Posted by David Arditti at 01:40 on 2010 Sep 09
I’m not sure about this, but the problem you describe looks to me to be more likely to be due to the focuser not being square to the tube than incorrect offset of the secondary. This is a hard fault to correct, if present, as most focusers have no capacity for adjustment of how they sit on the tube. However, it does not usually lead to serious optical problems.David