eric moerman
super member
Reged: 11/08/03
Posts: 279
Loc: belgium
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The titel says it al,if cost would be no object what would be the best material to make a tube.
Im specialy interested in thermal behavior of materials.
I have read somewhere in a tread somebody saying that carbon fiber would make to much tube currents when cooling down and when tempeture is changing.
I was pretty surprised about that buth now i can find this tread back anymore to aske this person about it  so im asking al of you what you thinking about it?
When you would make a telescope tube and you have the choice to make it out of aluminum(1mmthicknes),carbon or wood (2mm baltick burch plywood)what would you choose to have the best thermal behavior inside the tube?
Any idee's are verry welkom.
Greatings,eric
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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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Hi Eric!
The lightest, strongest and strongest construction material is composite and uses a 1/4" thick structural foam core called Divinycell.
Carbon Fiber is a boutique material now. Because it is esthetically pleasing it has become popular for use even in applications where it's not needed.
Carbon fiber is a wonderful material for many applications. It is light, stiff and strong. Pound for pound, it is 9 times stronger than aluminum and 30 times stronger than steel. It doesn’t corrode. It doesn’t fatigue. A well made structure for an OTA is easy to develop and will last longer than the metal part it replaces.
The downside is that the the volume of structural material in a tube is so small, that the reduction in weight is slight. Worse yet, the loads imposed on a tube are relatively light so the increase in structural benefits are hardly likely to be realized. And, of course there is the insane cost!
Most telescope structures will be more than adequate in strength and extremely light when made using cored composites employing Divinycell cores and conventional woven fiberglass.
Divinycell comes in an assortment of thicknesses starting at 1/4" but the most popular and most easily attained is 3/8". You can also buy it with premade slits that allow it to be wrapped in right radii around a mandrel.
For my small structures I use a 1/8" thick cork-like material that is attached in half inch squares to a flexible scrim. It looks like fly screening and allows the user to press the core material down into deep molds.
The use of core materials can be very advantageous. When core material is used to double the thickness of a structure, the relative stiffness increases 7 times. The strength increases 3.5 times while the weight only increases 1.03 times. When core material quadruples the thickness, the relative stiffness increases an incredible 37 times, the strength increases 9.2 times, but the weight only increases a mere 1.06 times.
These statistics are right out of the Composite Engineering Handbook, a Bible for anyone who is involved in the design and construction of composite aircraft. At $200, I don;t have a copy myself but technical data like this is available at vendors like FiberGlast.com
If you wrap two layers of 8 ounce bi-directional fiberglass, with it's weave at 45 degrees to the center axis of the tube, then apply a 1/4" or 3/8" core, followed by another two layers of BID, you will have a structure many times stronger than your current tube and many times lighter.
The reason for tilting the two ply is that the angle forms two double helixes which offers the best combination of low weight,and both torsional and beam stiffness. You will not build a telescope tube with such dramatic mechanical stiffness than using this method. That it's also lighter by far makes the materials and method even more amazing.
Foam cores like described here are structural materials and not to be confused with the thermal foam insulation used to mold coolers and coffee cups.
Nevertheless, they are still closed cell materials and as such, have excellent insulation properties. A telescope tube made in the fashion described above and painted white will offer no thermal problems as it's specific heat is not adequate for storage.
There is a string I started here here at Cloudy Nights about how to build using this method. It was within the last 6 months.
I need to leave for work or I'd go find it. If you haven't found it soon, I'll go dig it out.
Regards
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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eric moerman
super member
Reged: 11/08/03
Posts: 279
Loc: belgium
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Hi Art,
thanks for the reply,i already expected you to be the first 
I happen to have enouf carbon fiber for free to make a tube for my scope buth its only enouf to make 1 layer.
So i wonder if its possibel to make a tube of 1 layer carbon and maybe on the inside puth corck for insulation.
