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confusing....?
Hi leinad - yes, it does become extremely confusing when one tries to fathom the absolute depths of collimation!
All the links suggested (including your own) are good to digest - but it can also cause added confusion which is often semantics as much as specifics. I've read (and heard) some heated arguement on these matters over the years: with people taking up positions that ultimately betray their own lack of understanding of certain aspects.
Which is to say pointedly that I'm no expert and many things still have me scratching my noodle: but I reckon that if you can get your scope to perform well with no (or very little of the) nasties like coma etc, and star-testing gives good diffraction ring images you are fairing way better than 90 odd percent of AA's!
Just a couple of things I've gleaned by re-reading the links provided in this thread and bouncing them off my own thoughts/experiences:
Bartels (imho) seems to be giving the methodology for "full" offset collimation whereas Carlin is opting for "partial" offset.
There "final View" diagrams are different, Bartels one being what you seem to be getting frustrated about you not achieving; whereas Carlin's equates with your attached image - figure where you stand there!
I repeat, these are non-expert opinions of mine but I've been led to believe that partial offset is preferable to full offset - the reason I thought was something to do with the amount of impedence the secondary creates for the primary (my presumption!)
In Bartels article there is the confusion of him stating that his "controversial advice to many amateurs" is not to square the focusser with the tube (ie mechanical collimation) giving one of his feasons as the difficulty of adjusting the focusser alignment - but in his "how-to" launched into shimming the focusser!
It would seem axiomatic to me, that Bartels' method of mechanically adjusting those 2 spider arms he mentions in his article, creates the full offset of the secondary and (possibly) creates the situation in his final collimation diagram wherein the reflection of the focusser tube is moved "up" towards the reflection of the secondary mirror (the thing you're getting frustrated about.)
Conversely, in Carlin's article (the first link Rajah's given you) he describes the partial offset methodology - in particular re-read paragraphs 4 & 5. This I presume produces the scenario in his "final collimation" diagram, which is similar to your own!
Finally, do what Rajah suggests and just get out and scope it out, if you're getting coma "comets" you'll know it and dong a star-test on a clear still night will give you more info.
I hope this hasn't been too long-winded: your problems are, I assure you, the same for many of us no matter how long we have been "at it" - it's just that most won't admit that collimation confuses them, and near-enough is good-enough.
Hopefully none of the above has made you more confused: I think it has helped me by articulating it - as I said, rajah's last bit is the most important
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