I seem to have found the source of my drift problem, although it is perhaps slightly early to tell for sure. Unfortunately, the answer raises a number of other, potentially scary, questions.
The Story
I removed the Edge 11 and mounted the Sky90 refractor. Got everything balanced, etc, ran my tests. Still drifting, same as before.
At this point I still had the 350 point TPoint model in TSX, created with the original complete Edge + refractor setup. It was still helpful in pointing, but I certainly didn’t expect it to be useful in ProTrack. I decided to rerun a model since the 350 point model certainly no longer applied to the simple refractor setup, and I was guessing that might be the next thing Daniel would ask for. I ran a 50 point model to start, and WTF? the polar alignment is WAY off, 11 tics in azimuth and 5 tics in altitude (11 arcminutes and 5 arcminutes, I think each tic is 1 arcminute). The earlier responders were right about the alignment; I had ignored that issue since I “knew” it was OK, I had just aligned 2 months earlier.
I ran several more 50 point models while I got the alignment adjusted, then ran a 128 point model for now. Alignment is “Good enough” in both axis; I will do a more complete model when I get back to the desired configuration. The drift is completely gone in all directions (East, West, and South targets). 10 minute exposures are great even without PEC or ProTrack. Hallelujah!
Questions
So, this raises questions. The prior polar alignment was done November 12, with 7 or 8 50 point runs followed by the 350 point run all run the same night. Somehow, in two months the alignment shifted that much.
1) I have occasionally bumped into the OTA or counterweight shaft while working in the small area in the dome. Would this be likely to cause an 11 arcminute shift?
2) Would typical maintenance tasks such as greasing the gears, adjusting camera positions, or rebalancing be likely to cause this?
3) Can the polar alignment be impacted by the weight of the payload/counterweights? The original system was about 50-60 pounds of OTA and cameras; I needed both counterweights and the extension shaft to get it balanced. The refractor-only setup has maybe 10 pounds of payload, only needs 1 counterweight up near the mount, no shaft extension. Perhaps the heavier system can make the pier “bend” somehow?
4) Scary – can a mount settle or shift this much over 2 months? Might it still be shifting?
The concrete base is 18 inches square on top, about 28 inches square at the bottom of the hole. The hole is 42 inches deep. The base and pier are isolated from the building. The pier itself is 8 feet long, 8 inch diameter steel pipe filled with sand. I am located in Arizona, with desert “caliche” sand/clay in a flat yard. The cement was poured in January 2012, good polar alignment was done 9 months later onNovember 12, then it is bad two months later in January 2013. Is this normal for settling? Might I hope it is done, or should I expect continuing shifting?
4) Other than running a new TPoint model, is there an easy way to verify polar alignment precisely? I expect to want to verify alignment every week or two, at least until I am confident it is stable.
My next task is to remount the Edge and verify it is working correctly. I will run new TPoint models there to see if alignment shifts. Assuming things are under control there, I will remount the whole system and recheck.