[HeavyT]

Thanks all this is good advice.

I think I've figured out my acquisition problem. I'm imaging the Tarantula and went for longer subs to get more of the outer faint nebulosity. I kept the ISO low to try not to blow out the stars and more importantly the core. I can stretch the core without noise and it looks good. When I bring up the mid tones they come out and so does the noise.

So I guess I need to do some exposure bracketing - take a new stack at higher ISO to get the faint stuff up out of the noise, then mask in the core over the top?

I was worried about the core, and forgot that noise lives in the shadows. Until recently I had only been apply linear exposure stretches then raising the black point until the light pollution and noise went away. This works ok for very bright nebulae like M16, M17 etc and I've been able to reprocess those and extract more mids without noise, but the fainter objects like Helix and Tarantula are very noisy, so I conclude I've not managed my histogram well.

I'm very amateur and until doing astro never really post processed any photos, except in Instagram haha!

Thanks again for your assistance, you've helped me work out where I've gone wrong 🙃 argh learning curves!