I'm curious, are you using Pixinsight's background suppression on that or perhaps a large scale unsharp mask?

Hi Grant, I used NoiseXterminator, blurXterminator with luminosity masks when using curves & MultiscaleTransform .

Not sure what a multiscale transform is in this context, but I assume thats why you are getting a slight darkening around the SMC with the sky background at the edge of the images being brighter. Had wondered if it was galactic cirrus, but its a good way from the galactic plane and, looking at a pic I took of the SMC some years ago, I couldn't see that variation in the sky brightness, so I assume its in the processing somewhere.

Hi Grant, yes I know what you mean. I’ll have to go through the processing again. I think it came as a result of the Automatic Background Extraction. I’m going to try again without it and see how it comes out. 

Hi Grant, I've taken another look at the initially integrated LRGB image. The halo is visible there prior to processing, other then integration, I had to use the provided calibrated images, as the darks were not available. So it could be an artifact of the lens or the calibration, or real, however the halo seems rather odd. I'm having another go at the processing I'll post once it's done.

Yeah, am having a similar problem with some data I collected just before morning twilight kicked off. I was so tired I forgot to collect darks and flats..... Does make sorting out the vignetted corners hard.