These are two very nicely processed images Dave, the Cen A is particularly pleasing to the eye with excellent resolution and colour balance, very well done
I think 1000mm FL is actually pretty good for mono CCD work as you would be getting an image scale of between 1.2"/pix and 1.7"/pix with the popular CCD cameras available today, which is the ideal range really. Becasue of the colour matrix on DSLR's though your effective resolution will be less than this and yes a longer focal length would be better (say 1500mm?) I would think?
Mike