OK here are 14 frames of Corona Australis @ ISO400 @ 240 sec each and converted from RAW to Jpeg using the Canon proprietary software. Dark framed subtracted with Jpegs. No flat field so you can really see the vignetting.
Number 1 image is straight from DSS and into Photoshop. When it came out of DSS it was totally white, I couldn't see any stars. When I opened it into PS it looked similar to other preprocessed TIFFs I have opened in PS. The size reduced and quickly leveled and curved processed image (number 2) looks "sort of" ok (except I haven't colour balanced it) at first glance, but it really is bad. The third image is a crop about 2/3 the way out to the top left corner and the fourth is from near the center. You can see the problem in the center one. The dark subtraction didn't work properly and dead pixels were smeared during the alignment and stacking process. It is generally a pretty grubby image. There are also a lot of artifacts that screwed up the registration processes and therefore the stacking process.
You can see the
same image here using RAWs rather than Jpegs. And here is a
crop of the same corner but a bit further towards the corner