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Could weather factor in? The pier expanding contracting differently to the concrete its in? Might be some room for the heavy load to expand the cavity its in then settling its center of mass diferently, water drainage in the soil? Just thoughts heavy traffic nearby shaking and settling the ground. Got a laser distance meter? Maybe you can measure the distance from the topside of the pier from the four directions out to the walls and track the distance to see if its moving and by how much or are you talking to a minuscule amount through the scope? Can you use the scope to measure and track the amount its out to make sure its only a small amount random in a circle or if it is leaning further out? The bottom of the pier would be experiencing a large lever action force. Its most likely every tims you place your rig on the pier its not in the precise position everytime, If your mount just slips over fours bolts there is a huge gape there of each hole and bolt, might not seem like much but its a source of alignment play even if its small but its always there. Bolts can also stretch with the stress too are the mating surfaces painted or greased look for everytime that can be squished or bent or wiggled in any way, close enough is not good enough. Also can you do anything to keep the setup in place, thats the purpose of a proper pier isn’t it?. My instinct is pier construction , thermal expansion, if the mount it taken inside it will be a different temperature to the pier and during a session the thermal expansion differences I would expect show in alignment from start to end of a session. Test yourself your process, set up take image at default “aligned”position without moving anything. Then take it down and put it back up again and take another shot, theoretically should be identical but my guess is you’ll see different just in that alone, meaning the parts have enough give in them to through it out even if you think they dont. (Ie you are condensing your imaging session task to setup and shutdown only not doing any alignment or moving anything, likewise setup and take image then leave setup for a day or two take same image again should be identical still, but if pier tilting it’ll be a lot out, might still be issues with thermal expansion too. Experiment and try to find if theres a problem but most likely suspect is you pulling down and setting up as its a big change you make every time. Its impossible to pick something up and put it back precisel the same, people are horribly slack and imprecise and you’re creating movement thats not meant to be normal operation so its most likely the culprit. Theres just so many factors that could be all adding up. Whatever you can test you can then address and reduce or remove that factor. I was able to retain great alignment with my cgem setup by spending the time locking off things i never needed to move so when fitted together were physically limited in precise position and parts that moved I used masking tape across moving parts with a thin line marke so I could orient them accurately each time I pulled down and each time I started everything was in pretty much the same spot, not precise but as precise as I could get and plenty for my needs, the balancing became the biggest variable for my imaging.
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