Secondary mirror moves with colimation

[Sconesbie (Scott)]

Hi everyone. I colimated my collapsible dobsonian tonight and was outside viewing. I noticed that afterwards the mirror had moved slightly and not in centre any more. It was lucky as the clouds came in and I had to pack up. My question is that any small bump seemed to move the alignment when I was colimating it and the mirror works loose a bit. Is there a way to secure it better?

[gaseous (Patrick)]

If you make sure the central bolt is firmly fixed, your collimation knobs (if you have them) are fitting snugly, and the spider vane knobs and fixing screws are all secure, your secondary shouldn't have any play in it.

[Sconesbie (Scott)]

Thank you. I hadn’t touched the central bolt at all and will certainly tighten that one. The spider ones seems pretty good but will check to be sure.

[gaseous (Patrick)]

If my scope is badly out of whack, I normally start by having the central bolt/screw just loose enough to enable the collimation screws to be adjusted. Once collimated, gently tighten the central bolt/screw just enough so everything is firm, ensuring the collimation remains intact. That central screw/bolt also determines how far "up and down" the tube the secondary mirror is, so it's a process of fine adjustments at first, then you should be more or less right with a few minor tweaks every viewing session thereafter. Good luck!