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Hi, Ben,
Just back from 3 weeks of European castles and cathedrals, and hopelessly jet lagged and struggling to think straight.
In the attached, the top panel shows the very bottom end of the histogram for my M83 after stacking. (The horizontal scale is 0-4096 out of 65535, so we're looking at the blackest of blacks out here at the farm).
The point labelled with a yellow arrow is what I call the foothill of the histogram. The histogram shows that there are no pixels (or hardly any, say less than 0.1 percent) that are darker than this.
The bottom frame is after correctly setting the black point accordingly. The foothill is now almost (but not quite) hard against the y-axis. Thus we know that we have not clipped the data, but there is no milky haze either.
Your histogram looks very different. It seems to have two quite separate contributions to the dark background. It can happen if you are doing a mosaic for example, where some frames were done under moonlight. In the case of a mosaic, you fix it by normalizing the panels to each other, effectively choosing a different black point for each panel. But in your case, you are not doing a mosaic, so it is hard to see how it happened and hard to see how you could fix it. One wonders about some faint extraneous light source which is only illuminating part of the image, but with a rather hard edge. Vignetting could do it, but I don't see any evidence of that in your image.
The above comments apply no matter what package you use. The histogram indicates a problem with the underlying data at the time they were taken.
You'll have to experiment where to put the black point to produce the best results. If it was vignetting, I'd start with placing it where you indicated. I had a go using your 8 bit JPEG and it looked very promising. Should work much better on the real data.
Very best,
Mike
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