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Ok. If the tube has a maximum possible length of the light path (1530mm), then with a 39mm illuminated field the tube length would have to be shorter I assume. Using the diagram from last night, the tube length should be 1530-585(the point at which 39mm illumination occurs)-150(focuser)-100(2"diagonal)=695mm (or 945mm at the point where the light enters he eyepiece - counting the focuser + diagonal as part of the tube). That seems very short for a 1530mm fl scope, or have I completely missed the point?
Regards
David
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