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Originally Posted by Gargoyle_Steve
The more mass that is in the base, the less the weight at the focuser will matter. Design it so that you can add/remove additional "balance" weights if needed, or make the sphere itself heavier to maintain even weight distribution at that end. Metal sphere for instance??
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I think on the constructors site it mentioned that the centre of gravity of the ota should be at the centre of the sphere. This means that at the focuser end you have a lever of comparitively very long length working against a much shorter one of half the radius of the sphere. A small change of weight at the focuser requires a change many times that in the bottom of the sphere.
The simplest solution maybe is to weight the sphere to balance for the heaviest forseeable load at the ep end, and use removable weights there (at the ep end) to balance for anything lighter.
This is the way I went with my dob. Im balanced for my 31mm nagler, and I move a speaker magnet up the tube to compensate for lighter eyepieces.