I've been trying to finish collimating my 8" home-built dob (commercial mirror), and I've discovered that the autocollimator is extremely sensitive to wobble in the focuser. I've got a helical focuser from Novak, and as I rotate the focuser, the autocollimator passes through both bright and dark stages. During the dark stages I can see the image of the spot on the primary that is reflected back to the secondary by the autocollimator, and it passes through the field of the secondary like a satellite passing across the sky.
I put a bit of teflon tape around the autocollimator to avoid using the screws in the focuser/adaptor; tightening and loosening those screws causes visible change in the autocollimator image. But even with a snug friction fit of the autocollimator, I get the same behavior. If I stick the sight tube in, and then look down the length of the telescope, I can see wobble in the angle of the sight tube as I rotate the focuser, particularly in one spot in the rotation. (The 2 - 1-1/4 adaptor seems pretty snug in its own right; I haven't tried teflon-taping the sight tube yet.)
Is there any way of getting my helical focuser to stop wobbling? I'd really like to get my rig to where the autocollimator will be steady enough to let me zero in the alignment.
What can I do, what should I check?
Thanks to all,
Ben