Incomplete Application of Flats

[varmint]

I'm working on processing an image of M51 and am having some difficulties applying my flats.

I've attached a screenshot showing a single post-calibration light frame, and the Master Flat I generated using Vicent's tutorial.  While the calibrated light frame does appear to have reduced the affect of the optical train artifacts, I'm wondering why it didn't get completely cleared out of the light frame?

Is it a bad flat, or are there some parameters I should look to adjust during the Master Flat creation?  I only took 10 flat frames, so maybe there wasn't enough data to make a solid flat?  Also the Flats were in the 13,000-30,000 ADU range, so I felt some might be a little low, but should be in the linear region.

Thanks.

[Nocturnal]

Actually the image looks pretty flat to me. What artifacts are you seeing in the result you expected to be gone?

[varmint]

Sorry, I should've previewed the JPG before posting.  In the "flat frame" on the lower right you can see a few dust motes (donuts).  They don't show up well in the calibrated image, but they're still there, they just look a bit dimmer.

I'm attaching the combined luminance frames the little dust motes are most notable in the middle/lower.  Inspection of the flat frame has them in only a couple of spots.

[varmint]

I think I found my answer.  I'm processing my R, G, B files and finding that the focus is a little bit softer on the R, G, B's than the Luminance...  The R, G, B's are cleaning up fine, it's only the Luminance that has the donuts.

Drat.