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Hi Steve
You don't mention what size mirror , focal length etc. Also depends on what sort of camera you envisage using with the scope. If an DSLR you need 55mm of extra intravel from the visual focal point. If for an CCD camera then they only need about 12 / 15mm intravel usually. So I would imagine that 20/25mm of focuser drawtube travel should be plenty enough.
Those distances get skewed around if allowing for filter wheels or filter draws as well. Then the focal plane has to further out from the tube wall to accommodate. The filter draw is a bit more friendly to focal travel than filter wheels but it is all a compromise. A lowest profile focuser design as possible while providing 20 / 50mm travel to allow for cameras, good luck.
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