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Concrete slab, metal pier

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#1 FiremanDan



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Posted 23 September 2019 - 12:50 PM

Ok so about a year ago I scored a great deal. $500 for a SkyShed Pod XL1 (insulated) with visor, and a metal pier. It looks similar to the SkyShed pier but a little different.
I am finally inching my way towards getting this thing set up. I am getting ready to reach out to contractors to put down a slab. 10x12 or maybe 12x12. My plan is before they put the concrete down, I want to pour concrete for the pier. Then frame that off so the pier is separate from the slab. First of all, is that necessary for imaging? I gather that it is, but if I can get away with just bolting the pier to the concrete slab that would be easier!
Second, how thick should this slab be? I have read 4 inches should be enough since all it will do is support the Pod.

A pier question is, I plan to have a 10in or so sonotube pier below ground which the metal pier will bolt to. I plan on it being 24-36 inches. We are in Chesapeake, VA. There is a lot of clay and sand here and while freezing happens it looks as though our frost line is only 10 inches according to my googling. I plan to use 4 bolts sunk into the concrete and hope to do all of my leveling via those 4 bolts and nuts. My plan is to secure the AP adapter to the top of the metal pier. So the "rat cage" will be between the concrete and the pier. My thought process is those bolts are heavier and will have less flex. But I am concerned because there will be more total weight on them than a rat cage under the mount but above the pier.
The AP adapter has fairly small holes for the bolts, so I worry that those would flex too much.

So what do you folks think?

#2 greenstars3


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Posted 23 September 2019 - 01:51 PM

Hi Fireman Dan

I am also a fireman (vol) good to see another firefighter here.

As to your pier, you do want separation between it and the slab if you are going to image, if just visual a slab will be fine.

As far as the pier is concerned do not have the sonotube below ground, just build a box to secure it to at the surface and have the underground part of the pier set up in the hole, that way the pier is secure and you will not have to strip off the tube below ground nor will you have to backfill and tamp around the pier, it will be alot more stable if it it set in undisturbed    soil.

My slab retained a lot of heat and that thermal mass was holding about 7 degrees above ambient  all night so I covered it up with those cheap space blankets and an outdoor carpet and now it is only 1 degree above ambient all night with a 30+ degree difference between day and night temps.


ps - use some rebar in the cement



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#3 speedster


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Posted 23 September 2019 - 02:01 PM

Howdy Dan!


You might be interested in this:


https://www.cloudyni...er engineering


Great snag on the observatory.  Have you noticed that all the Pod owners in pictures have a great bug smile on their face?  Still waiting on mine.  My delivery quote of "Septemberish" has become "unknown at the moment" but I'm sure I'll have a big smile too when it gets here.

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#4 Hawkdl2


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Posted 23 September 2019 - 02:04 PM

Dan,  Here's what I learned from my build.  First, I have a 24" x 3' concrete pier that is rebarred dircetly into fractured granite - it's not going anywhere and is as stable and vibration isolated as possible.  The top of the pier sits about an inch below my concrete deck.  I have 4 x 5/8" SS bolts in the pier that attach to my aluminum pier.  On top of that I have a rat cage with 3/8" bolts.  The rat cage has about a 2.5" gap between two two plates of the cage.   After I built this, it seemed to me that the consensus is that there is no real need for the top of the pier to be perfectly level.  I also learned that even 3/8" bolts with a span of ~2.4" have a measurable amount of flex.  So, I reduced the rat cage to be adjuster nut-to-adjuster nut (i.s. as close to together as possible) leaving only the gap created by the adjuster nut on the bottom of the top plate to the nut on the top of the bottom plate.  The top plate is now "pretty close" to level, but only as level as the pier is.   What I found is that my PE and tracking improved!  Removing most of the flex from the rat cage really helped. I also have not seen any problems with my pier being ever so slightly not-dead level.  


I had to leave the nut-to-nut gap because I have most of my wiring, USB hub, etc., inside the pier and need the gap to run the wires out.  As soon as I get around to it, I plan to remove the rat cage entirely  and route the wiring through the side of the pier. 


The point is:  I now agree with many others that you do not need any sort of rat cage. Just get the top of your concrete pier close to level.   BTW,  I did that too by mounting the bolts in the concrete with a plywood template/holder, and used a level on the plywood to tap it pretty close to level. 



Edited by Hawkdl2, 23 September 2019 - 02:50 PM.

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#5 macdonjh


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Posted 23 September 2019 - 03:23 PM



The rat cage won't cause you any problems, the forces put on amateur astronomy piers are tiny compared to what the materials can handle; even considering the tiny deflections we should allow in our designs.  That said, I still don't like rat cages.  We can do better.  So:


speedster's link is a good thread, informative reading.


+1 to keeping your pier's foundation separated from the slab your POD will sit on.  It's best practice.  It doesn't matter when doing visual, but if you're going to be imaging you'll have to put a delay on your exposures to allow you to leave the POD before starting to collect data.  Otherwise your footsteps can ruin a sub.


+1 to not leaving Sonotube below grade, and to pouring any concrete against undisturbed soil.


+1 to a 4" thick slab being structurally sound for a POD.  However, you might want to pour a bit thicker to make sure there are a couple of inches of concrete below grade in case you get some erosion.  You can accommodate this by digging a shallow trench around the perimeter of your slab and using 2x6 lumber for forms instead of 2x4.  Then your slab will be thick at the edges and thinner in the middle.  Don't forget the rebar (#3 24" centers each way).  It won't prevent your slab from cracking, but it will keep the cracked pieces aligned so the cracked slab doesn't separate and heave creating gaps and tripping hazards.


Making your pier plumb (and thus your mount level) by using the anchor bolts at the base is just fine.  Same effect as leveling the mount at the top of the pier.  It'll look good, too.  You can even fill the gap between your metal pier and concrete foundation with mortar or grout for a finished look.


Good luck.  You'll really like having an observatory.

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