10" f/8 Have beam, will travel.

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[13]

It took 22 miles of cycling on and through snow before I had my builders straight edge profile safely home.  I wrapped it well to protect it against damage from, or to, my cycle despite the shrink wrap already applied.  The first shop had one length of 2m marked Eskimo but I could not confirm whether it had a central internal stiffening rib. Tapping with knuckles and fingernails proved nothing. The staff could not confirm reinforcement either and supposed it was just a long, empty box. I was determined to have the extra stiffness promised by the central reinforcing rib.

So I left the first place and rode on into the steadily falling snow to the next builders merchant some miles further on. The Svalk website showed a central rib on their straight edges. The new outlet's stock was clearly marked Svalk.

I still had to take it on trust that there was a central rib, but again I could not hear anything by rapping or tapping the tube. As soon as I had it home I pulled one end cap off with my fingers. No tools required. Just gentle rocking until it slid free. There was the central rib which had cost me many more miles, wet socks and very cold feet!

The weight is surprisingly low at 2.8lbs/1.3kg for my 2 metre (6'6") length of 100mm wide by 18mm deep profile. The finish is a smart silver and the end caps are black. I had sighted along it to double check for straightness in the shop. I feel very confident it can handle my 10" provided I triangulate the mirror cell support properly. No bottlenecks!

Don't laugh, but I have an idea about using a 30cm /12", lightweight, aluminium saucepan for the cell support structure. Which will itself be supported on a long hardwood or plywood block clamped to the beam. A deep pan will hopefully have some off-cut material  to provide my focuser arc.

I just need to find a suitably deep 30cm/12" saucepan as a donor for the necessary material. A shiny new one would be nice to keep the cosmetic appearance at a very high level. The trouble is that aluminium is out of favour for cooking because of the Alzheimer's association. An aluminium pan will be stiff, round and light. With excellent characteristics thanks to the seam-free curve between the bottom and sides. Ideal for distributing the changing loads which the primary mirror places on the beam. A simple tube will need strengthening not to roll from side to side and possibly distort with changing altitude. The saucepan bottom will provide the stiffness I require. If I can find one.

I clamped one end of the beam to a wooden stool and rested a laser level on the other end. The beam certainly flexes up and down when laid flat but is well damped. It has excellent torsional stiffness and is, of course, very stiff when resisting loads on-edge. The MkIVs 2' /60cm saddle will certainly help here. I think some internal stiffening for the cantilevered upper end of the beam will be useful. This should allow a minimalistic single beam to function adequately.

I now have Kriege and Berry's book on the Dobsonian telescope. The local library had emailed me to confirm it had arrived while I was out. So I rode to collect it after lunch. Though dated 2001 it really is full to the brim with excellent material, lots of B&W illustrations and tables. Good stuff! It is aimed at the builder of larger Dobs rather than my 10" F:8 tiddler. Though much of it is still highly relevant thanks to it being extremely thorough. Beams are mentioned but not in the context of minimalistic telescope tubes. I have only scanned the book so far. So I may have missed something.

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