30 replies to this topic

[mearnest]

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.

 

Thanks, Mike

[nitegeezer]

My scope does not live in the observatory during winter anymore, I just don't want to fight the snow.  I will put it on the pier in the spring and verify my alignment on the wedge.  I may have to tweak the alignment slightly and then not check it again and will pull the scope off in the fall.  I think there is something going on with your pier.

[AstroBrett]

A pier, like any structure, must have a properly designed foundation appropriate for the soil or base upon which it is supported. If this is not the case, then it can shift.

 

Brett

[terrypaula]

I have a setup on my deck. Everything is marked well all I have to do is  take everything outside and set it up to the marks.  The tripod never moves up or down.  It always seems to be level too.  But every time I check the PA upon setup I almost always have to adjust the dec.  Once or twice a season I'll need to adjust the RA after my initial, and by the numbers PA.  I figure it's because of climate changes throughout the year.

[ngc7319_20]

Sorry what does "PA" stand for?

 

What is the accuracy / step size on your encoders?

 

I recall some rule of thumb that goes like "for every 1 pound above grade, you need 10 pounds below grade."  Is that what you have?

Edited by ngc7319_20, 13 January 2021 - 10:49 PM.

[macdonjh]

Sorry what does "PA" stand for?

 

What is the accuracy / step size on your encoders?

 

I recall some rule of thumb that goes like "for every 1 pound above grade, you need 10 pounds below grade."  Is that what you have?

PA = polar alignment in this case.

[mearnest]

Sorry, Polar Alignment.  I couldn't answer your question about the encoders.  It's an eq6r pro mount if that helps.  As far as the rule of thumb your referring to, I wish I had known about that rule last yr.  I'm sure my pier doesn't meet that requirement.  Maybe that's the issue here. 

[macdonjh]

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.

 

Thanks, Mike

A lot depends on the soil your pier is in and what your frost line is.  If you have stable, rocky soil and your pier is anchored to shallow bedrock, your results are surprising.  If your soil is sandy and loose, you likely don't have enough concrete in the ground to keep your pier stable.  Another possibility: if your frost line is deeper than 30", your pier may be experiencing frost heave.

 

If you're dissatisfied with your pier, do you have any way to remove it and rebuild it?

[Cfreerksen]

That doesn't sound very deep for having 4' above ground. My tripod sitting on pavers set in the ground only drifted 8' over a 6 month period.

 

Chris

Edited by Cfreerksen, 13 January 2021 - 11:44 PM.

[mearnest]

No, there's not a good way to rebuild the pier.  It's in my back yard so it's not that big a deal to walk out and tweak it.  I was just curious if it was my pier or if this was normal.   If there is another observatory build in the furture, I'll be sure to back the concrete truck up next time.   This is my first observatory build and all in all I'm very happy with the performance.  My home made system to keep things dry is working like a champ, the roof rolls off very well and there's no leaks.  All in all I'm very happy with it. 

[deonb]

I've had weeks where it was stable, and then it started being off by 15'. Then next day fine. Day after that - 15' off again.

Turns out, we had a series of earthquakes. Check your local seismic history.

[rgsalinger]

"its only off about 9 or 10" ---- is more than enough accuracy for any conceivable use. At the same time, you don't say how you are adjusting the mount BUT seeing alone can give you that much variance between nights. With all four of my scopes ranging from 350mm to 2540mm getting within 2 arc minutes of the pole gives me round stars in 20 minute exposures.

 

Now if you are actually seeing field rotation that's another matter but 10 arc seconds is terrific alignment. 

[Rustler46]

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.

 

Thanks, Mike

Hi Mike,

 

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.

 

Side Note:

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

 

Best Regards,

Russ

[aa6ww]

Your pier sounds fine. It sounds to me like your mount is moving slightly over time. Id make sure its really clamped down in all directions and most importantly, the center bolt holding down your mount could be making your mount wiggle slightly. 

Since you have a pier, that center bolt should be tightened with a decent amount of torque, not handing tightening. Also, Id make sure the surface your mount sits on is flat and smooth, similar how people hyper tune the base of their tripods or the bottom surface the mount that makes contact with your pier.

The Latitude and Azimuth bolts that secure these types of mounts in place aren't very precise at all. This is why Hyper tuning is popular. 

 

...Ralph

 

 

 

 

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.

 

Thanks, Mike

[speedster]

Soil moves a whole lot more than people realize.  You may be going up and down 3" seasonally.  If everything moves together, no one notices but it seldom happens that way.  At 2.5' deep, you are still in active surface soils and things are going to move and there is no stopping it.  Some soils don't move much and others move enough to crack masonry and bind doors in homes.  It's not a matter of weight.  Active shallow soils have enough force to move most anything founded in them.  No substitute for going deeper and not forming anything below grade.

