Pier base size

[I Am Marsha]

I have a POD observatory that's been on a deck for several years. I'm going to remove the deck and have a cement slab to put it on. I want to put in an isolated base for a pier. How big an area should the cement base be to bolt the pier into?
Thanks for any info you can give me.

[sdbodin]

OK, I did not isolate my pier base, the pros don't in the big domes either, but rather integrated the base mass into the slab. The base is a cubic yard filled with rebar under the slab with the J-bolts cemented in place to hold the pier. I can have the scope imaging at 8000 mm efl and jump up and down and barely detect a wiggle in the image on the laptop. Of course, I normally just move gently around the scope when imaging, no sense being an idiot.

Clear skies,
Steve

[KathyNS]

I think mine is a 36"x36"x18" block, with its base four feet down to get below the frost line. On top of that is a 16" diameter column with its top 6" above ground, with J-bolts embedded in the top. A plywood pier column is bolted to the top of the concrete.

[I Am Marsha]

I live on top of a hill that is all ledge within 12 inches below the surface. That's the depth that I have to work with. If the base of the pier is 12" diameter then is that what the diameter of the footing should be also? Or should I make it bigger? The idea is to separate it from the slab so I won't be walking on it when I'm at my scope.

[KathyNS]

If you have bedrock 12" below the surface, then use that as your base. If you can drill into the rock and epoxy rebar into it, then your concrete pier will be well attached to something very, very solid.

The concrete column should be big enough that the bolts attaching the pier to it are well inside the concrete. You want the bolts to be 1" to 2" inside the concrete. So if the mounting flange for the pier is 12" in diameter, the bolts are likely at a radius of 5". So you would want the concrete to have a radius of 6"-7", so a diameter of 12"-14".