Steve, you mentioned f/4.7... I can't see any traces of difraction spikes on stars?
If you have't already, try to use external aperture instead of internal one - with my manual Canon lenses above 100mm (and
Tair 11A) the imrovement difference is drastic in terms of getting rid of colour bloating, CA and coma.
See here, it may be helpful:
http://www.iceinspace.com.au/forum/s...ht=canon+200mm