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You will need to move your primary mirror up the tube a bit. You're outside the focal point. The solution is to move the mirror up maybe 20 mm or so. Is this a solid tube DOB ?
A low profile focusser might help instead.
This situation is often seen on DOBs when folks attempt to do DSLR photography. The Focal plane of the camera cannot be brought in close enough to reach focus.
On my original truss scope build I just chopped 25mm off the struts. May sometimes mean you need an extender for some EPs and if the secondary is too small you may lose a little light but not a problem for visual.
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