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Tallish Wood Pier or Tri-Pier?

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#1 MattT

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Posted 02 April 2013 - 09:05 PM

My wife and I are moving into a townhome with a yard, where I'll have a decent view of the ecliptic from my own property for the first time in several years. (Woo-hoo!) I'm considering a pier for ease of setup and steady views with a scope mounted fairly high, with the GEM head at perhaps 6' elevation. I need height because I've found that even a couple of feet elevation makes a difference with local seeing. And, because the scope I expect to observe with most from home is a 5" f9 refractor and my stiff old joints do not want to crawl around on the ground to get to the eyepiece when viewing near the zenith.

Since this is a townhouse I do not want a concrete pier, for several reasons. I've read threads here describing piers made of pressure treated 4x4s and 6x6s bolted together, and may try that. I've also read elsewhere about a pier made of 4x 2x4s screwed and glued so there's a vertical hollow in the middle, which after installation is filled with sand to dampen vibration. Sounds interesting but maybe a little light for a 5-6' high pier....if I go that route maybe I'll use 2x6s or 2x8s.

But I also recall a magazine article I read years ago, describing a sort of "Tri-Pier." IIRC it was made of 3 2x8s, arranged in a narrow tripod configuration with the lower end of each board in its own sand-filled hole. Any thoughts on that general idea?

Any comments, experience or links that might be helpful to someone thinking about a semi-permanent, tallish pier to carry a 40" long, 20lb refractor, mostly for visual use?

#2 Raginar

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Posted 02 April 2013 - 09:36 PM

The hardest part with a long wood pier as you're describing is it's susceptibility to warping. The longer it is, the more likely it'll do it.

You could try a telephone pole? Something thick enough might not have any issues.

What about a removable metal pier? You could sink a concrete 'pole' 4-5' deep with bolts for holding the metal pier in place.

Or, just get a really nice tripod and use markers to make it easier to setup.

#3 rlandsboro

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Posted 02 April 2013 - 09:43 PM

The tri-pier made from 2x8's sounds interesting - but I think your most likely causes of movement will be from inadvertent collisions with the pier, and from wind. Bumping one 2x8 might cause more problems than bumping a more massive single-piece pier made from glued and screwed-together lumber. Similarly, a single massive pier might have less cross section to catch wind. Sounds like a fun project - keep us "posted" :)

#4 MattT

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Posted 03 April 2013 - 06:57 AM

I could try pressure treated 4x4s or 4x6s for the legs of the tri-pier.

One question comes to mind....how deep would I need to sink a) a single post pier, or B) the legs of a tri-pier? I'm in California where we get frost every Winter but only occasionally, 27F is an unusually low temperature and the ground never freezes to any depth. I recall an old rule of thumb to sink a post 1/2 as deep as its height above ground, but not sure that applies to telescope piers. If I need to dig a 6' hole for my 6' pier this project might not be viable with the amount of money and effort I'm prepared to invest!

Second question... are there particular angles that work best for tripod legs? I wonder if certain angles are more or less prone to resonance and vibration. The tri-pier I saw in that old article had a much narrower spread than a typical portable tripod, each leg on the order of 10 degrees from vertical, which would be attractive both for the reduced footprint and less trouble with the scope meeting the pier when pointed near the zenith.

If I built a massive narrow-angled tripod out of 6' long 4x6s, braced the legs near their bottoms and just set it on or a few inches into gravel, how do you think the stability would compare to a good portable tripod with a much wider spread? If it was merely "as good," such an arrangement would give me the convenience of weatherproofness and quick setup, along with better clearance for the refractor than a portable tripod. And, I might face less grief from the HOA since it would be obviously non-permanent.

Just thinking out loud....comments are invited!

#5 stmguy

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Posted 03 April 2013 - 09:05 AM

I've used a tripod for several years and now I have a pier , no comparison as far as I'm concerned..the pier wins for me anyway

I have a 12" concrete pier in the ground with a "big foot" on the bottom of it with about 40" of 6" well casing bolted to it

You can probably get away with just a wood pier sunk into the ground....others have done that and are happy with it

Norm

#6 Raginar

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Posted 03 April 2013 - 06:57 PM

Honestly, you're talking about maybe 150 bucks in lumber. I'd avoid pressure treated since as it dries it'll be more likely to warp. Some nice dry cedar maybe? It'll cost you... but maybe it'll stay straight longer. Say... 4 6x6s screwed together?

#7 Goodchild

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Posted 03 April 2013 - 08:26 PM

You could do 4 4x4's together and then at the base (maybe 2 feet) attach some 2x8's to make it wider and sink that part in the ground. I would use treated lumber.

I think the key to keep posts from warping is to look at the end grain of the post. You want the grain to be as near circular as possible. If the post is cut off of center then you increase the likelihood of warping. However, 4 4x4's bolted together is not likely to warp.

#8 Patrick

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Posted 04 April 2013 - 12:38 AM

I have a wood pier laminated from 4 - 6x6 pressure treated posts. The posts were glued and bolted together with 1/2" by 12" bolts. The posts are sunk into a concrete foundation 30" x 30" x 4' deep. I think the height of the pier is about 8' from ground level to the top of the pier.

The pier is very solid and has served me well for about 3 seasons now. Here's a Picture of it with my CPC1100 mounted on it with a wedge. No issues even with that load. :smirk:

Patrick

#9 MattT

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Posted 04 April 2013 - 11:26 AM

Thanks for the tips and images!

#10 mich_al

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Posted 04 April 2013 - 05:00 PM

I've got a 10' 4 6x6 pressure treated wood pier. It's pretty good. It also moves around a little day to day. I suspect temp / humidity / drying. if you"re thinking about wood you should also consider old barn timbers. That would eliminate many of the movement issues I am seeing






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