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Further to Doug's suggestions,after the focus tube is aligned with the centre spot, I would be looking for faint laser reflection from the lens centre, that should come back into the laser itself. (much fainter than a mirror reflection : after all, you are looking at an anti-reflection coating ;-) Maybe it is perfect : and the objective assembly is already perfectly square : in which case tilt the laser ever so slightly to find the reflection.
Some refractors have push-pull screws around the objective to enable better alignment if it is off.
Of course the laser itself must be perfectly collimated. A lathe chuck could be used as suggested, though I prefer to slowly rotate the laser in a v-block and point it at a target some meters away, to check and adjust until there is minimal movement of the laser spot.
Good luck with it,
regards
Gerhard
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