I have a concrete pier, the base being a random hole in the ground about 0.6m deep and 1m dia.
I often wonder why ppl make huge concrete bases of 1m deep or larger, but have been lately thinking perhaps this is a good thing .
I have found I that my polar alignment drifts out by some 20 arc/mins over a period of 4 months or so and am wondering why.
Doesnt sound a lot, but guiding gets progressively worse over this period (at 2250mm FL) and then snaps back to perfect after a retune.
The concrete is (past about 100m of soil) into shale thats seems stable. Im wondering if this drift is due to a small shifting base, or a fundamental ground shift depending say on moisture due to changes in weather?. Does the ground level generally, anywhere, change over time with the weather, and render any pier base construction mute anyway?.
Im not fussed with periodic polar alignment, but the need is a puzzle.
Given the moisture of late, some ground movement is expected with even moderately reactive soil. My brother lives not far from you and he has clay that is highly reactive with water and expands. Hence the movement.
Having the observatory on the side of a cliff as well.....
But the big lesson here with piers is go DEEP, dont go for a blob of concrete on or near the surface of the ground.
You want to know about movement, my house which WAS on clay and made of weather board, used to shift upto an inch between summer/winter, wet/dry. The front door had 2 positions for the lock striker plate, as the house could drift enough that you could open it one morning and not be able to close it.
Have just poured the slab for our new house, 50 piers ranging from 1-2 metres for a 180sqm slab. Even then, that is also why they use a waffle framework for the slab to minimise the contact points with the clay.
Well its not precession !
Thats about 33 arc mins per century not every 4 months.
So its most likely ground movement
Is the Obs situated on or adjacent to any backfill or terraformed land ?
Have you noticed if its in the same direction or if it varies back and forth with Winter and Summer ?
As the ground gets wet the soil (especially clay) expands, if your pier is between two areas that are exposed to moisture differently then that could be a possible reason.
Before you go digging up the Obs - it may be possible to change the dyanamics of your soil by adding some extra soil drainage - put in some agricultural draining pipe (convoluted poly pipe with lots of holes in it) and try to remove all the excess water.
I have done this and its amazing just how much water comes out.
Fix up any backyard drainage issues - eg off the obs roof, shed/house gutters, neighbours run off etc.
Also is there any disturbed soil nearby - pool, sewer, drainage, water mains, underground electricty, old tree stump from original site clearing etc etc - these could all be causing soil settling/movement many years after the work was done.
But bigger deeper footings are better - however if your soil is moving it may be the case that going deeper will not of itself necessarily solve the problem.
On the other hand repeatedly leaning against the OTA in the same place night after night after a few beers may be the culprit !
I would look at drainage and water runoff first - followed by light beer
My pier seems to move a bit as well Fred. My PME sits about 2m above the base of the concrete pier, which is founded in apparently massive sandstone. The lower half of the pier is about 500mm square. A steel pipe goes up the middle and carries the mount base.
I found my guiding was erratic of late (using an OAG) - after redoing some largish Tpoint models I found that alignment had shifted noticably. After realigning, guiding has improved again.The geology in my area has sandstone interbedded with shales and claystones so I guess the sandstone foundation is subject to either hydraulic effects along discontinuities in the bedrock or some reactive clays or both.
Seasonal checks on polar alignment are on the menu as a result.
I have used a laser for years mounted on my GEM and
it shines on an X on the nearby house.
I can see it drift over winter and summer months.
A quick re-set gets most of it zeroed out.
Another night with Drift Explorer and it's gone.
Fred,
20 arc min over 4 months seems like a lot to me. How many knob ticks does that equate to?
If I recall correctly, I got the results below 3 or 4 months after my initial PA.
************* AZIMUTH ************
MA: +183 seconds (+3.1 minutes).
Rotate axis East (counterclockwise).
For latitude -37.78°, the azimuth adjustment is 3.9 minutes.
Paramount: Loosen the East knob, and tighten the West knob 1.9 knob tics.
Sigma=16.578
************* ALTITUDE ************
ME: -147 seconds (-2.4 minutes).
The polar axis should be raised -147 seconds (-2.4 minutes).
Paramount: Raise the polar axis 1.2 knob tics.
Sigma=9.220
The soil under my pier is nothing special - mostly clay and topsoil.
I wonder if there could be something else going - there is a lot of maths and mechanics going on behind these numbers.
For this reason I wonder why we dont lay mini waffle slabs insteads of deep piers and blob footings. With waffle slabs they "float", so movement is restricted to the vertical plane, whereas deep piers can suffer movement in both vertical and horizontal planes. But the slab would have to be of sufficent area to spread the load.
I have not thought about polar alignment drift before because I have not noticed it. My wedge mounted LX200 standing on a pier bolted to a reinforced concrete 2.4 metre square slab just sitting on the local ground was aligned once in the four years I had it. It was parked after every use and restarted by going to a star.
The initial alignment took quite some time and I had to get my LX200 orthogonally true first. I was then able to align the wedge by rotating the OTA at the 90 degree position until it rotated about the single point of the south celestial pole.
I regularly pointed it at -90 and always got the same view of the south celestial pole (naturally rotated due to time). The FOV was about 30 arcminutes so my readout precision would have been about 1 arc minute so I can safely say that my long term polar alignment was within an arc minute.
