Cheap truss struts - extendible washing line props?

[furrysocks]

I am building an 8.5" f/7.6 newt (possibly two) and looking to put together something along the lines of this... http://rfroyce.com/f8_truss_tube_newt/index.htm, and blogging the build here...

I initially planned to build a full length tube for <£10, but to improve portability I'm going for a truss design, hopefully <£25. I know there are several factors to consider in selecting a strut material (weight, strength, etc...) but I'm looking primarily at cost.

Being of a relatively long focal length, I think I'd struggle to get full length struts in my price range, so I'm looking at my options...

Extendible washing line poles come in two sections, typically powder coated thin-wall steel tubes, and are available at about £4/each on eBay. With careful tweaking of the design (tube lengths, weighting, etc...) I should get a two pair of struts (one small dia for center to secondary, and one large dia for center to primary) from a single washing pole, requiring a total of six pair, or three poles for one scope. I could therefore have the struts for both scopes in the binoscope in six washing poles for max £24, delivered.

Not the lightest, but cheap.

Any thoughts on this? Anything cheaper I've perhaps missed?

Matt

[furrysocks]

Net curtain rod? Perhaps too thin...

[furrysocks]

PVC conduit - £30 for 90m of 20mm? Perhaps not rigid enough...?

[Moonshane]

I think that it would work well. the weight would be higher than ali tube but not much. aluminium tube is not dear though. you can buy 2.4m lengths of 5/8ths tube for about £3.50 plus overall delivery £12. http://www.aluminiumwarehouse.co.uk/Aluminium-Round_Tube/c120_131/p17850/Aluminium_Round_Tube_%286063T6%29_%3Csmall%3E%3Csup%3E5%3C/sup%3E/%3Csub%3E8%3C/sub%3E%3C/small%3E_in_x_16_swg/product_info.html

[furrysocks]

Aluminium broom handle - 1.2m in length. Can get 16 delivered for <£30.

[billhinge]

google carbon fibre tubes, probably what I would use if I were doing this e.g. first one in google search http://www.easycomposites.co.uk/Category/Carbon-Fibre-Products/Carbon-Fibre-Tube.aspx

I'm quite keen to do some DIY with carbon fibre mouldings but I haven't worked out what yet

[furrysocks]

I think that it would work well. the weight would be higher than ali tube but not much. aluminium tube is not dear though. you can buy 2.4m lengths of 5/8ths tube for about £3.50 plus overall delivery £12. http://www.aluminium...oduct_info.html

Cheers, Shane. You reckon the steel washing line props would work well? I'm not sure I can get the cutting list to bisect the lengths to get the split in large and small diameter that I want. So am looking at aluminium...

Not sure what those broom handles are in terms of diameter - perhaps larger than the 5/8ths you linked to - thanks for that, though. Good to know it's not that expensive.

[furrysocks]

google carbon fibre tubes, probably what I would use if I were doing this e.g. first one in google search http://www.easycompo...Fibre-Tube.aspx

I'm quite keen to do some DIY with carbon fibre mouldings but I haven't worked out what yet

I got the impression carbon fibre would be at least twice the cost of steel or aluminium, if not more - might be wrong though.

[billhinge]

Probably, personally I would go for the material to do the job, if it costs a bit more then so be it within reason ie if it costs me £70 rather than £30 then thats OK. just wait a bit longer till you can afford it

Obviously different if it cost £200, but thats just my opinion so feel free to ignore

[furrysocks]

Probably, personally I would go for the material to do the job, if it costs a bit more then so be it within reason ie if it costs me £70 rather than £30 then thats OK. just wait a bit longer till you can afford it

Obviously different if it cost £200, but thats just my opinion so feel free to ignore

No, I agree - but I think that if folk have reasonable success with wooden broom handles, then alu/steel is going to be fine. CF would be better, I agree.

This scope build is about minimising cost - I could spend >£100 on the job and justify it, but I'm trying desperately not to. I've gone from solid tube <£10 per scope, to a truss design at somewhere around £25 per scope tube - just to be able to fit it into the car. So you see, I'm already blowing the "budget" away once... going all the way to CF in one fell swoop...?!

I was considering the Serrurier truss design, with center tube, but if I can get 1.25" aluminium broom handles, 1.2m in length for £1.50 each, delivered, then I might just go for a single truss design, and make primary/secondary stages just a little longer to let me use the 1.2m lengths. Part of the reasoning behind the Serrurier design was that I could use shorter lengths, cutting to suit from off-the-shelf parts, given the long focal length of the scope(s).

Alu broom handles are currently top of the list... if I can get the lengths to work out.

[furrysocks]

Pound shops may do steel/alu broom handles...?

[furrysocks]

I think that it would work well. the weight would be higher than ali tube but not much. aluminium tube is not dear though. you can buy 2.4m lengths of 5/8ths tube for about £3.50 plus overall delivery £12. http://www.aluminium...oduct_info.html

£3.42 + P&P per length for the 5/8 tube. Robert Royce used 1/2x1/2 channel, which at 1/16" thickness, comes in at £2.56 + P&P per length. By timming the end to a flat tongue, a very neat low profile can be achieved. At this cost for just over 8 feet, it rivals the £1.50 inc P&P for 4' 30mm aluminium tubing, which is perhaps a tad oversized.

Details of Robert Royce's construction here... http://www.rfroyce.com/f8_truss_tube_newt/construction_pictures_tube_struts.htm

[furrysocks]

Note - I'm swinging back towards the double truss (Serrurier) design, rather than single truss to enable a dobsonian or split-ring mount now, and perhaps tripod later.

[furrysocks]

Home Bargains - washing line props - £1.99 each

1210mm @ 18mm, black, 311g

1190mm @ 22mm, green, 384g

Quite heavy.

Include P&P, the 1/2" U-section aluminium channel is probably twice the price of this, but better suited. The 1/2" aluminium tubing (either 1.2mm or 1.6mm) is a little cheaper than the channel. Their postage (£14.95) is quite steep, but they're not going fit through a letterbox.

Aesthetically, the channel with a trimmed tounge end may be slightly better then tube clamps or crimped tube ends. For comparison, the 1/2" channel works out at about 180g for a 1.2m length. Just on paper, calculating other weights give about 130g for the thinwall 1/2" tube and 170g for the thicker wall. It's about 190g for the 4 foot 1.25" broom handle but that would mean a wall thickness of 0.65mm - so I've maybe gone wrong somewhere.

I'm just talking out loud - I don't know what to do, yet. Aluminium is going to be lighter than steel, not too much more expensive and the channel would give the lowest profile finish.