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Doug Culbertson
Pooh-Bah
Reged: 01/06/05
Posts: 1353
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Maybe this would have been better posted in the Observatories forum, but I have been wondering about this since reading a couple of wood tripod threads, and planning ahead for my own observatory.
Since it seems to be widely accepted that wood tripods display better vibration damping characteristics than metal tripods, how come you never hear of anyone using a wood pier for thier mounts? I would think that a good thick wood pole, perhaps a length of telephone pole, sunk deep enough into concrete would make for a pretty decent pier that would be free of vibrations. So, what am I missing? Other than the threat of termites, perhaps? 
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Doug
Midway, FL
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bseltzer
sage
Reged: 10/28/07
Posts: 306
Loc: East S.F.Bay, CA
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The long and short of it is wood is dimensionally unstable. Over time, wood crawls all over the place. This characteristic can be reduced with various treatments and finishes, but never eliminated.
-Bert
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8" F4 Newtonian (Royce Mirror)
SV 105 APO Raptor
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dancerchris
member
Reged: 08/12/09
Posts: 12
Loc: Southern, CA
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Plywood is dimensionally stable. A torsion box structure out of plywood is very ridgid, dimensionally and thermally stable.
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Televue Genesis
20mm, 9mm and 3-6mm Naglers (and zoom)
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 2704
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Yes, over time, it might be, however that is what latitude and azimuth adjustments are for.
From night to night, and even month to month, a wood pier would not change shape enough to affect anything.
I have known of a couple of people that used old telephone pole sections for peirs in their back yards, and they seemed happy with the choice.
Best would be to build a beam by laminating say 4 2x4s together so that deminsonal changes will want to cancel each other out (or use the box section plywood mentioned already).
Modern paint can make woods almost impervious to the elements that it would be exposed to for observatory usage.
Wood is cheap, can be worked with hand tools, can be EASILY cut or drilled in to accept trays or other accessories, and can be stiffer and less prone to vibration on a pound to pound basis.
After seeing a telephone pole sectoin pier once, I almost considered doing it for myself.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7811
Loc: Franklin, Ohio
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HERE's My Wood Pier thread. I've found that the pier is plenty stable for my mounts. The only negative is that the low resonance frequency of the pier is not as good as a steel or concrete pier, but then the pier's performance depends on how tall it is. As far as dimensional stability, with 4-6x6's laminated with glue and bolts, it's not going anywhere. I'll let you know more after it's gone through a freezing/thawing period this winter. 
I had slightly different goals when I built my wood pier and if I were building a real observatory, I don't think I would have hesitated building the pier with concrete block or steel. But, overall I'm happy with the wood pier for the money I've got invested in it and it's serving me well. My main goals were to isolate the pier from the deck and to be somewhat asthetically pleasing and at a low cost. I think I spent around $600 for everything, including the hired labor. The hardest part was digging the hole, which I was glad to pay somebody else to do. 
Patrick
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10" f/6 Truss Tube Newtonian
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 2704
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While I do think that wood can be used to make a pier, the CHEAPEST and BEST piers that I have seen in observatory use have been concrete.
Use the same tubes that are used to build house piers in pier and beam house construction. Any company that does home leveling can sell you one. They are basically just big cardboard tubes. They can even be left in place and painted.
You can also simply sink your mounting bolts right into the pour. Use a plywood template to anchor them in place the correct depth. In fact, if you want get fancy, you can even put nuts on the ends and wrap the theaded rod with paper so that the rod can be removed or adjusted for length in the future.
A concrete pier is EASY to make, cheap to make, and perhaps the most solid to make.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)
The night sky is my mistress. She seduces me away from all other lovers.
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7811
Loc: Franklin, Ohio
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Quote:
A concrete pier is EASY to make, cheap to make, and perhaps the most solid to make.
It really depends on how tall it is. Anything over 4 feet and it's probably better to use concrete block.
Patrick
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
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Vixen GP2 Photo Guider Mount
My Astronomy Pages
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Doug Culbertson
Pooh-Bah
Reged: 01/06/05
Posts: 1353
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Thanks for the replies, everyone! I keep thinking that I want to use a good thick wood pole, like a telephone pole section. These can be purchased pretty reasonably around here every couple of years when poles are replaced. Fortunately, here in N. Florida I don't have to worry about issues like frost heave. We may get four or five nights in the upper teens to twenties, but those are few and far between.
OTOH, it does seem simple enough to make one from concrete too. My problem with either method is that I am currently using a NS11GPS, but you never know when I may get the urge to upgrade to a C14, which would require a taller pier. I would hate to have to replace wood or concrete!
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Doug
Midway, FL
NS11GPS/Feathertouch/Starizona Virtual View
100ED/Giro II
Some eyepieces and a barlow
1 Thermacell®
Life's too short to drink cheap beer
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roscoe
super member
Reged: 02/04/09
Posts: 192
Loc: Northwestern Mass
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You could always build the lower section out of concrete, then use a wood or steel pier extension to get to the right height for your current scope. That way, even if you get dob fever, you're safe.
If you use wood, try to get some stock that has been stored inside, or at least under cover, for a long time, as it'll be more dimensionally stable.
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7811
Loc: Franklin, Ohio
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Here's a photo of my wood pier with my AP gear sitting on it.
Patrick
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Celestron Regal 8x42 Binocular
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oldsalt
Astro Philosopher
Reged: 02/12/05
Posts: 8677
Loc: Pa - between starparties
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The biggest problem, especially in Florida, would be termites. An old telephone pole would work, but be sure it is treated well. Also you do not want one that has been kreosoted.
