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Hi Ian,
Best tip from the excellent andy's hot glass site on collimation (link already posted) is to have the scope horizontal when working on secondary mirror to prevent it falling onto primary. Having said that, I have found that I can loosen the three adjusting screws on the secondary of my GSO 12' dob enough to adjust the mirror without it falling off. I always suspected that the central screw for adjusting the depth of the mirror is the one that ultimately holds it on - reassuring to know that there is a clip that does this job. Anyway, don't panic, but keep it flat to be sure!
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