[codemonkey (Lee)]

Originally Posted by Merlin66

Lee,
Sounds very much like atmospheric chromatic aberration....
What was the altitude of the FOV when imaging - obviously the higher the less atmospheric effects.
http://commons.wikimedia.org/wiki/Fi...sion_spots.png

huh... that's very interesting. I wonder if that could be it. Images were captured with NGC 253 being between about ~45 and ~65 degrees.

Originally Posted by LightningNZ

NGC 253 is pretty high overhead for the much of the night. Unless these were taken not long before dawn, I'd not have thought the atmosphere would have been too bad?

Seeing was pretty rough, but I'm not sure if that's the same thing?

Originally Posted by Amaranthus

Seems mostly likely due to a slight misalignment of the chrominance channels. How are you stacking them Lee?

I'm registering them all against the same original B frame. All images are registered using PixInsight's default StarAlignment registration settings. Each colour was stacked separately using PixInsight's ImageIntegration module: Average combination, Additive with Scaling normalisation, Noise Evaluation weighting and Winsorized Sigma Clipping.

Originally Posted by Rex

Hey Lee, just a thought and it may be irrelevant, but are you refocusing after you change to the filter for each channel? If not that could be your problem because not all filters are parfocal. Like I said just the first thing that jumped into my head when I read your original post.

Good thinking Rex. I haven't been and this is one of the things I thought as well, but I was thinking the stars would be bigger / more diffuse whereas in the RGB integrated image it just looks like the green stars are longer. What I should (and will) check is the difference between a star in the red and green subs using pixel math as that should give a clearer idea.