Ambjörn Naeve discovered that if you put 2 parabolic trough concentrators in series at the right distance apart, you can get a burning point of light. Professor Naeve, of the Royal Technical Institute, in Stockholm, Sweden, has given me permission to use his images here to show his device. He showed that by bending 2 pieces of reflective sheet material you can mimic a paraboloidal reflector. It is much easier to bend 2 pieces than to bend and cut and bend and cut many times to make a parabolic dish.
Here is the proof
Here is Naeve using the device to melt copper!
Here is Naeve's diagram of a Solar wheelbarrow
I believe that Ambjörn Naeve made a great breakthrough back in the 1970"s when he discovered and worked on these concepts.
http://kmr.nada.kth.se/wiki/Main/PointFocus is his story of the discovery and how he tried to spread the knowelege.
For me, I think the Naeve cross holds the potential for people to design a combined trough solar cooker with a 2 hour cook time (or a 3 hour cook time)
(Which would be excellent in its own right) and then "reverse engineer" a dish from it.
This combined trough or dish with a 2 or 3 hour UNATTENDED cooktime would be a wonderful breakthrough for solar cooks all over the world.