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G'Day Brendan,
The problem is not so much to do with the f ratio or focal length of the scope but the way it is built. Just becasue you can achieve focus in one scope doesn't necessarily mean you can achieve focus on another scope.
Many people have to "shorten" their Newtonian scope to achieve focus with a camera... usually this means mounting the primary mirror a little higher in the tube to move the focal point further out of the focussing tube.
Al.
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