The invention relates to systems and methods for controlling angular position of a pivotable platform relative to a radiation source. In one embodiment, an apparatus for controlling angular position of a pivotable platform relative to a radiation source includes a frame, a thermal actuation element coupled to the frame, a pivotable platform positioned between the radiation source and the thermal actuation element so as to at least partially block energy from the radiation source from impinging on the thermal actuation element, and apparatus for pivoting the pivotable platform in response to a thermally controlled deformation of the thermal actuation element.