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Concrete telescope pier? Or?

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#1 Greg Stone

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Posted 08 January 2010 - 11:43 PM

I'm mulling over making an outside, permanent, telescope pier. About 15 years ago I had a steel one fabricated for my little observatory - not too expensive then, but these days we're talking $1,000.

Then I saw this video:

http://www.youtube.c...h?v=FhovY5K74HE

Hmmm. . . does this make sense? A Concrete pier? Why not use Sonotube - say 12-inches in diameter? How high could I go? For what I want it would have to be about eight feet with three feet of that underground, for a pier height of five feet above ground. Is that doable this way? A good idea? I'm no engineer - not much of a mechanic. I have no idea. But it seems like it would be pretty easy. And all I would need is a bolt coming out the top that I would attach a UA Unistar (all weather) mount to. What's wrong with this idea? Any and all comments and suggestions welcome!

#2 MtnGoat

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Posted 09 January 2010 - 12:53 AM

Not much wrong with it. Piers are really fairly easy. I'd dig a hole a two feet wider than your sonotube diameter, your depth seems right. fill it with some chicken wire, rebar pounded into the hole, or scraps of range fence. not a lot, just a few winds. drive a couple t posts near the center, get the long ones so they go 3 or 4 feet up into your sonotube.

start mixing and pouring. don't make the concrete too sloppy. slightly under a gallon per bag of 80lb ready mix works for me. fill 'er up to within about 6" of ground level.

give it an hour or so until it's partly set and doesn't give when you poke at it. you need to let it set a bit so when you put the sonotube on it and fill, the weight of the fill doesn't squish the concrete at the bottom out.

brace up the sonotube with three two by fours nailed into it before you start filling so it doesn't tip over.

that's about it...now start filling the rest of the way. tap on the tube with a 2x4 scrap, small sharp raps all over, now and then to get the mix to settle and fill properly.

for the top, have your bolt or bolt pattern ready to go by making a template for it out of a scrap of ply. you basically install the bolt in the ply, sticking down below it as far as you want. this end will go in the concrete. any bolts you use should have nuts or j bends on the end in the concrete so they'll stay put when it sets. above, have what you need sticking out. trap the plate from both sides with a nut on each, the bottom one will stay with the concrete, the upper one will be unscrewed to take the plate off.

so when you get near to the top when filling, now you just set the ply plate with the bolt(s) on top of it, and tap it down, tap the tube, tap it down, tap the tube, ect. hopefully you put in a circle of three blocks on the ply to go on outside of the sonotube to make sure your ply template goes on all centered and nice.

the key for the pier portion is making sure the sonotube fills right (tapping!) and you get the bolt plate on right.

then wait a couple days before using it. cover it with a tarp maybe and spray the footer a tiny bit to keep it moist. after unwrapping the sonotube (which sits *on* the footer, not into it), you can shovel some dirt back over the footer and get back to ground level again.

and that's it. it sounds scarier than it is. I'm no expert, but once I tried it I was hooked...and added two more. The last one was three hour job from shovel to done.

here's scopehenge. the black one is 4" pvc with a three bag footer and a t post up the middle, pvc is filled too. the left one is a 10" sonotube and a 6 bag footer. The right one used the 16" diameter bottom foot of a wastepaper basket for the pier form, has a huge footer (3' foot circle 3' deep) and an 8" bolt pattern. What will actually wind up there, and soon, is a GOTO'd Starfinder 16 GEM mount to hold up my Zhumell 12" reflector.

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#3 Greg Stone

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Posted 09 January 2010 - 08:19 AM

Thanks - that's extremely helpful. Of the piers you've built, do you have a preference? The concept of using a PVC pipe and filling it with concrete appeals to me. I tried PVC pipe many years ago and found it wouldn't work at all, but I never thought of simply filling it with concrete.

#4 EddWen

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Posted 09 January 2010 - 11:40 AM

Scan through the "Observatories" here. Lots of stuff on concrete piers. I have two 12" concrete. Esay to make and cheap.

#5 MtnGoat

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Posted 09 January 2010 - 01:23 PM

Greg, don't really know that I have a favorite. The PVC works fine at this diameter, but I don't think it is solid enough for more than what you see on it...a small refractor and a light mount used mostly for visual work. I do like the height it came out at, now I tend to run an Orion short tube 90mm refractor or my 90mm Mak on it and it's perfect. The 4504 mounts can be had for 50-75 bucks if you watch Craigslist, and after some simple rework actually function better than you'd expect, especially after ditching the long 4.5" reflector they come with. With a short tube, they're OK. Good GOTOs, actually.

The midsize pier has worked out quite well and will wind up with the LXD75 on it as soon as the Starfinder GEM gets here. I may copy it one more time to have a place to pop down the SN6 once the mount/pier musical chairs is done.

The fat short pier was the fast one, not because it's short, it actually has way mor concrete in it. By number three, and given the ease of working with concrete, my confidence level was pretty high. Dig, toss in fencing, fill, wait, add form, fill, tap down plate. Bing bang boom. You don't need to over think things for a basic pier, the keys are simply making sure you don't make the mix too wet, getting a decent footer size, and getting the pier form on it straight. On the footer, I like to go about 3x the column width, and 2' to 3' feet. If it looks about right, it probably is.

#6 Gregg Lobdell

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Posted 10 January 2010 - 02:43 AM

I'm not a mechanical engineer, but I'd suggest some reinforcement in the pier as well as the footer. Roll some fencing several inches less than the sonotube in diameter, or use 3 or 4 columns of rebar tied together with wire in a pattern similar to a mirror cell. Otherwise what MtnGoat says sounds excellent.






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