laser guided weapons delivery systems are now well known . such systems , termed laser designator / sighting devices , are optical devices . one of the major potential sources of error in optical devices is misalignment of optical elements , particularly in helicopters and tanks where operating environments are quite severe . alignment is maintained by boresighting , using optical instruments , called boresighting modules , which can be coupled optically with the misaligned optical device . since these modules usually include prisms , beam dividers , mirrors and lenses , vibrations and other mechanical problems have been sources of error . while vibrational and weight load errors have been considered , the problem of errors due to thermal device has not been solved . the problem is solved herein by a boresight device incorporating a zero expansion , glass ceramic , optical transfer tube .