Hi Don, as I understand it, due to the optical arrangement proper alignment of a catadioptric or pure mirror lens is very sensitive to error. As the optical path is folded upon itself so much, a slight aligning error gets magnified several times. That surely doesn't help in building a good cat lens. And it might have been also misaligned from factory or previous owner.
AFAIK I have read good things about the Tamron SP 350/5.6
catadioptric, but I never tried it myself.
Good light!
fra
