Collimation , what's that ??
Now If herd that term some where before .
I do remember what it is i think.
O that's right that what I did a long time ago when i first made my scope with the comical mirror that does loose collimation
Seriously I used an old Synta Cheshire eyepiece that came with my old synta refractor and it's only 2" long.
My newt is an f6 though and was easy to collimate.
I did a friends 12"GSO f5 and found it doable but hard with this short unit.
I also use a laser colimater as well.To check center alignments .
Interesting to note on the friends GSO the focuser was out of alignment and cocked to one side.
We needed to shim underneath it to get the laser dot in the center of the diagonal. It was out not because of diagonal off set but from side to side and by about 15mm.
Check those focusers.
I would say the tube was slighly out of round near the end and this played a part in the focuser mounting.
Start at the focuser and work back to the primary mirror.
I use my 11/4" cheshire in a 2"focuser .
It will need to be a snug fit or the cheshire will cock over to one side when tighned.
If it's a slopy fit as mine was , shim around the eyepiece with paper , but if you have to go around more than once with the paper strip make sure you stop where you start with the paper so the thickness is the same all round.
It worked for me I think
Mark