sky saturation using the colour module. Tried this on an image and it worked OK.

In Pixinsight, the dynamic background extraction tool should tidy up the problems automatically. There is also a very capable gradient removal tool in IRIS, but working out how to use it is somewhat challenging.

Any idea what causes the problem? - its always easier to tidy this sort of thing up at the source if you possibly can