A concentrator having a reflective surface (1) and a pair of flexible members (2) supporting the deformable reflective surface so as to define its shape. A mechanism (3) is movably mounted to a vertical support and has one end of each of the flexible members (2) attached thereto. A controller drives rotation-changing linkage and tension-varying linkage so as to move the mechanism (3) to impart a toric shaped curvature on the reflective surface (1). This toric shaped curvature minimizes astigmatic aberration due to obliquely incident radiation and changes as the solar point moves throughout the day.