Disclosed is a lightweight reflecting system for the concentration of solar radiation in which the reflecting optic is approximately sinusoidal in cross section and is maintained in this shape by minimal contact with a support frame. The reflector can be a flexible rectangular sheet deformed into a generally sinusoidal trough by compression between fixed or adjustable end supports. The sinusoid can be adjusted to be less than half a cycle by torsion applied to each end of the flexible sheet. A reflective film, polished metal, or similar material on the reflector surface can concentrate sunlight along a diffuse band rather than a sharp line, providing improved concentration levels for photovoltaic cells. The reflectors and their support frames can be mounted singly or severally on a tracking device that maximizes instantaneous power output by orienting the collectors toward the sun.