Looking closer at your picture of the dovetail I cannot see a thumbscrew or similar to prevent the dovetail from sliding out completely.
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This is not the case with the Losmandy made dovetails, and frankly without one you are inviting trouble.
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Replacing it with a safer dovetail design would not only provide more stability but safety.
Paul
No the dovetail is very long so you don't need much pressure to keep the OTA in place.? No stop needed whatsoever.? My problem is that the scope popped out because I had it clamped against the base not in the shoe.? A stop would have made no difference, it was not sliding out, it fell out completely so it would have missed the stop if there were one.
My guess about the OP's problem is that it was the same as mine.? I can't imagine that once you have the dovetail in the shoe and tightened properly, that it would ever come out.? Unfortunately he did not mention if he was using the Vixen shoe but based on what he described I suspect he does.? In that case the stop that he put in won't fix his problem.