Thanks for the ideas Bob & Kip. I have not paid a lot of attention to the temperature and whether it affects the shape of the wedge. It could be making a difference. I will be sure it is well cooled and experiment with the focus as I move the scope around. Is it possible to stress the secondary mirror while collimating? It has always appeared that one screw seems to be fully tightened and the two others somewhat loosened when the diffraction rings are centered? Willie Jones