Abstract: The invention concerns a guidance, or tracking, arrangement which automatically adjusts the alignment of a radiation responsive system in accordance with the position of a movable radiation source. The arrangement utilizes two bodies able to transmit heat expansion forces into linear movements to rotate a rocker about an axis perpendicular to the path plane of the source. The bodies are partially shaded such that exposure to the radiation source controls the extent to which the bodies rotate the rocker. Resilient support means compensate the arrangement for undue forces arising from expansions of the bodies.