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William Optics GT81 Focus Tube Wobble

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After an imaging session last night I was getting some minor star elongation, which at first I put down to my usual nemesis - polar alignment - but on this occasion I knew it was quite good.  At the end of taking the equipment down afterwards I then noticed some slight play with the focus tube and am thinking: (a) it shouldn't be there and (b) it's the cause of my wobble and hence comma stars.

I have just looked at all the parts and tightening up the small plastic grub screws on the outer tube seems to help - always wondered what they were for!  As far as I can see they are probably there to help damp any final play in the the tube?  However, I now see there is also a small alan grub-screw marked 'TENSION' underneath along the R&P focus mechanism, next to the lower lock screw and am wondering what this does + should I tighten it - any ideas / experience of this please or other ideas? 

Thanks, Graham 


IMG_20160827_173249555 (Medium).jpg

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Yes the little plastic screws can adjust for the slop. If you don't care for the rotating, just tighten them up all the way and secure the locking screw.  The tension allen is for the drawtube tension. You can stiffen it for heavier loads. I had to adjust my 81gtf tension when I added a filter wheel. Works great now.

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