I was in fact using a IR/UV cut filter. I think that the major promlem may be the fact that I did not use good data for the processing, that is to say the dark frame was not suited for the capture. I should have shot a dark frame or 2 when I was packing up, but to be honest I was pretty disappointed that the sky deteriorated so quickly and I took my ball and went home
.
I did a process without the dark frame and the image was much lighter, but the hot pixel at the bottom was very distracting. As I didn't prepare a dark frame I used one I captured last month, but it was for 60 seconds not 120 and I seem to remember that it was much cooler as well. The result from Registax was MUCH darker than I have previously got. While this seems to be causing some artifacts, it seems to have brought out some good data as well.
Probably should just shoot a more appropiate dark frame, don't you think?