 

Your PA may be bouncing around due to how you are trying to measure it rather than your pier moving.  Several methods can theoretically get perfect but in reality just get close and then vary about the ideal due to their precision.   You can tweak it all night and always be close be never be perfect.  It's the difference between accuracy and precision.  ASPA can be this way.  You might try doing a PA by plate solving and record how far off you are.  Then, come back and do it again a few days later and see if you are off the same amount.  You might find things are not moving and that ASPA, or whatever method you are using, although accurate, is not precise to the degree you are trying to achieve.

[jfgout]

"its only off about 9 or 10" ---- is more than enough accuracy for any conceivable use. At the same time, you don't say how you are adjusting the mount BUT seeing alone can give you that much variance between nights. With all four of my scopes ranging from 350mm to 2540mm getting within 2 arc minutes of the pole gives me round stars in 20 minute exposures.

 

Now if you are actually seeing field rotation that's another matter but 10 arc seconds is terrific alignment. 

+1 for that.

 

Not sure there is anything wrong with your pier.

 

jf

[mearnest]

Thanks guys for all your thoughts.  I really appreciate it. 

 

Mike

[mich_al]

Couple of other things that I don't see mentioned.  Did you pour the concrete into virgin soil ?  IE dig the hole and fill with concrete or dig / forms / concrete / backfill..  Also what about your 'rat cage' ?  I see many that are several inches of 'too small' rods/bolts between pier & mount adapter that can introduce plenty movement with temp etc changes.  Also, even a sizable steel pier will expand & contract with temp changes IE night to day.

Edited by mich_al, 14 January 2021 - 01:59 PM.

[SteveInNZ]

If that was my setup, I'd be measuring the PA as part of my setup routine. I think the free version of SharpCap will do up to the bit where it tells you the error and only takes a minute or two. I wouldn't bother realigning as the error itself is largely inconsequential and not worth the time chasing perfection.

However, I would record the value in a spreadsheet and then graph it. You may see that the PA is entirely random, or cycle with the season or your pier is very slowly falling over.

For me, better to measure and know.

 

Steve.

[robbieg147]

9 or 10 seconds is quite accurate that's better then the Polemaster is quoted to, it's hard to say if the difference is being caused by your polar alignment method or movement in the pier?

 

As long as the pier is solid during imaging your pier may not be ideal but don't think I would worry about, I would always check the PA anyway.

[t-ara-fan]

about 4' and about 2 1/2' below ground. 

Sounds top heavy to me.  Did you drill a 6" hole into undisturbed soil for the pier, or dig a hole with a shovel and backfill with soft dirt?

 

Frost line?

 

Did you pour concrete into an open hole, or is there sonotube underground. Sonotube apparently rots away leaving a gap between your concrete and the soil.

[mearnest]

I drilled into undisturbed soil.  It had a high content of clay too.  I did use a sonotube  but it was not extended into the ground.  As far as the hole itself, I just dug a hole about 12 to 14" in dia. about 2 1/2' deep.  The sonotube was level with the ground, not going down into the hole.  I had it suspended in air (with 2x4's) while getting it level in all directions.   Once it was level, i used 1/2' rebar to make a cage that went inside the tube from the top to the bottom of the hole in the ground.  So the concrete was poured with no part of the tube below grade. 

[rgsalinger]

Unless the drift is cumulative - increasing night after night, the you will be heading down a real rabbit hole trying to make the pier better. As I said earlier, there's nothing to be concerned about, my PA changes by much more than that.

 

So, I have a PierTech pier attached to a 48x48x48 concrete isolated pad. Yet still if I use PEMPRO to measure my PAE, it will vary from night to night but it never ever gets beyond 2 arc minutes. So, I don't worry and I strongly suspect that you are in the same boat and it's not leaking. 

 

Rgrds-Ross

[KLWalsh]

Any chance you have some residual error (cone error) between your scope and your polar axis?
I.e., if the optical axis of your scope is not precisely parallel to the polar axis when it’s pointed at the NCP, then when you do a polar alignment it will only be correct for that evening.

[rgsalinger]

AFAIK, it is not correct that cone error will cause polar alignment to shift. You are aligning the MOUNT to the pole, not the scope to the pole. Can you provide a citation for that position? Many people have as much as 30 arc minutes of cone error (as I did with a Meade F8 ACF scope) and my PA sure didn't seem to shift during the runs. 

Rgrds-Ross