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There are no winners in war, only bigger losers.
Jim
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mish
member
Reged: 02/10/09
Posts: 76
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As Jim has stated, the biggest problem with wood is that it will decompose if given a chance. So you need to protect it from termites and such (which often involves chemicals that you don't want your kids around), and you definitely need to protect it from wet/dry cycles, which will degrade wood very quickly.
So if you have intermittent high ground water problems, then wood isn't a great choice.
Wood is almost as stiff (i.e., ratio of stress to strain) as concrete, but it's a LOT lighter than concrete, so it exhibits more high-frequency behavior, but that shouldn't be a problem. Like concrete, wood tends to damp vibration pretty well by virtue of its composition (in concrete, damping occurs at the aggregate/cement paste interface: in wood, at the lignin-cellulose interfaces where cells are glued together to form wood fibers), so it should work well enough.
The main reason I would tend to avoid timber piers is that there are no good ways to treat wood that are guaranteed safe around inquisitive young ones. Old treated wood members used arsenic salts, more modern ones use copper, but neither is what you'd like to have your kids leaning against. You could use old-growth redwood (heartwood), which is about as decay-resistant as timber gets (with no treatment beyond a good sealer), but that costs a fortune, and really, old-growth redwood belongs in parks, not under a telescope.
And finally, the bolts that hold the mount to the timber pier will likely be attached to end grain (i.e., they will be pointing in the same direction as the grain of the wood). That's a bad idea if the wood dries out, as those bolts will loosen as the wood fibers shrink away from them.
So if you're going to use wood as a pier, get plenty of sealer, and don't shy away from putting plenty on the top surface -- the cells will soak it up, but just add another coat and you should be fine.
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Doug Culbertson
Pooh-Bah
Reged: 01/06/05
Posts: 1353
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Some excellent replies, thanks! Yeah, here in Florida termites are definitely a problem. I live in a rural area and my property is about half wooded, so I have them all over the place.
BTW, Jim,I have had my dealings with kreosote, and I am 100% in agreement with you on wanting to avoid it. While I don't have small children anymore, I also don't want to be in constant contact with too many chemicals either. I have seen untreated telephone poles for sale, or at least untreated as far as kreosote is concerned. That's more or less what I had in mind. Mish also has some good points about bolting the mount to end grain though. That's what I love about CN, someone always gives you more to think about! 
BTW, Patrick, that is an impressive wooden pier you built!
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Doug
Midway, FL
NS11GPS/Feathertouch/Starizona Virtual View
100ED/Giro II
Some eyepieces and a barlow
1 Thermacell®
Life's too short to drink cheap beer
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oldsalt
Astro Philosopher
Reged: 02/12/05
Posts: 8677
Loc: Pa - between starparties
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Doug, you may be able to find some cypress poles. They are usually untreated and could work. Unfortunately, you will probably have to look long and hard to find them. Another problem is knots if you do find them. Just a thought , if you do decide on wood.
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There are no winners in war, only bigger losers.
Jim
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7811
Loc: Franklin, Ohio
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Regular pressure treated lumber is ideal for a wood pier application. As long as the timbers are encased in a concrete footer they should last a very long time.
Patrick
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
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Vixen GP2 Photo Guider Mount
My Astronomy Pages
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HandyAndy
professor emeritus
Reged: 01/11/08
Posts: 652
Loc: West Midlands and around
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Hi,
Even with a treated post its only the surface which is effectively treated. You must re-treat any cut ends.
Before I stick any wood pole in concrete I want to last I stand it for a few weeks in a bucket with Cuprinol treatment to allow it to soak right into the whole timber which is in the concrete and will get splashed by rain of the ground with microbes in it. A bit of patience pays long term dividends.
Cheers. Andrew.
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Doug Culbertson
Pooh-Bah
Reged: 01/06/05
Posts: 1353
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Andrew, thanks for that information. I am not familiar with Cuprinol but, then, I'm not exactly a woodworker either! I have found all of the responses in this thread very helpful, though I am still undecided as to whether I will eventually end up using a wood pier, concrete, or steel.
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Doug
Midway, FL
NS11GPS/Feathertouch/Starizona Virtual View
100ED/Giro II
Some eyepieces and a barlow
1 Thermacell®
Life's too short to drink cheap beer
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macular hole
super member
Reged: 08/12/09
Posts: 148
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A couple points on treated wood-
1. Modern treated wood is available in solid posts up to about 10"x10" by 8 ft long.
2. The treament actually does penetrate the entire board.
3. For the best longevity in an inground application, it is best not to use a concrete base.
4. The end grain will be soft, while still wet, but will harden up when it dries completely.
5. You must use stainless steel hardware or galvanized hardware that is rated for modern treated wood. Otherwise the hardware will rust and fail.
6. It is completely unnecessary to paint, or retreat modern treated wood. The end grain does not need to be soaked in anything to increase longevity.
&. I have pulled 4x6 treated fence posts that were placed in well drained soil, without concrete bases, that looked as good as they did the day they were placed in the ground 23 years ago. That includes the exposed end grain at the top and bottom of the posts. In fact, I reused all of them for another fence.
Edited by macular hole (09/18/09 10:31 AM)
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2274
Loc: Kent, UK
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how about an old railway sleeper? in the UK we get them used from garden centres and reclaimed building materials yards. Maybe 2 bolted together, last pretty well for ever.
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Jc
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 733
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What about AZEK? (synthetic wood)
Greg N
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