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Equipment Discussions >> Reflectors

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Aleko
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Reged: 08/01/10

Loc: Georgia
Best tube materials?
      #5671695 - 02/10/13 01:02 AM

Curious as to what materials are considered to be the best for making a solid tube for a Newtonian reflector. Thinking mainly along the lines of thermal properties, coming to ambient temperature quickly. Older scopes seem to be fiberglass, sonotube, Bakelite, steel. I imagine recent productions use other materials. What is now considered the state of the art?

Thanks,
Alex


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derangedhermit
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Reged: 10/07/09

Loc: USA
Re: Best tube materials? new [Re: Aleko]
      #5671746 - 02/10/13 02:27 AM

Coming to ambient temp quickly is good, but materials that do that tend to also continue to cool and drop below ambient faster and farther than more insulating materials. As far as I can tell, this is as much of a problem as being above ambient. I'm not sure what state of the art is, but most larger Newtonians use truss structures, with or without a cloth shroud, and almost all of the inexpensive small to medium Newtonians use steel tubes, more for matters of cost and appearance than for providing optimal views, I suspect. ATMs still use Hastings (or other) aluminum tubes, lined or not, along with concrete form tubes made of rolled paper, and also there is the Protostar phenolic tubes. The last would be my preference.

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JM La Galette
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Reged: 11/24/07

Loc: France
Re: Best tube materials? new [Re: derangedhermit]
      #5671832 - 02/10/13 05:28 AM

Sandwich carbon composite seems to me the best material for tubing, and carbon composite for truss.
Light, stiff, stable in temperature with near to zero CTE, (no focus change during night).

JMarc


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Jon Isaacs
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Reged: 06/16/04

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Re: Best tube materials? new [Re: JM La Galette]
      #5671911 - 02/10/13 07:19 AM

Quote:

Sandwich carbon composite seems to me the best material for tubing, and carbon composite for truss.
Light, stiff, stable in temperature with near to zero CTE, (no focus change during night).

Here is my home made tube. The whole thing should be finished end of this year.

JMarc




For visual, focus shift is not an issue. The insulating properties of Carbon fiber maybe OK, in the refractor world and SCT worlds CFRP tubes mean slower cool down.

Jon

Edited by Jon Isaacs (02/10/13 07:20 AM)


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: Best tube materials? new [Re: Jon Isaacs]
      #5671925 - 02/10/13 07:50 AM

This has been discussed many times. The Protostar tubes are excellent--they do not have a lot of thermal mass, nor do they overchill via radiation to the cold sky. They are lightweight and competitive in price with metal tubes. Sonotubes (concrete form tubes) are a good low cost solution, but you need to find quality tubing. Some of the stuff sold in the big box stores is too flimsy to work well. Any metal tube should have a thin layer of insulation inside to avoid over chilling. Uninsulated steel, as used in most Chinese scopes will form dew or frost inside the tube under adverse conditions, and can lead to dewing of the primary and secondary mirrors.

JimC


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dpwoos
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Reged: 10/18/06

Loc: United States
Re: Best tube materials? new [Re: Mirzam]
      #5672217 - 02/10/13 11:53 AM

Quote:

Sonotubes (concrete form tubes) are a good low cost solution, but you need to find quality tubing. Some of the stuff sold in the big box stores is too flimsy to work well.




The ones that I have gotten from places that supply the concrete/building trades are excellent - night and day from the cheap stuff sold at Home Depot and Lowes. Sonotube makes a great tube, and in fact I finish mine with clear coatings so that folks can see that they are homemade. Pretty cool for public observing - I can't count the number of times I have heard stuff like "Honey, I bet you could build that"!


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Aleko
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Reged: 08/01/10

Loc: Georgia
Re: Best tube materials? new [Re: derangedhermit]
      #5672490 - 02/10/13 02:41 PM

Quote:

Coming to ambient temp quickly is good, but materials that do that tend to also continue to cool and drop below ambient faster and farther than more insulating materials. As far as I can tell, this is as much of a problem as being above ambient.




Thanks, I did not know that. It might explain why the old 50-yr-old homemade scope gives outstanding images long before other scopes have equilibrated, but later the images soften up with more tube currents.

Alex


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careysub
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Best tube materials? new [Re: Jon Isaacs]
      #5672923 - 02/10/13 07:36 PM

Quote:


For visual, focus shift is not an issue. The insulating properties of Carbon fiber maybe OK, in the refractor world and SCT worlds CFRP tubes mean slower cool down.

Jon




Is this a known issue?

It looks like the CF transverse thermal conductivity is actually about twice that of phenolic/paper so it would not seem to be the case for a solid CF tube.

Of course many CF tubes use a low density core which would be very good insulators. Should these then be cork lined to avoid thermal problems?


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