Ambjörn Naeve is a professor at the Royal Institute of Technology, in Stockholm, Sweden. In the 1970s, he discovered that if you put two parabolic trough solar concentrators in series at the right distance apart and in the right orientation, you can get a burning point of light. He has given permission to use his images here to show his device. He showed that by bending two pieces of reflective sheet material you can mimic a paraboloidal reflector. It is much easier to bend two pieces than to bend and cut and bend and cut many times to make a parabolic dish. Here is the proof.
Here are more images. (Click on the icon at the bottom-right corner of any image to enlarge it.)
Unfortunately, the Naeve Cross does have disadvantages. Compared with a paraboloid, more mirror material is needed. Also, since, the light is reflected twice, more of it is lost. The device does not share one of the main advantages of simple trough cookers, their tolerance of movements of the sun parallel to the length of the trough. This can allow a trough cooker to work for hours every day without any need for tracking the sun. But movement of the sun parallel to the length of one trough of a Naeve Cross is perpendicular to the length of the other, so the Naeve Cross has to be moved continuously or frequently to track the sun, like a paraboloid.
Nevertheless, Naeve's device is potentially very useful. Here is his story of the discovery and how he tried to spread the knowledge.
There are now commercially available cookers that use the Naeve Cross principle. See: 
- ↑ In Swedish, the name of the institute is "Kungliga Tekniska högskolan". Its initials, KTH, are to be found in many of Naeve's files.