I dont worry about strengt so much,even a wooden tube is more then strong enouf for a scope buth if i can reduce the weight of the tube on the upside the balance point will become lower and so i can make my forkarms shorter .
You also now that its the best way to stiffen up a mount by making the arms as short as possibel.
Maybe its bether if i would rebuild the arms of my fork using this carbon?
eric
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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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Hi Eric!
If you use carbon fiber, one layer is all you need but you'll need one layer on each side of the core. The difference in weight between fiberglass and CF tubes is hardly noticeable.
CR is expensive stuff and I won't encourage you to make your first composite device with such exotic material.
I suggest you save it until you have more experience working with this technology and have accumulated the extra CF you need to do things right.
Building the fork is remarkably simple. You would cut and bond the same geometry as you did the metal one. I would add bulkheads this time, not foam, to the interior.
Must go to work!
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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Stefan Rostyne
Pooh-Bah
Reged: 10/19/04
Posts: 1026
Loc: Assenede, Belgium
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Eric,
If thermal insulation is what you need, nothing can't beat sealed wood for a tube. wood is also a closed cell structure. Aluminum actually is to be avoided, unless you're looking for tube currents all night long.
If you're looking for serious strenght, I would suggest to go with Art's layered composites. Composite is also a *bit* lighter than wood. Carbon is very aesthetic, and if you want a beautyfull shiny, bright tube on top of that wonderfull mount you made, then you *must* go for carbon. (Especially if you get the stuff for free   )
The small one you see below in my signature is completely made of wood, sealed of and impregnated with epoxy - I simply used wood as a part of the composite  - It is by far the best telescope tube I ever made (-last it dropped from 1 meter high on the concrete floor, nothing happened, just had to re-collimate...). The empty tube weighs less than 1 kg, and that is, baffles and all, just made of 4mm meranti ply (uit de Gamma!!) If you go for 2mm birch, you could even save a substantial part in weight, yet loose no strenght when you impregnate it with epoxy. Here you can see how I made it.
For a next project, again I will go for the skeleton build, but I would leave the plywood walls and immediately lay the veneer on it. Loosing some more weight... Where do you find such thin birch, over here?
If your carbon is not enough for more than 1 layer, then why don't you just cary it upon a layer of the birch? It will insulate at the inside and look like plain carbon on the outside?
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
22" f/3.5 dob project
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Alan French
Night Owl
Reged: 01/28/05
Posts: 4009
Loc: Upstate NY
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The issue with aluminum is not the material, it is the paint. A painted aluminum tube generally has high emissivity, so it cools very rapidly and can cause problems both with tube currents and with dewing of the optics. Unpainted aluminum has very low emissivity, and works extremely well. For appearance, you can give it a nice "jeweled" finish with a very simple jig.
Anyway, that's my choice for future projects.
Clear skies, Alan
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Anonymous
Unregistered
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Found a really good telescope making engineering Internet page and the guy is a real mechanical engineer (See: http://www.tfn.net/~blombard/book/MechanicalDesign.htm). He discusses most of what I do in some of my articles, but with more knowledge. We seem to disagree on some things like tube color and materials, but agree 99% otherwise. I may have to add some stuff to my articles to clarify things about when I say that a black telescope cools down faster than a white one. The black paint I use is a brand and type that has high emissive properties and the white paint has very low emissive properties, such as titanium powder white. It is expensive and not easily found in hardware stores but it can be found.
Don Parker and I used a temperature/IR measuring gun on my aluminum telescope before and after painting. We charted the time and temperatures at different times of day and night, before and after painting. I don’t care how much theory is used we concluded that after painting the inside of the tube titanium powder white and gluing cork to that, then a light black coat, with a coat of high emissive flat black my 12.5-inch tube cooled down to ambient air temperature at least 100 times faster than when it was unpainted. I remember doing this before with my Cassegrain by replacing the fiberglass tube with aluminum and the difference was light night and day. In fact I was ready to pitch the Cass out for a Newt before changing the tube.
