10" f/8 A tube by any other name?

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Once I decided I might build a Dobsonian out of my 10" F:8 I realised that I had now made an even more difficult job for myself. The narrow, offset beams, relative to the cells, did not offer any easy way to add altitude bearings. The offset between the centres of mass and the beams would cause all sorts of balance problems.  So what to do?

The automatic choice would arguably be to make a smart, truss tube Dob. Okay, I thought, start looking for some suitable truss poles. How difficult can it be? Hours [and more hours] of browsing later I'm looking at well over a hundred pounds equivalent before freight charges. I chose 25mm diameter in aluminium, with a nominal 1.5-2mm wall thickness. Many metal stock holders won't entertain private sales. Their suggestion of approaching a local smith or engineering company to order on my behalf is unlikely to make the tubing any more affordable. £50+ for a 3 meter length would need hundreds of quids worth before I started cutting it up into useful lengths with huge potential wastage.

An alternative is to use the Porsa system and buy square truss tubes from their storage frame, building materials. Well over £120 equivalent plus £25 in freight but at least I get a choice of colour. Elox silver, black or white and the lengths I want. Not certain I like square truss poles but it makes for much easier construction without the need for clamping. A simple bolt would do as it wouldn't need to be dismantled for transport.

Then I remembered that round curtain poles come in 25mm diameter x 1.5mm, thin wall, aluminium tube. Or rather did, according to my completely fruitless, online searches for Café style poles. The prospect of touring every charity shop on Fyn in the vain hope of sourcing suitably matching material will make it a very much travelled pile of tubing. I drew a complete blank at the 3 nearest and the next candidates are over ten and over fifteen miles away! This too proved a complete waste of time. Though I did discover some 2m long x 25mm diameter tubing in a builder's merchant. Sadly they sell at 140DKK each or about £14 per 2m length with a pretty 'elox' silver finish to the 1.5mm wall aluminium. Eight of those tubes for a truss is about the same cost as the Porsa system in 25mm square tubing. I would want to be much more sure of the validity of my design before launching myself in that direction.

Cardboard tubing might still beckon if I were willing to ignore the massive weight [and cost at Danish wholesale-resale prices!] Knowing how heavy and cumbersome my DIY laminated cardboard tube turned out was fair warning not to follow that route again!

Aeroplane plywood is still only a distant hope with exclusion barriers to private sales by the only Danish stockists. Or, put up with rip-off retail charges per sheet and obscene delivery charges. Though I could always go direct to the Dutch stockists and save a fortune. They say they would just roll the sheets and pop them into a long box. A skill well beyond the grasping Danish re-salers who want £100 per sheet for delivery regardless of quantity ordered. Thereby pricing themselves safely out of any chance I would ever trouble them with an order. Another, more sympathetic, builder's merchant manager hopes to be able to supply at more modest prices if I am prepared to be patient. We shall have to see what turns up from that source.

An alternative is to buy some more builder's straight edge beams and arrange them firmly at the sides of the cells [pots] to support the altitude bearings directly. Two more beams, at top and bottom, would make a fairly affordable and hopefully rigid tube. Though its beauty would definitely lie in the eye of the beholder. I also have some slight doubts about the sharing of the tension-compression loads of the beams via the relatively thin pots/cells. Perhaps I should relax my objection to plywood and build the cells as boxes of 1/2" 12mm waterproof birch plywood?

Here's a superb [prize winning!] 10" F8.8 Dobsonian by Mike Lockwood: It's even longer than mine but balances very low to the primary cell thanks to its much lighter secondary cage! My heavier pot/cage would need a very much taller rocker box. Possibly leading to very different handling qualities when pointing the telescope.
 
http://www.loptics.com/ATM/telescopes/10in_F8p8/10in_F8p8.html

Discussion with the owner/builder suggests I could usefully increase the size of my truss tubes and beef up the rocker box for windy conditions.

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