Re: Changing polar alignment...Ground shifting?


Dale Ghent
 

Ouch. If you really want to know if it's your pier then you can get a digital inclinometer and periodically measure the pier on two axes to get an idea as to what's going on.

I'm going to auger out and pour a pier footer in the upcoming spring and I've been taking heavy notes from a good thread on the CN observatory forum called "Pier Engineering":

https://www.cloudynights.com/topic/652025-pier-engineering/

The basic takeaways are from the author's experiences are:

1. Auger, don't dig out, the hole for the pier, and remove all loose material from the side wall and bottom
2. Pour the footer into the hole, using the hole itself as the form. You can use a sonotube form or whatever for the top several inches to give it a finished aesthetic above ground, but for the majority of it you want direct contact with the surrounding compacted soil.
3. Mind your frostline
4. A bunch of other things that should be considered aside from basic hole digging and concrete pouring

The reasoning is that you'll have concrete directly in contact with the existing undisturbed and compacted soil instead of loose fill. There is also no sonotube that will ultimately decay and leave voids between the footer and the surrounding soil. These voids invite shifting, and the lack of loose fill surrounding the pier means that it will be better supported and more stable.

I'm no soil engineer but it makes sense and, in my case, I really have to nail it on the first try because the place where I want to put a pier is the only location in my yard that I get the most sky... so a do-over would mean a less ideal location, even if it's just a few feet to the side.

Hope the cause of your tilting has an easy solution.
/dale

On Dec 29, 2020, at 22:04, Tom Blahovici <tom.va2fsq@videotron.ca> wrote:

Hi
I thought I would ask the experts here about my shifting polar alignment.
5 years ago I rented a backhoe, and dug a hole 7 feet deep in my backyard. I then poured a concrete pillar in a form 3'x3' by 6 feet deep. On this I have a paramount pier.
All the earth was filled in and I have used this mostly over the winter. Last year all was fine. This year I had about 1 minute of arc drift in each axis using pempro.
Recently I started noticing that my stars appeared trailed during long exposures and autoguiding was acting up. PHD 2 told me my polar alignment was off by 4 degrees. . So today I checked my polar alignment with pempro and sure enough, 4-6 degrees off in drift. I have put it back to within 1 minute.
I thought I was golden with such a pier. Recently we had a lot of rain and then the temperatures have been going between + 12C and -12C. Could this be a factor? How often do you check your alignment?
Thanks

Join main@ap-gto.groups.io to automatically receive all group messages.