truss poles for 12" Dob: what size works best?

[janoskiss]

Found a local supplier for aluminium tubing to use for a 12" truss Dob (thanks Vermin!). For those who have done this before, what size tubes do you recommend for the truss poles? These are the sizes available from the supplier:

Outer diameter : wall thickness (in mm)
25 : 1.6
25 : 3.0
32 : 1.6
32 : 3.0

[AndrewJ]

Gday Steve

Just for info ( as yr in Melb ), i have found Capral Aluminium to be an excellent source of small dia alum tubing, but if you want larger sizes
( including true 2" OD that fits into 2" fittings ), you can also try
Light Spars of Australia in Moorabbin. They do a whole range of thinwall, high tensile alum tubing for yachts and hanggliders etc.
They have a range of inch size tubing that fits one inside the other with minimal slop. Very good for making miscellaneous bits.

Andrew

[Vermin]

Changing the outer diameter makes a bigger difference than wall thickness when considering mass vs stiffness.

For a 12" (f5?) 25:1.6 should do fine, 32:1.6 will be better, but probably not required, someone with Krieg's book (I sold mine), should be able to tell you for sure.

[janoskiss]

I've got the Dob book, but I forgot to mention, I'd be after a 6-pole design (like the Lightbridge) rather than an 8-pole one described in the book.

The 32mm tubing looked more confidence instilling than the 25mm. :confuse3:

[Vermin]

As a very rough approximation, 12" : 16" = 6 poles : 8 poles (same ratio, 3:4) so use what is recommended for a 16" scope with 8 poles.

Stiffness v's number of poles is probably not a linear function but by using at least the minimum diameter recommended for the 8 pole 16" you should be ok.

[Starkler]

Steve I'm intrigued as to what kind of design you have in mind :)

[janoskiss]

As simple a design as possible Geoff. I'll post more on it later.

The scope got here today. It's a white one! As soon as I looked at the beautiful white tube I was having second thoughts about hacking her to pieces. But I think I'll still go ahead with it. I'm not sure whether to get a pro to cut the tube or if I should give it a go with the angle grinder.

And I got it with a Crayford for no extra charge. :) And it's a good Crayford! not like the bumpy & sloppy one in my 8" Dob. :D After having tried quite a number of them I consistently found that there are two models:

Good GS Crayford: EP securing screw on top (12 o'clock)
Bad GS Crayford: EP securing screw on the right (3 o'clock)

Good = smooth, no barrel slop even with tension off (slip under weight yes, but slop no) :)
Bad = opposite of good :mad2:

[acropolite]

Steve 25mm x1.6 aluminium tube will be fairly rigid, but it may be better to go to 32mm. What size is the Lightbridge, it looks like around 25-30mm; whatever that is would be a safe choice.

[Satchmo]

The formula in The Modern Dobsonian for pole diameter is actual pole length divided by diameter should be about 80. Changing the thickness does not make a whole lot of differnce to stiffness compared to changing the diameter. 20mm poles are therefore ample for a 12" F5 scope . It is only supporting a top end weighing a handful of kg. I'd avoid 6 pole trusses though.

Mark

[janoskiss]

Thanks Mark. I will be using the original OTA (with a good length of the the midsection removed), so the secondary cage will probably be heavier (OTA being steel) than on custom made Obsession style Dobs. Hmmm... 8 truss poles could get pretty crowded.

[Roger Davis]

Take note that when you use 6 instead of 8 truss tubes the protrusion of the truss rods into the light path is possible. The Meade Lightbridge, for example, has an oversized primary mirror box with a smaller secondary cage. This enables you to "toe in" the truss rods without impinging on the light path.
Re the focusers, I am looking at five at the moment where the orientation of the eyepiece retaining bolt is totally random ... go figure.

[janoskiss]

Thanks Roger. I noticed that I will probably have to mount the mirror end of the poles further away from the tube. I'm still trying to figure out how exactly ... Kriege & Berry has given me a few ideas. It would be nice to attach the poles so they are completely clear the solid tube. That way I could align all the hardware (and temporarily attach with double sided tape say) before cutting the tube. But some intrusion (into the tube not into the light path) might be desirable to stop the whole thing becoming too bulky.


Hmmm. Maybe it really is just luck of the draw then after all.

[ving]

gunna hack the tube yourself!
man are you brave! and i thought davo was brave cutting a hole for a fan!

[Roger Davis]

Well, if brave is measured by the cutting of tubes, I've now cut three holes, 150mm X 200mm into the tubes of three brand new C14"s.
This is a harrowing experience not for the faint-hearted, it gave me the screaming meamies to gouge into a perfectly good tube. As to why? To fix mirror slop.