Hi all.
I know most of you don't use refractors, especially...achro refractors for astrophotography like I do but I found this pretty interesting.
I've spent a
great deal of time collimating my 150mm refractor until it's as close to perfect as I can get it.
The other night after setup I decided to do a few experiments & one of them was to inspect my collimation by way of toucam & laptop on a star.
After getting canopus on the laptop I defocused to see if I could see any diffraction rings, & after adjusting the toucams brightness/gain etc etc they were as plain as day. No central obstruction with a refractor as you know but there is a central round 'blob' of light in the middle of the diffraction rings to go by to see if collimation is out. (Airy disc?) This central blob was not quite in the middle I noticed. It was perhaps 2mm from central.
I took the dew shield off to get to the objective cell push/pull screws & after doing the 'stick finger over front of scope to work out which screw to tweak' got a screw-driver and did so.
After turning the appropriate screw about a 16th of a turn I noticed no (laptop) visual difference....I turned the screw a 1/2 turn....no difference......

turned the screw THREE full turns

........NO difference! took avi of saturn & stacked/processed/reviewed.....looked normal to me!
Conclusion?....

SURELY by turning that screw as far as I did would have thrown collimation way out?
I should have taken photos of the collimation while I was at it & next time I will.
This has me thinking I know LESS about refractor collimation than when I first started out.

I'm thinking this has everything to do with the draw-tube not being 'square on' & less to do with the objective being 'square on'..
Interesting..