So I have a 6" concrete pier with rebar inside, above ground about 4' and about 2 1/2' below ground. Its holding an eq6 mount with a sw 120 triplet. Nothing fancy. My observatory works pretty well with one exception. Almost every night I have to readjust the PA just a little prior to each session. Not much, sometimes its only off about 9 or 10" from the previous night, other times I can go for 3 or 4 nights without having to adjust the PA at all. My observatory floor does not come in contact with the pier at all. I was under the impression if the mount was on a pier you set it and forget it. Is this normal, or is my pier not as stable as I thought?
Any thoughts would be greatly appreciated.
Sorry you are having issues with your polar alignment shifting. I have found that my pier, a 4-inch, thick walled galvanized water pipe, set in concrete is rock solid. The below-ground part is perhaps 3 feet deep, tapering to about 1 foot diameter at depth. At ground line the concrete expands to around 3 foot diameter. At depth the soil is decomposing sandstone, with a loam and clay mix nearer to ground level. I figure that 4 foot tall pier has around 850 lbs. of concrete below ground.
Now your pier has a cross section of 0.20 square feet [π x r2 = π x 1/42 = 0.20 ft2]. So figuring concrete at 116 lbs. per cubic foot we have:
- Below ground: 0.20 ft2 x 2.5 ft x 116 lbs/cubic ft. = 58 lbs.
- Above ground: 0.20 ft2 x 4.0 ft x 116 lbs/cubic ft. = 93 lbs.
Add weight of telescope and mount: say 30 lbs.
Total above ground = around 120 lbs.
So here you have twice the weight above- versus below-ground, with a longer lever arm. So unless your soil is well compacted around the below ground part of the pier, you can have trouble. As others have mentioned the composition of the soil has a bearing on stability. Hind sight is such a powerful tool.
Now as a possible solution, if you can remove enough of the floor around the pier for access, you might try after the fact compaction of the surrounding soil. Depending on soil composition, there is an optimum moisture content to achieve good compaction. But if you could compact the soil around your pier to a 2 foot depth, for a foot around the pier, this might help with stability. So you could excavate to the two foot depth, and lay in successive 3-inch layers of compacted earth. Hope this suggestion will be of use. Now if the change is just 9 arc-seconds, and doesn't continue to get worse, that may be OK as is.
I have carried my own pier (all 850+ lbs.) to two different locations as I moved residence. In each case I dug a hole wider and deeper than the concrete pod. This was then rolled into the hole. Next adding soil while rocking the pier to and fro brought it up to ground level. Then the 4-inch pier was braced into a vertical orientation. Lastly thin layers of soil were added around the concrete and firmly compacted. When finished all the way up to ground level, the pier was very solidly attached to the ground in compacted soil. That's how I did it. But there will be no more moves like that. It stays where it is from now on. LOL