13 replies to this topic

[Jacob.Redshift]

So I am investigating some materials for truss tubes for a 25” f5 dobsonian. So we are talking around 10 feet, for the sake of visualizing this.
I keep thinking that fiberglass should do well. But, am I missing anything?

 

1. The modulus of elasticity is slightly better than aluminum. (If I read my sources correctly.)

2. The weight of fiberglass is about 50 percent lighter. (Actually, it ends up similar in the end because the thickness for fiberglass tube at 1.5” OD is about twice as much as for an aluminum tube.)

3. Fiberglass’s Thermal coefficient is lower than aluminum, so it should bother focus and collimation less.

4. The price of fiberglass tubing is significantly cheaper. Like half! $500 cheaper.

It seems that if I use fiberglass tubes instead of aluminum, I would have a more rigid system, with similar weight, slightly better thermal properties, and save $500… 

 

So, is there a reason there aren’t a million dobs with fiberglass trusses? Am I missing a calculation or other consideration somewhere? Or is it time for a fiberglass truss revolution? 

 

Thanks!

 

 

[triplemon]

Fiberlass may have a lower specific weight than aluminum. But if you were to make a fiberglass pole anywhere near as thin as the aluminum ones, it had no rigidity against denting or squeezing that thin wall.

 

Aluminum truss poles usually 0.7 to 0.9mm thin.

 

And no, a set of aluminum tubing isn't that expensive. Just shop around a bit.

Edited by triplemon, 24 February 2025 - 10:08 PM.

[Sarkikos]

I built a 6" f/8 Newt in the early '70's.  The tube is fiberglass!   The scope is long since taken apart and the tube is lying on a shelf in the garage. 

 

I know it's not the same thing, but there it is. 

 

Mike

Edited by Sarkikos, 24 February 2025 - 10:13 PM.

[Bob4BVM]

Same reason i would not use CF tubes- too fragile to crushing and denting impacts.

I don't think FG  or CF would be lighter than Al for same stiffness

 

FG and CF are my preference for other things, like fishing rods, noted for their flexibility, totally opposite application, but that might tell you something.

[drspiv]

I used to do racecars: most of the time when someone asked for carbon fiber they actually wanted the properties of fiberglass. Much like carbon fiber, and everything else, its properties are dependant on the engineering: you want extremely flexible? There are swim fins that can fold back on themselves. Want extremely stiff? The most rigid monocoque racecars, or even boat hulls.

Fiberglass tends to be a little more resistant to impacts (which made it great for body panels), but not as much as aramid (which we defaulted to for skirts and front facing things that would take rocks).

In short: if folks can make carbon and fiberglass work for mountain bike wheels (they do) I'm sure there's an engineering solution for telescopes.

Fun trick: start with a thin fiberglass tube, fill it with stiff polyurethane foam. It will still be very light weight, resist dents and crush better, and make the total length somewhat stiffer in the process.

[TOMDEY]

Fiberglass is typically brittle and "mushy", despite the numbers. it's typically very anisotropic in tube form. For those reasons I wouldn't trust it.    Tom

[Phil Perry]

In https://www.cloudyni...be-rabbit-hole/ , there was a proposal to use a FG tube (not truss). One of the votes against it was that the FG would have more thermal mass than the Al. Plus, FG is nasty to work with.

[triplemon]

Fun trick: start with a thin fiberglass tube, fill it with stiff polyurethane foam. It will still be very light weight, resist dents and crush better, and make the total length somewhat stiffer in the process.

Make sure you have a layer of circumferential fiber then too. Or that foam will burst your tube long before its done expanding.

 

I mean, if you had the time and money, you could likely engineer something. But given the small cost and weight issues with aluminum poles, there is a very limited possible upside with that. So why ?

 

You can cut tube cost and weight in half by making it a string scope:
https://www.sidereal...ogy.com/28inch/

 

 

Edited by triplemon, 24 February 2025 - 11:49 PM.

[Oberon]

You can cut tube cost and weight in half by making it a string scope:

Sadly no.

As a matter of principle, weight for weight, a string scope cannot be as stiff as a properly built truss.

PS. when this claim came up before on CN, Albert Highe noticed and quietly corrected the proponent via PM.

 

[a__l]

 and save $500… 

 

 

I bought carbon tubes for my 24" f/3.3 in China for about half that amount
It's about half the weight of the black anodized aluminum ones I had on this scope before. I'm quite happy with the replacement. Nothing has cracked or broken in a few years.
Question: Why would you invest in a 25" f/5. A heavy, tall scope that you'll get bored of quickly.

[Jacob.Redshift]

Thanks for the input and advice everyone. Much to think about! 

[Jacob.Redshift]

I know it's a beast of a scope, but I couldn't turn down the opportunity. The price was right. And I've never looked through anything over a 17.5" scope. 

[Jacob.Redshift]

 

You can cut tube cost and weight in half by making it a string scope:
https://www.sidereal...ogy.com/28inch/

 

 

Mostly, the price for aluminum tubes at 10ft lengths, with shipping, becomes very expensive. 

[symbiosis]

Same reason i would not use CF tubes- too fragile to crushing and denting impacts.

I don't think FG  or CF would be lighter than Al for same stiffness

 

FG and CF are my preference for other things, like fishing rods, noted for their flexibility, totally opposite application, but that might tell you something.

I used CF in my 12" f/5 and it works great. Plenty stiff and much lighter than aluminum. I just have a rule about not running over the truss poles at star parties. No issues.