Anyway, I need to find out where I got those paints!
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NHRob
Post Laureate
Reged: 08/27/04
Posts: 4111
Loc: New Hampshire
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How about an aluminum tube with the new Protostar tube liner?
Rob
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Rob ... my avatar pic is me ... on a good day!
**********************
Canon 12x36 IS binos
Spooner 6.5" f/8.6 dob w/Dobstuff mount
Past Scopes:
Starstructure 14.5", Starmaster 14.5", 10" f/6 Zambuto homemade dob
AP130EDSF, AP130EDT, Traveller, AP152 f/9, 6" AP Christen triplet,FS-102(x2),FC-100,FS-152,TOA-130,TEC-140(x2),TV-101, TV-102,TV-76,TV-85, WO Z80FD, SV105/LZOS, WOLZOS100/800, TEC-6,TEC200/15,MN56,APM130/1200/LZOS,Mewlon180, Jaegers 6" f/15 achro, Palermiti 6" f/15 achro
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Alan French
Night Owl
Reged: 01/28/05
Posts: 4009
Loc: Upstate NY
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Hi Jeff,
If you have a high emissivity tube, you need an insulating layer on the inside. Otherwise you will have trouble with tube currents, and, on some nights, dewing of the optics. My wife's 6" f/8 in a painted aluminum tube featured a secondary that almost always dewed over, and a primary that frequently dewed up. Lining it with cork and removing the paint fixed the problem. (The cork alone might have done the trick - but the paint was in sad shape.) With smaller scopes, it seems they cool quite effectively by conduction and convection, but I can see that high emissivity could aid the process.
We often have dropping temperatures much of the night, and the cooling of the optics seems a bigger issue than cooling of the tube.
Clear skies, Alan
(Remember the good old days of NVN?)
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NeoDinian
Experienced Postmaster
Reged: 10/05/05
Posts: 14043
Loc: Rockford Illinois
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Instead of painting the Aluminum tube, take it to a plater and have it Anodized. You can have that done in a wide variety of colors nowadays... It would last longer than the painting, and not have the thermal problems associated with painting.
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Neo... (Jeff)
-=-=-=-=-=-=-
10" LX200-GPS/SMT UHTC "Draco"
 Rockford, Il.
NeoDinian's Eye on the Sky!
Coming soon:
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Alan French
Night Owl
Reged: 01/28/05
Posts: 4009
Loc: Upstate NY
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Anodized aluminum has a much higher emissivity than untreated aluminum. I remember Telescopics sold Newtonians with anodized aluminum tubes and thinking how dewy they would be here (no internal insulating material, as far as I could tell).
Clear skies, Alan
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Jeff Morgan
Post Laureate
Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
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I'm using a powder coated aluminum tube with a ProtoStar liner. The liner has improved performance greatly. My observing location in norther Arizona does not feature dew (although I have gotten exterior frost, especially in January and February), but the seeing here is terrible. After sundown the temps go down like on the ice planet Hoth. Even my 4" APO was plagued by it with maximum magnification being about 200x (and not too pleasing at that!). With the liner I am almost at that point. The Hastings tube I have is not perfectly round by any means, during the next scope maintenance session I am going to pull the liner and affix some more foam spacers to insure I have liner-tube separation (air space) at all points.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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Neo, have you any idea what it costs to anodize an aluminum tube that size?
I suggest you check around. First find someone who has tanks big enough to do the job. Then be sure to sit down and hold your breath when you ask "How much?"
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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Anonymous
Unregistered
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Quote:
Hi Jeff,
If you have a high emissivity tube, you need an insulating layer on the inside. Otherwise you will have trouble with tube currents, and, on some nights, dewing of the optics. My wife's 6" f/8 in a painted aluminum tube featured a secondary that almost always dewed over, and a primary that frequently dewed up. Lining it with cork and removing the paint fixed the problem. (The cork alone might have done the trick - but the paint was in sad shape.) With smaller scopes, it seems they cool quite effectively by conduction and convection, but I can see that high emissivity could aid the process.
