Hi,
I found out that it is not a good idea to correct mecanically exactly to the center. I always correct max 75% of what it should be and so I get better results then correcting 100% eg. putting the star exactly in the crossing of a eyepiece with a corsshair or when using a CCD camera.
Centering mecanically 100% could resuklt in overshooting. Do not ask me why
