After a long time I finally decided to remove my CR-6 refractor objective lens and try to reduce the undercorrection that borders between 1/4 to 1/3 wave. The lenses were easy to remove from the cell and I marked everything including the position of the spacers. The original spacers are made of some material that looks and feels like duct tape. The thickness of the spacers is .2mm according to my digital caliper. My first try was with foil paper (about .03mm) spacers. Testing with an artificial star revealed that I had better spherical correction but the images were awash in blue. The next set of spacers were made from car trimming tape that has exactly the same width as the original spacers but with a .06mm thickness. Star tests with these spacers revealed a still improved spherical correction that was now clearly in the 1/4 wave area but the out of focus image showed the purple point besides a red point and the diffraction rings just coming out of focus more prominent on one side. Since the telescope was very well collimated, the only reason for this could be lens decentering. So, back I went and took the lenses out. Very carefully repositioned all spacers to make sure that they were all equidistant from the lens border and made sure that the crown element was as well centered as I could make it. Result... Now the blue dot out of focus is in the center with a hint of the red dot behind it. The telescope now snaps into focus and chromatism is very well controlled, in fact better than ever for this scope. There should be centering screws for all achromatic objectives bigger than 4 inches. How much three threaded holes with small adjusting screws can cost?