We often have dropping temperatures much of the night, and the cooling of the optics seems a bigger issue than cooling of the tube.
Clear skies, Alan
(Remember the good old days of NVN?)
Yeah, I forgot to mention the cork. I always line tubes with cork, but my 12.5" f/7 tube the cork is beginning to come unglued. Of course, that is after that is after using for over 20 years. I took the cork out of my 16” but it is a truss tube with a 33” bottom section (primary end). I have never had any dew problems with either scope. At times the secondary will get some dew but that is easy to take care of. Hum, I forget to say the truss is covered with Nylon.
True that metal tubes will cause weird temperature related problems. The temperature in metal tubes actually gets cooler than OAT so if the dew point is anywhere near the OAT the dew is always a problem.  My problem with writing about it is that as an engineer I always assume others will understand everything I infer! Then again, I am only a wanna-be mechanical engineer 
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Anonymous
Unregistered
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Quote:
How about an aluminum tube with the new Protostar tube liner?
Rob
I forgot to say that I alwasy use cork as a liner, but sicne never hearing of the Protostar tube liner I cannot say. It is probably a good thing to use since their other products seem to be very good.
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Alan French
Night Owl
Reged: 01/28/05
Posts: 4009
Loc: Upstate NY
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The unpainted, raw aluminum tubes almost never get covered with dew. Because of their low emissivity, they can only get slightly below ambient air temperature. High emissivity materials can get farther below ambient air temperature and are more prone to dew.
Breezy nights are nice because the forced convection keeps stuff warmer and dew doesn't usually form.
Clear skies, Alan
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Anonymous
Unregistered
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Quote:
Instead of painting the Aluminum tube, take it to a plater and have it Anodized. You can have that done in a wide variety of colors nowadays... It would last longer than the painting, and not have the thermal problems associated with painting.
Yes, and I actually used an 8-inch with an Anodized tube that was he greatest. The guy had it at the 1994 RMTC and had used my article to design his telescope. Actually, old Bob Cox was my inspiration for it. The problem with Anodizing a 92” long, 15” diameter aluminum tube would be cost – I suspect. 
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Anonymous
Unregistered
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Quote:
I'm using a powder coated aluminum tube with a ProtoStar liner. The liner has improved performance greatly. My observing location in norther Arizona does not feature dew (although I have gotten exterior frost, especially in January and February), but the seeing here is terrible. After sundown the temps go down like on the ice planet Hoth. Even my 4" APO was plagued by it with maximum magnification being about 200x (and not too pleasing at that!). With the liner I am almost at that point. The Hastings tube I have is not perfectly round by any means, during the next scope maintenance session I am going to pull the liner and affix some more foam spacers to insure I have liner-tube separation (air space) at all points.
No amount of engineering can help in a bad location. The problem with many of us is we live in the worst places for observing. I lived in south Florida where seeing was great but at times the dew could be very bad. Now I live in central Florida and seeing here is even better than south of Miami. However, before I discovered (with some friends help) my seeing was not good and after moving my telescope from next to the house all changed. The roof was causing bad seeing. If you are low and with mountains around then you know what your seeing problems are.
Anyway, will have to Google this ProtoStar liner, sounds interesting.
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Alan French
Night Owl
Reged: 01/28/05
Posts: 4009
Loc: Upstate NY
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Most of us should probably consider moving before we worry about what telescope to buy or what type of tube to use, at least if we are interested in the planets.
Clear skies, Alan
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Qkslvr
Pooh-Bah
Reged: 06/23/06
Posts: 1316
Loc: NE Ohio, US
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You know there are some fancy coatings available???
http://www.jet-hot.com/Pages/coatings.html
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Mike
Onyx 80ED/N8/CG-5/40D
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