Ideas for a tube material for a solid tube OTA?

[Harpage]

I've been thinking of turning my Heritage 130p into a 5" imaging Newtonian and right now I'm just adding up all the costs to see if it's cheaper to do so or just buy a 6" Newtonian locally. But basically, I was just asking what you would recommend for a solid tube? It seems like Sonotube or concrete forming tube is very common in the US, but it doesn't seem to be available in the sizes which I need, and appear to come in only very large sizes (>300mm, several metres long). Is PVC pipe a good option?

[hamishbarker]

galvanised steel duct is very cheap and readily available at ventilation suppliers.



But you do need to reinforce around the focuser for an astrograph to prevent local flexing of the tube due to the bending moment of the focuser and camera. I'm in the process of doing so to my homemade dob/newtonian because I do spectroscopy so the load on the focuser is a lot more than a camera.

[bojan]

Cardboard tube (paper), soaked/impregnated with epoxy or polyester resin is also good.

[John K]

A cardboard display tube is your best bet. Ideally an off cut. Add internal spacer/cut out sections to strengthen further or at each end.

Painted with a mat paint inside and a nice gloss paint on the outside.

Otherwise Carbon fibre or find someone locally to simply roll an aluminium tube.

PVC will flex too much.

Good luck.

John K.

[Harpage]

I have managed to find some stove pipe for fairly cheap prices online. They say it’s made from stainless steel, but don’t mention anything about thickness or whatever.

Would these be a good alternative to cardboard tubing? It’s pretty hard to come by because they often come in sizes way large for my needs, and I need to get a quote for every place so shopping around will be fairly time consuming.

[Sunfish]

I have some of that Stainless stove pipe I have been using to make big gutters. It is helically constructed and welded and not very high quality stainless . So it has a spiral weld on the surface. I am not sure if this would create an issue but I would be interested to hear if it could be used.

[doug mc]

A helical seamed structured tube would be much stronger than a single seam one. Bit ugly though.

[Sunfish]

The stainless stove tube I have is 1mm thick and 225 diameter and a little over 1m long once you take off the sleeve. A bit heavy but very robust . I wonder if it would make a good f4.5 newt or if it is too flexible .

[Sunfish]

The weight of the 1.1 m .225m ss section including the collar is around 7.5 kg , a little heavy but if only 700 mm was used for an f4 that would equal 4.7kg for the tube alone less the penetrations.

[hamishbarker]

1mm is a bit thick. stainless has lower yield stress than carbon steel. I have a 0.55mm straight seam galvanised steel tube on my dob, $27 from the local duct shop. now locally reinforced at the focusser by riveting a ring of 5mm flat aluminium.

[Sunfish]

Thanks for that advice. I think if I take my ss tube to the sand blasters that would,reduce the thickness a little . The first use might be an f7.2 newt as I have all the parts and so the front section , or the whole middle section, may be simply open aluminium struts.

I have also looked at carbon fibre as I have been doing a little epoxy fibre glass work lately and it could not be too hard to use the tube and some plastic sheet under foam as a mandrel form