Star collimation question.

I've carefully collimated my 8" f/6 (using a sight tube and cheshire) and subsequently checked this with a holographic laser, and both agree. All looks good, secondary centered under focuser, etc, etc.

Then on a bright star overhead at ~270x, defocused star on one side of focus shows misalignment, but not awful. Central obstruction is definitely not centered, though. However, on the other side of focus it is really bad, central obstruction is way off to one side.

I've looked through lots of star collimation resources, none of which seem to discuss differences in inta/extra focal images. Any thoughts? Thanks for the help!

-Tim

Tim,
How are your tolerances in your focuser for your eyepieces? Is there any play? My 8" Newt low profile screw in focuser had a tight fit with the 1.25" eyepiece adapter fitted, but with the 2" eyepieces the scope would seem to show miscollimation spikes. I quickly noticed that upon tightening the set screw the eyepiece would kick over. Just another thought...Brett

I'm no collimation expert, but when you collimated with the Cheshire and laser did you have the tube pointing nearly straight up, as you did when using the "star overhead?" I wonder if the mirror (or maybe some other component) is moving a bit as you change tube orientation.

I would guess your focuser is not square to the optical axis. (or, your collimators are not being held squarely in the focuser). Just a thought