On a related collimation issue... I have a question: If collimation appears lined up well when looking through the focuser (I have yet to star test because of weather), but putting in a laser collimator I get a de-centered laser beam (by about 1.5 inches), can I assume that my focuser tube is not quite aligned?
It is entirely possible. One way to check that is to remove the spider and make a mark on the inside of your OTA opposite the focuser. The easiest way to do that is tape a piece of white paper to the inside of your scope. Then take a strip of paper that wraps all the way around your scope. Mark where the paper strip overlaps itself, which will mark the "beginning" and "end" of the paper and give you an accurate measurement of the circumference of your OTA. Then, holding your two marks together, fold the paper strip in half to give yourself an accurate measurement of half the circumference of your OTA. Then, starting at the center of your focuser, hold/ tape that strip to the inside of your OTA. Then you'll have a mark opposite your focuser. Finally, measure from the end of your OTA, parallel with the length of your OTA, to the center of your focuser and transfer that mark to the opposite side as marked by that strip of paper. Now you have a mark, or target, 180o opposite your focuser. You can now use your laser to help you adjust your focuser until the laser hits that target. Reinstall the spider and recollimate your scope as normal.
When I give the focuser tube a little pressure from one side (from the ground side, pushing skyward when the scope is aimed at the horizon), it pushes the laser more toward the center of the primary-- AND when pushing like this the beam on the secondary gets pushed more toward the center of the secondary. It's definitely not hitting the center of the secondary by 3/8th of and inch. And it's not off center on the optical axis toward the mirror or away from the mirror, but is off center toward skyward when the scope is horizontal. Before I start shimming it, I considered the laser may itself be off (I have yet to use it on another telescope, as it is second-hand and came with the dob I'm discussing here. I don't have another one). I even rotated the laser inside the focuser to see if it spiraled around, but it seems pretty dead on and not floating or moving or arcing anywhere no matter what orientation it is sitting within the focuser.
Sounds like your focuser has a sloppy fit between the body and the draw tube. I had the same problem with the plastic focuser that came with my Orion XT6i. Just like you're contemplating, I shimmed it with plastic sheet from a local hobby store and solved the problem.
Here's my action list, play by play.
1. With my eye dead center of the focuser, I have aligned the secondary mirror so I see the primary perfectly centered. Like I said, no cheshire, but I've got a decent eye for that sort of thing.
2. Then, I aligned the primary so it is aimed squarely at the secondary. I got the central mark to appear dead center on the secondary.
Everything appears nicely placed concentrically set out from there-- BUT I throw in the laser collimator, and it's 1.5 inches off from center.
I think your theory below is correct: you're sighting down a "crooked" focuser. Besides, even with a "good eye for that sort of thing", a laser shines a straighter line than you can hold with your eye.
The first time I did the above procedure, I backed off from my eye-balling confidence and did the procedure only using the laser-- from aligning the secondary so it centered the laser dot on the primary, and then aligning the primary to center the reflected laser dot back into the collimating eyepiece target. BUT, when I pulled out the laser collimator after using it to center everything, looking down the focuser tube clearly shows misalignment of both the secondary and primary. Pretty heavy misalignment.
That's the correct order for collimating a Newtonian. If you could post pictures of what you describe as "pretty heavy misalignment" it would help.
I'm thinking the only thing left is the angle of the focuser tube pushing the laser beam off kilter, whereas my eye isn't affected by the slight angle of the focuser tube.
Another clue-- the laser aims and strikes off-center of the secondary, actually in the skyward side
Anyone experience this before?
Thanks for sticking with me on that run-down!