dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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I'm ready to start building my OTA but I ran into a snag. I calculated the amount of wood I need and the dimensions and everything. Then I went to see about getting the wood. However, I weighed how much I would have to buy against how much I would have to waste and found that I don't think solid wood is worth using. I then decided a sonotube would be ok afterall. However, I would still like to use wood but my only other option is going to be plywood. Because of the weight issue and the amount it would take, I think I'm pretty much restricted to 1/4" plywood. My tube will be just short of 55" long and each panel for the octagon would be 7.5" wide. If I use 1/4" plywood, do you think it would be rigid enough? If not, would 1/2" plywood be rigid enough? BTW, I would be gluing the panels together and installing a few basswood baffles on the inside, mainly for bracing purposes.
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Building a f5.24 10" Dob
in an octagonal wood tube
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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A little more info on how I would build it... the tube would set in a cradle that was clamped around it. The altitude bearings would be attached to the cradle, not the tube itself. The cradle would be about 2 ft long so each end of the tube would overhang a little over a foot outside the cradle.
Here's a picture of a 12.5" scope that belongs to my astronomy club. The construction I intend would look something like this...
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Building a f5.24 10" Dob
in an octagonal wood tube
Edited by dreamregent (10/27/09 08:01 AM)
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1459
Loc: Highland Park, CA
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1/4" plywood is plenty strong for an OTA. Most of my scopes are made out of the stuff.
my 8" f/6 Springfield, I used Indian Rosewood plywood, and baffled the inside for strength as well. Each baffle was reinforced with fiberglass, too. That tube turned out to be WAY overbuilt. I could probably stand on it, but haven't wanted to try. Most recently, I made an 8' tube out of 1/4" walnut plywood for a 6" f/15 Jaegers lens I had laying around. Baffles in that one too.
I also used 1/4" teak plywood for a 12.5" f/23 Cassegrain. Made two tubes, the first with baffles and the 2nd without. The one without baffles is MUCH easier to get rid of thermal plumes via a fan. The one with baffles perpetually had thermal plumes all night long, so I don't use it anymore. The tube without baffles has longitudinal corners for the ply facets to glue and screw to. I should take a pic of the construction and post it sometime.
Hopefully, you'll get some additional replies from other woodscope builders here. Coopering solid wood is another possibility. If you have a band saw or a thin table saw blade, you can minimize the waste you'll produce. but a coopered tube will likely need a form to build it up and keep it straight.
Plywood tubes are easy to glue up. If they don't have too many sides, all you need to do is tape up all the joints from the outside while they're all laid out flat, turn it over and run glue down the joints, then roll the whole thing up and glue and tape the last joint, until it dries.
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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AV in CMH
sage
Reged: 11/07/05
Posts: 463
Loc: Columbus Ohio
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Hi,
I have made a few tubes from 1/8 baltic birch ply and all have turned out very well. Others have made tube from 1/16 ply, two layers, with much success.
I have found that some 1/8 plywood shests are more flexible than others so test before you buy.
Here is a picture.
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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Thanks guys... That settles it. I think that I'm going for 1/4" cherry plywood.
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Building a f5.24 10" Dob
in an octagonal wood tube
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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I made a coopered tube for a Galileo scope replica which employed sixteen staves of red alder. I wrapped the tube tightly with nylon monofilament to exert even pressure on the assembly during glue-up and then rolled it back and forth several times over a flat surface until all the staves squirmed into proper registration. It came out surprisngly straight and round. My tube has a much smaller diameter than the one you're going to build, but was a bit longer before I trimmed it to final length. I don't know how the monofilament trick would work on a larger tube such as yours (my staves were only about 1/4" x 3/8" each), but it might be worth a try.
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Questar 3.5 standard - pyrex and BB coatings
Powerguide II
8mm, 12mm, 16mm, 24mm and 32mm Brandons
modified Bogen 3030 w/ homebuilt wedge
Homebuilt Galileo scope and very large and ugly homemade tripod
other odds and ends, including iPod Touch with StarMap Pro (what a marvelous combo)...
