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Robin, as a matter of interest, is the movement of the laser consistently in the same plane or does it tend to be all over the place? If it is in a consistent plane, then the question is, does this correspond in any way with the weight distribution on the tube. If, for example, the movement tends to be left to right, then flexure in the tube probably isn't the culprit. If it is up/down, perhaps it is.
Peter
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