It is possible to use two parabolic troughs, curved in perpendicular directions, to bring sunlight to a point focus as does a paraboloidal reflector. The incoming light strikes one of the troughs, which sends it toward a line focus. The second trough intercepts the converging light and focuses it to a point. This diagram shows the principle.
Compared with a single paraboloid, using two partial troughs has important advantages. The troughs are "single curves", which can be made by bending a sheet of metal without any need for cutting, crumpling, or stretching. Also, the light that reaches the target - the cooking pot - is directed approximately downward, which reduces the danger of damage to the eyes of anyone nearby. On the other hand, there are disadvantages. More mirror material is needed, increasing the cost, and the light is reflected by two surfaces instead of one, which inevitably increases the amount that is lost.
Experimental arrangements of this kind have been made, and have worked well. The two troughs have been held in a fixed orientation relative to each other by being both fixed to a wooden frame, The whole assembly of frame and troughs has to be moved to track the sun as it moves in the sky. Here is a photograph of the assembly.
However, this idea does not yet seem to have been tried in a practical cooker.
Lunenberg Solar Concentrator uses this principle in its solar furnace, the company has a patent on this arrangement for reflectors. Prometheus Solar Forge [1]
Interesting. I wonder if the patent (or patent application) would hold up in court. The idea has been publicly known since the 1960s, which would make it unpatentable. Details might be patented, but not, I think, the basic concept. DOwenWilliams 20:36, 17 April 2013 (PDT)