Many thanks to the folks here and on Cloudy Nights for pointing me in the right direction for fixing my coma problems.
Installing a Baader MPCC Coma Corrector, and the helpful information about collimation has all but eliminated coma from my images - right out to the edge of field. If you use a DSLR for imaging in a Meade SN-8 (or I'd guess any Newtonian) you MUST get the MPCC. It didn't cost a lot.
The next thing I have to do is to acquire a guidescope. The few remaining aberrations in my images now appear to be related to tracking.
This example is an unguided image raised from a stack of 44x30secs ISO1600 with Noise reduction on. Canon 350D (unmod) in LXD-75 SN-8 with the MPCC Coma Corrector.
You can view a
larger version here and see a closeup of the
stars here.