A heliostat having a frame structure for supporting a mirror. The frame structure is pivotable about at least one axis of rotation. A liquid ballast system is mounted on a side of the frame structure that is remote from the reflective surface of the mirror. The liquid ballast system includes at least two tanks, and an imaginary line between the tanks extends at an angle or perpendicular to the axis of rotation. An arrangement is provided for moving ballast liquid between the tanks in an actively controlled manner.