Abstract: The alignment of a collimated light beam is monitored by diverting a sample portion of the beam through an auxiliary optical light beam transmission system which converts the sample portion into an annular collimated light beam and transmits it coaxially with the main beam. The annular beam is reflected back through its transmission system by a fixed reference annular mirror that permits the passage of the main beam through its aperture. The retransmitted annular beam is divided to sensors that detect its angular misalignment and lateral displacement as a true measure of main beam alignment relative to the fixed reference annular mirror. End to end main beam alignment is accomplished by servo controlled optical elements that respond to the outputs of these sensors and to the outputs of sensors that measure alignment of the main beam input.