The reflector-frame is build for equinox. This means the reflector has to change its shape. This is only possible by shaping the reflector after an other parabola for each seasonal inclination-angle of the sun, i.e.
But the centre of the reflector and the position of the focus are not allowed to move. Otherwise it's not possible to obtain a sharp focal point. The paraboloid has to perform the same change of inclination in order to stay directed at the sun.
In the course of the seasons the incident angle of the solar radiation varies + / - 23,5° in relation with the perpendicular to earth-axis. That way the focus stays fixed, which is very useful, as it means the cooking-pot doesn't have to be moved either. During the day the concentrated light will only rotate around its own centre but not move sideways in any direction. The distance between focus and centre of the reflector depends on the selected parabola. The focus is located on the axis of rotation to prevent it from moving when the reflector rotates.
That way the reflector always maintains it's gravitational equilibrium and the mechanical tracking device (clockwork) doesn't need to be driven by much force to rotate it synchronous with the sun. The axis of daily rotation is located exactly in north-south-direction, parallel to earth axis and runs through the centre of gravity of the reflector. The sunlight that falls onto this section of the paraboloid is reflected sideways to the focus located at some distance of the reflector. The inclined cut produces the typical elliptical shape of the Scheffler-Reflector. The reflector is a small lateral section of a much larger paraboloid. Additionally the reflector is adjusted to the seasons by flexing it in a simple way. The best solution was a eccentric, flexible parabolic reflector which rotates around an axis parallel to earth-axis, synchronous with the sun. To make cooking simple and comfortable the cooking-place should not have to be moved, even better: it should be inside the house and the concentrating reflector outside in the sun. The locally available materials must be sufficient. At the same time the device should be build in a way that allows it to be constructed in any rural welding workshop in southern countries after a certain period of training. The basic idea that lead to the development of the Scheffler- Reflectors was to make solar cooking as comfortable as possible.
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