need advise constructing observatory

I all... I am a long time amateur astronomy (30+ years) getting my first permanently installed scope in a roll-off roof observatory. I have purchased a 14.5" classical Newtonian which will be housed in it (on Astromart! . The pier that came with it is about 7 or 8 inches in diameter (and weighs about 80 pounds!) We are having a concrete floor poured, with a separate concrete base which will be poured into a hole in the middle for the pier to be bolted to.

Here's the big question..... How deep should we dig the hole?

I assume it should be a few inches or so wider than the base of the pier. The scope if f4.05 so it's a very short system and will not need to be very high at the top. (The pier is 22 inches long.) This should help with stability, but it is a very heavy scope, with a fiberglass tube and large cast iron fork mount.

Suggestions?????!!!!!

Thanks! and Clear Skies to all!

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I think the general rule of thumb for setting fence posts is one third underground and two-thirds above. Fenceposts are usually set in a 1 to 2 foot bed of gravel for drainage; I would think you'd want to do that and then pour about a foot of concrete over the gravel to form a solid base for the pier. Harold's suggestion to use 'spikes' for lateral support sounds like a good idea too.

Go over here:

http://www.astropier.com/installation.html

for a good description.

You will need lots of concrete very deep if you want long term stability.

You will need to make two pours. ONe for the pier, and then you wrap the pier base in something so that it is separate from the floor. Then pour the floor. If you try it in one pour, footsteps will transmit to the pier.

Good Luck,

Alex

MANY THANK YOUS TO YOU ALL! Building permits!?! What's that??? No -seriously - we are WAYYYY out in the country on agricultural land and no permits are required except for homes here.

I am already planning to keep the pier base separate from the floor. I am in central Florida, so instead of having to worry about a freeze line I have to worry about annual summer flooding and sheet flow. I have had a dump truck load of clean fill brought in, am getting it compacked now, and then the concrete will be ordered. We are having concrete brought out for other things on the property, so we will have it delivered in a concrete truck and that way it is mixed better, etc. anyway. I have been hoping I can get the floor and pier base poured at the same time (hard and expensive to get concrete trucks to come way out here in the country). I hope to start by first digging the hole for the pier base, putting a pipe with rebar in the center in the hole, then forming the floor with extra wide (deep) boards for the boundary between the two. The scope I am setting up has a base plate for the pier to attach to with 8 large all threads welded to the bottom which will go around the pipe and into the concrete while it is still "hot". This is where it gets tricky. I will have to set the base plate in place, then have them continue the pour for the floor. In case it isn't obvious, we don't do wood floors here between the constant moisture and tons of termites.

This is a real nice scope - the one I've dreamed of having for decades - and definitely NOT the usual portable type. I have a really dark back yard, so I want to get it set up right the first time and be able to relax and enjoy observing from then on!!! Thanks a bunch for all the good advise. I'll let you all know how it goes.

FYI, pictures of some astronomy gatherings we have had here can be seen at
http://www.palmbeachastro.org/ under recent events at Kye's Star Farm.
Clear Skies!