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Hi Dave,
I also have a 12" LB and have made many mods in an attempt to solve this problem. I have heavier springs and Bobs Knobs.
I have found that the problem really is due to the poor design of this scope. Who in their right mind would build a truss with only three sets of struts. Four sets would have been the go. If you collimate your scope and push or pull the top end it will go out of collimation, this is due to several different deficiencies in the design.
If you look closely at the clamps that lock the struts to the bottom end of your scope you will notice that there is a round lug on the cast section of the clamp that comes into contact with the cast ring that the truss tubes locate in. This happens before the clamp has locked onto the ends of the truss tubes.
The holes in the truss tubes that the locking screw passes through are a fair bit larger in diameter than are the locating lugs that are meant to locate and lock the ends of the truss tubes.
As these lugs are to long and bear on the bottom ring before they clamp the tubes and because the holes in the tubes are larger than the diameter of the said lugs, there is scope for enough movement of the tubes to totally stuff up the collimation of the scope, just by grasping the top end of the scope and moving it around the sky.
People that don't use high magnification with these scopes, such as 6mm or 4mm eye pieces are blissfully unaware of this shift in collimation.
There is only so much that can be done to remedy this problem. I have found that by accurately measuring the length of these lugs and accurately measuring the width or depth of the struts, I was able to grind or file the lugs. To the extent that now when the clamps are screwed up tight, the clamps actually firmly clamp the struts. This has made a great difference to how well my LB maintains collimation, however if it had been fitted with four sets of truss tubes in the first place and if the holes in the tubes actually matched the diameter of the lugs on the clamps then the problem would not exist.
Well thats my rant for the day.
Regards
Trevor
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