AP 1200 cement pier question

I'm building a observatory in my back yard and I was wondering if anyone had any experience pouring a concrete pier for a AP 1200? I'm going to to mount a AP 160 or a OMI 12.5" Cassegrain, both weigh the same. I'm using a 12" sonotube and going into the ground 5' and out of the ground 4'. I'll have close to 3/4 cubic yards of concrete in the hole. Will this be good enough or is it under built? Thanks for any help. Martin

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Dig out the base as much as you can, and as deep as you can. Our pier bases were 4x4' wide, x 18" then rising up in 18" sonotube column, steel reinforced, to ground level, where we planted bolts and steel piers.

Your "concrete" should be an inverted mushroom if you can do it. Big, deep, fat base, then a column coming up.

Remember when you are pouring that the concrete will flow--so do not try to just pour down the tube into a wide hole--the concrete will flow right out to the wide hole and won't come up the column.

Isolate the pier from the rest of hte floor.

We did not have to worry about frost--maybe somebody can advise you on that.

Alex

Below is a picture of what I did. I installed a 30"x30"x60" deep column of concrete in the ground and brought the concrete about half a foot above grade. From there I had a 75" tall tapered steel pier made for me. The steel pier is 16" wide at the bottom and 12" wide at the top. I also have an AP1200gto installed on it and it is rock solid.

My soil is a pence sandy loam in northern Wisconsin with a frost depth of 42"-48". No movement at all over two winters so far and I doubt it will ever move.

http://kriv.net/observatory/12.jpg

Mike