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"Nothing exists but atoms and empty space. Everything else is opinion."
Titus Lucretius Carus 99-55 B.C.
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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I'm sure my naivete is showing here but...what is nylon monofilament?
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Building a f5.24 10" Dob
in an octagonal wood tube
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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Fishing line. It's available at any sporting goods outlet in various strengths, and a spool of the stuff (a couple hundred yards) costs only a few dollars. Get the cheapest stuff you can find for this application, maybe off a bulk spool at the sporting goods store. Monofilament is very stretchy, and multiple wraps around an object will exert considerable, evenly distributed clamping pressure. I'd suggest something in the 4-to-8 pound test range for your project. Most glues won't bond very readily to the monofilament, so the fishing line is easy to peel off after the glue cures (I used plastic resin glue because it sets very slowly, which allows plenty of time to get all the wood parts in place, it cleans up with water, and it's quite rigid when it finally cures). My edit adds a picture showing the monofilament wrapped around the tube of my Galileo scope at glue-up.
Edited by ColoHank (10/27/09 12:28 PM)
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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I think tape masking or painter's tape would work best for your octogon. Once you get your strips for the OTA cut and beveled lay them down on a table side by side with the outside faceing up. Press the boards tightly up against each other. Place stips of tape across the boards leaving overhangs on both side. Flip the whole thing over and then you should be able to "roll it up" into the octogon shape. Mind you I've never tried this with an 8 sided object but I know it works with a 4 sided one.
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Orion XT8 - Orion StarMax 102 EQ
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1459
Loc: Highland Park, CA
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I did this with a 9-sided African Padauk tube I made for a 5" f/5 Jaegers lens I have (but haven't finished the OTA!). This tube isn't plywood. I resawed the padauk on my band saw and thickness sanded it to 1/4" with a drum sander:
5" f/5's tube construction
That thread also has a pic of my 8" Springfield with the Indian Rosewood plywood tube (also 9-sided). This scope is 28 years old now, and still strong.
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
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These are all good solutions. I'll add that you can use wood veneer on your sonotube. It will make it stiffer, and looks great.
Ross
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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Quote:
I think tape masking or painter's tape would work best for your octogon. Once you get your strips for the OTA cut and beveled lay them down on a table side by side with the outside faceing up. Press the boards tightly up against each other. Place stips of tape across the boards leaving overhangs on both side. Flip the whole thing over and then you should be able to "roll it up" into the octogon shape.
Like this...
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---------------------
Questar 3.5 standard - pyrex and BB coatings
Powerguide II
8mm, 12mm, 16mm, 24mm and 32mm Brandons
modified Bogen 3030 w/ homebuilt wedge
Homebuilt Galileo scope and very large and ugly homemade tripod
other odds and ends, including iPod Touch with StarMap Pro (what a marvelous combo)...
---------------------
"Nothing exists but atoms and empty space. Everything else is opinion."
Titus Lucretius Carus 99-55 B.C.
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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...and this. Then, wrap with the monofilament.
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---------------------
Questar 3.5 standard - pyrex and BB coatings
Powerguide II
8mm, 12mm, 16mm, 24mm and 32mm Brandons
modified Bogen 3030 w/ homebuilt wedge
Homebuilt Galileo scope and very large and ugly homemade tripod
other odds and ends, including iPod Touch with StarMap Pro (what a marvelous combo)...
---------------------
"Nothing exists but atoms and empty space. Everything else is opinion."
Titus Lucretius Carus 99-55 B.C.
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Yup, exactly! Is there a post about that scope somewhere? I'd like to see how it turned out.
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Orion XT8 - Orion StarMax 102 EQ
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Pedestal
Post Laureate
Reged: 03/11/06
Posts: 3860
Loc: Smoggy Bottom, Baytown,Texas
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Take a look at my thread Rita for my version of a wood tube.
