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Collimation: is my focuser out of whack?

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Been trying to collimate my 10" Newt of late. Thought I'd nailed it using a Cheshire / sight tube combo, but star test tonight showed a load of asymmetry. Checked collimation again and secondary tilt seemed out; readjusted it to get the primary centre mark on the crosshairs, tweaked primary as usual - but still no good on star test. Went back to the sight tube and noticed that the primary centre mark was off the crosshairs again. I hadn't moved the secondary mirror.

I have a suspicion that either the crosshairs in the sight tube aren't actually intersecting at the dead centre of the tube itself; or that the focuser isn't holding the sight tube (or eyepieces) centrally, causing collimation errors.

Anyone experienced this kind of problem before? Any hints before I assume I need to replace anything?



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Could it be slop in your focuser causing this ?  I found the best collimation solution I ever invested in was a solid focuser.

Whats the focuser on the scope like

Do the eyepieces lock  with a compression ring type or just a side screw ?

Do the eyepieces fit snugly or with loads of slop

Does the focuser rack in and out without slop ?

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Yeah, it's possible. The focuser is just the standard one that came with the scope (Skywatcher Skyliner 250PX Dob), eyepieces just held in place with a couple of screws. I'm thinking I should try a precision centering adaptor first; then maybe think about getting a better focuser if that doesn't work. Sounds like I'm thinking the right kind of thoughts, anyway!

Thank you


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