a little help please

[Strider_]

I have been collimating my 12 inch dob using a saxon laser collimator for about 12 months now. I watched a video on youtube showing my how to perform collimation. I had been turning my scope horizontal when doing this aligning the secondary then the primary. All was well... so I thought, my views to the best of my knowledge had been fine although I have nothing to compare it to? My issue is, when I last collimated my scope I turned it verticle with the collimator still in the focuser, the collimation was no longer centred on the target, the more verticle I moved the scope the more the dot moved. The collimator was secured tightly and could not move. The primary lock knobs were also secured tightly, as far as I can tell the secondary is tight also. Any ideas people? Rubbish collimator? Loose mirrors? Any help would be greatly appreciated.

[Visionoz]

Probably a loose/sloppy focuser unit - check it by holding the end of the collimator gently and try to push it very very slightly - if it does move then there should be a tensioning screw/knob underneath the focuser that you can tighten (not too tight though) which will stop the slop - and then collimate again

HTH
Cheers
Bill

[dannat]

Is it a solid tube or truss style?

[Robh]

Hi Dylan,

As Bill mentioned, it could be focuser slop. Try winding it right in, insert the collimator and see what happens.
Daniel's question. I think he is implying there may be some lack of rigidity if your scope is a truss type. I have a Meade truss dob but haven't found this to be an issue.
You mentioned the issue occurred even with the locking screws fastened.
The primary mirror is also held in place with 3 clips, usually plastic. The advice usually given is to have the mirror only loosely held in place by the clips so that warping doesn't occur. In fact, sometimes they are too loose, so that the mirror will shift about from a horizontal to a vertical position. Your main problem is collimating the scope in a horizontal position. Presumably to prevent a screwdriver falling down the tube. I always collimate near vertical to reduce the possibility of primary mirror slop changing collimation. Normally, there are few objects that you will view below 30 degrees.
A set of Bob's knobs for the secondary mirror will eliminate the issue of dropping a screwdriver down the tube while collimating.

Regards, Rob

[Brian W]

And I take the Goldilocks theory into collimation and collimate at 45 degrees.
Brian