Wow have things changed!
This is the improvement from using way more
images in the stack and from using a 1250mm focal length SCT compared to a 600mm FL refractor for the previous M51 images.
This is an even greater stack of 21 images
now using a Nikon D5000 which has 12.2 Megapixel sensor and improved noise reduction Although I took dark frames, I didn't use them in the stack.
M51 with an 8" SCT and focal reducer at F/6.3
this is a stack of 21 images. The more images you have to stack, the smoother and less
noisy the images becomes.