...capture the Sun.
We cut a half-parabola, calculated on millimetric-squared paper, out of
3/8" plywood. The lower edge was carefully rounded, so as not to lose
the precise form
The ply had mounted about its axis of rotation a stout steel rod. Driving a vertical water-pipe into the ground, we arranged it such that the ply could swing around in a circle.
We filled the space under the swinging parabola, first with rocks, stones, gravel, to economise on the cement, which is coming next.
Piling excess cement onto the heap, we then skimmed it all around with the jig. The pivot must be well-adjusted - we thought of using a bicycle-hub...
When dry, sanded, gloss-painted, the parabolic mould is fairly tight to the ideal form.
You could, of course, operate on a big board, perhaps with fancy expansive foams - mirrors with an interesting focal length, i.e, those where you can be at a safe distance from the action, are surprisingly flat, but also thus lack some of the auto-rigidity of deeper dishes... Any pool-player should know the rules of equiangular reflection, so you can work out focal length using only geometry, no maths needed. Er, good job...
We burnished aluminium kitchen foil onto the furniture-spray-polish-lubricated mould with the back of a spoon. The pieces will overlap more or less at hazard, but this matters not. The foil has a shiner side, so obviously...
We then applied moultes layers of papier-maché, using PVA glue, with a touch of Teepol, or any detergent. Strips of newspaper espouse the form easily, so it goes quite fast.
Even 39" dishes needed some support to be durable. We applied old rope, the tattier the better, and then continued with the paper, tucking it in well with a stiff paint-brush. .
At first, naïvely, we applied the rope in a radial pattern. Error. By far the most effective
re-inforcement was achieved with patterns resembling Fuller's geodesic domes, or the structures of Pier Luigi Nervi, an unsung genius of whom I have more to say, and about whom much more to find out.
Chunky fish-net drapes nicely over the form... Many-smaller-lines seems to be advantageous.
The finished dish de-moulds with ease. The edges can be trimmed with scissors - mark the cut-line whilst the dish is on the mould. Use the pivoting template, jacked-up a bit. The central hole could even be cut out thus, with a Stanley-knife.
We never tried this, but in fact for holed mirrors it might be best not to fabricate the centre at all - a step in the former would give a cylindrical plug on top of the mould, easy to work around. Economy, even if you're only saving an 8" hole, and ensures concentricity...
We slit some 3-core flex and applied the PVC sheathing around the edge. This greatly strengthens, and gives an all-together more serious air to the device. Rigid 'Acorn' water-pipe would be even better.
Oh, yes, and paint the back generously with one of those old end-of-tins somewhere in the garage.
This technique could be scaled up - if our calculations are correct, under African skies you would need a 12' mirror to power a 10 cubic foot cold-store down to -15°C.
The mirrors would be made in-situ. All that is needed is the half-parabolic template, and, most important, the know-how. But I've just written that...
And once made with skill the same mould can serve for dozens, hundreds of identical paraboli.