Performing a lobotomy on a sonotube

[pmrid (Peter)]

I've been lingering over a decision to abandon my old hot-water system OTA in favour of a lighter truss design but before going that far, thought it might be feasible to get the saw/grinder out and cut holes in the sonotube to create a sort of truss design. I wonder if anyone has ever tried something similar.
You'll get the sense of what I'm thinking from the attached rough 2-dimemsional image.
It is a Dall-Kirkham scope and the tube has to be 1200 long so I imagine it would need the strength of a central rib section at about 400mm or so. The bulk of the remaining weight in this little project is in the mirror/cell/focuser/baffle assembly at one end (about 12 Kgs) and the secondary/spider assy at the other (about 3 Kgs). The existing tube weighs 12 Kgs and I would like to be able to sacrifice as much of that as I can without losing structural integrity by inducing flexure.

I'm hoping that by keeping the sonitube "trusses" to about 80mm wide and the end and "rib" sections being about 150mm wide.

Any thoughts?

peter.

[richardo (Rich)]

Hi Pete,
always interesting to put thoughts into practice.
Experimentation is the way to progress and learning.

Your thought with the 'sono tube' is not really going to give you any strength.
I originally used a sono tube for my first tube on my F4 10" Newt and found that things seemed ok during the first winter (I even had the tube coated in resin).
Further testing past this experimental stage and into summer, I found it had some small amount of flex when imaging @ 1.3 arcsec/pix and trying to auto guide out past 2min. I'm not sure if the actual tube took on moisture due to a sort of capillary action with the extreme heat here drawing moisture or what. You wouldn't think it would with the tube being sealed. But something changed and flex became an issue. Perhaps atmospherics.
I discarded this tube for a composite one.

If you have the time and don't mind spending a little money after the chop to the tube, you could try using epoxy resin and carbon cloth to strengthen it. But don't expect great results for imaging especially if you have a good deal of weight hanging off the focuser and a fairly heavy mirror and cell... might be ok for visual though.
Perhaps for the time and money, it might be worth seeing how much a Carbon fibre tube from say 'Public Missiles' would cost... the AUD is just about right now.... here's a link
https://blastzone.com/pml/

What ever you decide, it's still good fun to try things.

All the best
Rich

[pmrid (Peter)]

Originally Posted by richardo

Your thought with the 'sono tube' is not really going to give you any strength.

Thanks Rich. I wondered about that. But I have a cunning plan for that. I saw a wire truss design recently that impressed me with it's simplicity - applied to this scope (see the pictures of the "after" version), I am planning to run a stainless wire in a continuous loop through small pulleys at the apex of each truss triangle - add a turnbuckle to tension it all up and once it's in tension, all the forces of compression and expansion should be in equalibrium. I wish I knew what the dickens I was talking about but with all the learning I have gained from endless episoneds of New Inventors, I am convinced this could, just could, work.
Peter.

[richardda1st (Richard)]

Good luck with the cable tensioning idea, it might work.

Not knowing the rigidity of your tube and the tension you will need to apply to it is all guess work.

My thoughts would be that you may end up crushing the tube or at least cause it to bow outwards, probably at different amounts around the tube.

[pmrid (Peter)]

Originally Posted by richardda1st

My thoughts would be that you may end up crushing the tube or at least cause it to bow outwards, probably at different amounts around the tube.

Hmmm! Yes. Not such a cunning plan was it? OK -some bracing instead perhaps. It may turn out that the sonotube is now worthless but what they heck. I still have all the materials on hand now to do an aluminium truss tube so it is a case of nothing ventured ...

Peter.

[ZeroID (Brent)]

Jigsaws are dangerous things aren't they ?
You can cut a lot away real quickly.

How about bonding some alum extrusion (10 x10 Angle eg ) to the outside of the 'truss' struts. Epoxy or even silastic would stiffen it up immensely although it might defeat the whole weightloss idea. You'd have to figure out the loss/gain but you could also reduce the truss widths further.

In fact, taking it to the ridiculous. Use 25mm square or round maybe and create the trusses and secure them to top and bottom etc. Then cut away ALL the middle section. Hey Presto, instant truss scope !! All aligned as well.

Sorry, got carried away there for a moment.