Thanks Paul. I thought about the interference too and will check for that next time.
I also agree with the colour - it came out very blue and i'm not sure if it's the white balance settings on this modded camera. I'll check the colour temperature and maybe re-stack. I did notice the lack of orange stars, especially compared to my last image of M42 which was over 1.5 years ago.
Yes - it's been 1.5 years since I imaged this nebula so I thought it was time for it again
The last time was before IISAC2008 in October 2008..
Thanks again.