The 1" truss tubes are a bit too light (I use 1.25" poles on a 12.5" f/5 because I saw a difference), but that's not the cause of the problem.
When the scope points up, the secondary hangs below the spider and imparts no twist to the vanes.
When the scope points low, the secondary hangs beside the spider and imparts a twist to it.
A 3.1" secondary is not that light, if you count the weight of the holder.
The cure is either easy or complicated.
The easy cure is to tighten the spider vanes until they are very tight. The tighter they are, the less laser movement you'll see.
The harder cures come in many forms:
1) replace the spider vanes with the heavier ones that have 2 bolts per vane.
2) move spider vanes down in scope so secondary hangs much closer to the spider vanes.
3) Shorten the secondary holder so there is no space above the upper end of the secondary in the holder.
4) replace spider with a new design that looks like this: >o< You still get 4 spikes but the stiffness is much higher.
5) add a small counterweight to the center bolt above the spider that balances the weight of the secondary + holder.
6) Make sure the spider vanes, from the front of the scope, appear like a "+" or a "X" and are not joined at an angle. When the spider is joined at an angle, unless everything is heavy and stiff,
you will get collimation movement with altitude change. Rob Teeter experimented with this and found this to be true. Additionally, the collimation change has a lateral component to it, not just vertical.
All of this presumes you have:
--tightened the nut on the center bolt until it is very tight.
--filled up the space in the spider the center bolt passes through. I wrapped the bolt threads with plumber's tape. Others have used toothpicks between the center bolt and the spider.
But there should be no looseness whatsoever of the center bolt as it passes through the spider.
--made sure there is enough stuffing behind the mirror to press the mirror against the lip of the holder, even when the mirror is on its side rather than when gravity pulls it down against the lip at the zenith.
There should be no looseness here or collimation will shift as the scope points lower.
If the trusses bend, the UTA will drop. The likelihood is that the secondary will rise, causing the laser beam to go UP on the primary.
Since it goes down, I vote for secondary sag.
In the future, since you have 1" poles, I'd vote for collimating at a 60° angle so the error at low angles is reduced.
Edited by Starman1, 28 December 2018 - 12:09 PM.