Below a view of the south pole from the DSS. This is what I see (on a dark night) j2000
Barry
Last edited by Barrykgerdes; 21-09-2010 at 08:16 AM.
If that is the location for the SCP in year 2000, won't it have precessed about 3.3' since then? If so in which direction would this be in this photo? ie where on that DSS photo is the SCP now in 2010?
An interesting point from a tour of the Parkes Radio Telescope was that the surface of the land rises and falls with the moon forming land tides.
The land rises 1 metre in height when the moon is overhead.
I don't know if that could possibly be a factor (probably not) but its interesting.
We have had a wet last 6 months compared to previous dry years. The most likely thing.
Luckily its not much work to check your polar alignment from time to time with T-point.
If that is the location for the SCP in year 2000, won't it have precessed about 3.3' since then? If so in which direction would this be in this photo? ie where on that DSS photo is the SCP now in 2010?
Mike
Hi Mike
I expect so. That is a jpg I made a few years ago with the circles drawn centered on -90 at the time. I dragged it out to give some idea of the South celestial pole. I had a look at J1950 and J2000 and the stars incresed in RA somewhat.
Brett. My plot used to be a slate quarry, I dont know if slate is reactive?. Being right next to a cliff, mmm, solid slate, I dont see it cracking, but yes that might be a point. The next pier will be deeper, thats for sure.
Rally. Its on top of a cliff, next to a huge excavated hole (the whole street), that was a quarry, but some 35yrs ago, perhaps the ground hasnt settled yet from that activity. Its just dawned on me that the OBs is also right next to a really massive tree, I suppose its growth would constantly shift the ground.
I havent kept track of the drift to see if it goes back and forth with the seasons, good point, ill log that. Being on a cliff, the drainage is excellent.
The beer factor causing inadvertent mount-bumping might be a factor too.
Guy. Yes, AO gets bad very quickly with polar alignment drift, causing too many mount bumps, its the first indication of drift. Seasonal checks is the go, as you say.
Steve. A laser to have a quick indication of drift is a nifty idea. I use Tpoint to precisely measure alignment though, not drift, as it is quicker and also shows a report of exactly how many knob ticks to adjust the mount. Its usually done with 2 attempts within arc/secs.
James. Its 20 arc/mins in one axis only (cant remember which one) the other was 5 odd over the whole 4 mths. I have adjusted it a few times during that time, the last time being the worst, some 4 tics. Come to think of it, it has rained more over the last 2 mths than usual.
Dr Paul. A large slab, mmm, its not often you hear of that, wouldnt the slab have to be very large?. Then you would have to float a larger floor over it, to keep it isolated from walking in the OBs?
Barry. Well there you go, a slab. 2.4m square?, thats just 1.2 * 1.2m, I think that would be too small for a PME, but anyway it works for you so I guess a slab must be possible generally, so why isnt it more common?. Im looking for single digit arc/sec alignment too.
Greg. Thats interesting, but the Parkes scope would have an insane FL I suppose (mmm, I wonder what it is?). Yes, Tpoint is luckely quick and easy to use.
Ahhh the tree - how big, how far, proximity to the nearest root ?
I am guessing its probably a combination of things, but a tree is a living object - continually growing (or dying as they all are around here) , so its bound to have some influence on ground stability - just a question of how much.
I watch the carpark adjacent to large gum where I park my car shift around - over the last few years there is obviously a major root travelling under the bitumen and it just keeps lifting everything - by about 300mm close by and about 50mm up to 10m metres away !
A veritable Jack and the Beanstalk situation !
Not sure how you are going to deal with that one - a 3m deep footing might help. Oh - and I forgot to say I am busy that weekend.
Ahhh the tree - how big, how far, proximity to the nearest root ?
I am guessing its probably a combination of things, but a tree is a living object - continually growing (or dying as they all are around here) , so its bound to have some influence on ground stability - just a question of how much.
I watch the carpark adjacent to large gum where I park my car shift around - over the last few years there is obviously a major root travelling under the bitumen and it just keeps lifting everything - by about 300mm close by and about 50mm up to 10m metres away !
A veritable Jack and the Beanstalk situation !
Not sure how you are going to deal with that one - a 3m deep footing might help. Oh - and I forgot to say I am busy that weekend.
Cheers
Rally
Well, the tree trunk is 1m in dia, and about 6m from the pier , im beginning to think that might be the problem . Anyway, im not going to rebuild anything, just hope the council will allow me to cut it down sometime. In the meantime ill just align more often
I expect so. That is a jpg I made a few years ago with the circles drawn centered on -90 at the time. I dragged it out to give some idea of the South celestial pole. I had a look at J1950 and J2000 and the stars incresed in RA somewhat.
Barry. Well there you go, a slab. 2.4m square?, thats just 1.2 * 1.2m, I think that would be too small for a PME, but anyway it works for you so I guess a slab must be possible generally, so why isnt it more common?. Im looking for single digit arc/sec alignment too.
Now Fred. I said 2.4 metres square. That's 2.4Metres x 2.4Metres. No wonder you are having trouble with your measurements.
Here's a better suggestion. Have another beer and take up knitting.
Baz
Last edited by Barrykgerdes; 21-09-2010 at 01:29 PM.
.
. Anyway, im not going to rebuild anything, just hope the council will allow me to cut it down sometime. In the meantime ill just align more often 
I said 2.4 metres square. That's 2.4Metres x 2.4Metres. No wonder you are having trouble with your measurements.
