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When you say you tilt the secondary up to get the laser dot centred in the primary donut, do you mean you are tightening one of the secondary collimation screws? Tension between the screw and the mirror stalk should not normally work loose. One trick people use is to place a large nylon washer between the screws and the mirror stalk. The screws torque into the washer, then push onto the back of the mirror stalk. I find this holds the collimation more permanently on the secondary. You can also make a washer by cutting a disc out of a plastic milk container.
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