A rangefinder is described that uses the propagation time of a beam of infrared radiation to measure distances of up to 120 m with errors of less than 50 mgr;m. The instrument is capable of measuring up to five different distances per second by directing an amplitude modulated infrared beam to a retroreflector located at the far end of each path. The instrument is being tested with the expectation that such a system could be used to make precise and rapid measurements on a large radio telescope. Such measurements would be used to adjust the shape of the telescope reflector surface to correct for thermal and gravitational deformations taking place during astronomical observations. It is possible that the measuring system could be extended to relate the positions of certain moving parts of the structure to a reference system fixed in the ground, thus opening the possibility of making corrections to the telescope pointing.
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