Abstract:
Apparatus for maintaining the direction of a beam of electromagnetic radiation relative to earth horizontal comprising an optical reflector, a pendulum connected to the reflector and a mounting frame supporting said reflector and said pendulum by independent gimbal suspension whereby said reflector moves responsive to pendulum motion through one-half the rotational angle of said pendulum, correcting an optical path in accordance with changes in the position of the pendulum relative to the mounting frame. Apparatus units utilizing coherent light beams are a specific embodiment.