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Hubert
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www.smoggybottom.org
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1459
Loc: Highland Park, CA
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I would add that 2 inch tape is cheap, so run tape down the entire length of the joint between slats, in addition to wrapping it around the "latitudes" of the tube.
When you roll it up, you'll be surprised how tight the joint will become, and how well the tape will hold it while the glue sets.
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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I covered construction of my Galileo scope replica in this forum on 3/7/2009. I made the scope for our club's public outreach efforts in conjunction with the IYA2009 worldwide "100 Hours of Astronomy" initiative. As you might expect, the scope leaves a lot to be desired optically (pity poor Galileo), but it's a crowd pleaser. Moon craters and the Jovean satellites manage to manifest themselves despite numerous aberrations. Sorry about the picture. The thing just doesn't lend itself to very creative photo composition.
--------------------
---------------------
Questar 3.5 standard - pyrex and BB coatings
Powerguide II
8mm, 12mm, 16mm, 24mm and 32mm Brandons
modified Bogen 3030 w/ homebuilt wedge
Homebuilt Galileo scope and very large and ugly homemade tripod
other odds and ends, including iPod Touch with StarMap Pro (what a marvelous combo)...
---------------------
"Nothing exists but atoms and empty space. Everything else is opinion."
Titus Lucretius Carus 99-55 B.C.
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1459
Loc: Highland Park, CA
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Oh, I don't know. I think that's a gorgeous piece of woodworking! Did you turn it on a wood lathe to make it so round?
-Tim.
--------------------
"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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Nope. The tube was faceted (for lack of a better term) following glue-up, since the outer surface of each of the sixteen staves was still flat. I just hand-sanded the tube to knock off the little ridges where the staves abutted one another to make it round. The lens cells at either end started as solid blocks of wood and were turned on a lathe. Both were fitted with smaller diameter tubes, also of stave construction, that slide into the ends of the main tube with a friction-fit to permit focusing.
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---------------------
Questar 3.5 standard - pyrex and BB coatings
Powerguide II
8mm, 12mm, 16mm, 24mm and 32mm Brandons
modified Bogen 3030 w/ homebuilt wedge
Homebuilt Galileo scope and very large and ugly homemade tripod
other odds and ends, including iPod Touch with StarMap Pro (what a marvelous combo)...
---------------------
"Nothing exists but atoms and empty space. Everything else is opinion."
Titus Lucretius Carus 99-55 B.C.
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dawsonian2000
professor emeritus
Reged: 07/26/06
Posts: 574
Loc: Riverview, FL, USA
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Hi Dreamregent,
I think 1/4" plywood would work if you instill baffles in the mix, as you mentioned. Outside of using basswood for baffles, 3/4" baltic birch plywood would suffice as well. I consider making a wooden tube for my 10" f/5.6 Newtonian, but decided to go with a Sonotube. I like that I could easily rotate the OTA for more ergonomically acceptable viewing positions. Not being satisfied with how a Sonotube would hold up under those moisture laden nights, I decided to coat the outside of the Sonotube with a polymer two part resin, which work exceptionally.
Whoever built that Newtonian, I must say that you have an outstandingly well constructed instrument. Great design, finish, and execution!
Mel
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Clear Skies, Forever!
The Vega Sky Center
10" (254mm) F/5.65 Home Built Fork Mounted Newtonian Reflector
5" (127mm) F/9.4 Home Built Refractor (under construction)
3.5" (90mm) F/11.1 "Vixen-Spec Modified" Konus Refractor
3.1" (80mm) F/6 Scopos ED APO Refractor
4.5" (114mm) F/8 Tasco 11te-5 Newtonian (under restoration)
2.4" (60mm) F/16.7 Tasco 7te-5 Refractor
2.4" (60mm) F/15 Jason Discoverer 313 Refractor
http://www.vega-sky-center.com
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