I’m putting together some new “C” series lens tubes with a couple different focal length lenses, as well as replacing my M1 and M2 20mm mirror mounts with 25mm ones. This is being driven by the installation of Russ Sadler’s lightweight head, which uses a 25mm mirror and the 20mm lens tube. I have seen various diagrams on the placement of the O-rings in the tube. Some show it between the lens and the tube, others show it between the lens and the locking collar, and sometimes there’s no O-ring at all. In addition to the lenses, I’m putting in new mirrors & new mounts, and I have the same dilemma. Should the O-rings go between the mirror and the mount, or the mirror and the locking collar? Or not use them at all? I feel that it would be best to put them in, I just want to make sure I do it correctly. Any input would be greatly appreciated.
This is the 1.5" lens/tube and the lens retainer, right. If you notice it has it’s own teflon (I think) o-ring… On the left is a close view.
The one in the back is for the compound lens and I buy o-rings at the local hardware store and cut them to fit… they aren’t in the beam area…
In the right photo, the tool is nice to have… 
Did I miss anything?
So if I’m picking up what you’re putting down (which isn’t always easy for me) you put the o-ring in first followed by the lens and then the locking collar. I assume that would also be the case for mirrors as well. And yes, I did get one of the locking collar tools, I ordered it along with all the other parts for this project.
The Teflon piece is bonded to the screw in part… Down in the tube it’s just the edge of the tube…
I’d think you’d want it between the lens/mirror and screw in part as that is moving when you put pressure on it…
That’s how the Teflon part puts pressure on the lens…
I don’t have o-rings on my mirrors… they just sit there.
Only the compound lens is there need for an o-ring.
Probably wouldn’t hurt with the mirrors, but you want it to be stable with respect to the lens tube… so between the mirror and mount would be a poor selection IMHO…
Thanks Jack, you always manage to bring clarity to the subject at hand. Makes perfect sense.
I’m sure if you think about it, you’ll come to a reasonable conclusion.
A couple things I learned from Russ…
If you are having trouble with the end product figure out what the beams energy actually is doing to the material.
And to think about what’s happening, in most instances it’s simple and logical.
Plus you can get plenty of good feedback here… 
Take care…