Optical Echoes from the Moon

Without the use of Retro-Reflectors!

Experiments have been conducted to focus pulsed optical radiation on to the surface of the Moon and to detect the echoes.

A ruby optical maser radiating pulses of approximately 50 joules energy, 0.5 msec. duration, at 6943 A was used as the source. The transmitting optical system included a Cassegrain telescope of 12-inch diameter. The echoes were received on a Cassegrain telescope of 48-inch diameter, passed through an interference filter of 7A band-width and were detected with a photomultiplier tube of spectral response type S-20 cooled to liquid nitrogen temperature. The field of view of the receiving telescope was 0.2 milliradians.

The photoelectron count obtained in a 0.5 msec. interval at the expected time-delay was contemplated with the counts obtained in 0.5 msec. intervals where no echoes would be expected and where the only relevant contributions to the count were scattered light (photoelectric dark current was negligible).

The moon as it appeared on the night of May 9th, 1962

The experiments, conducted at Lexington, Massachusetts, on May 9, 10, and 11, 1962 gave positive results as indicated in Table 1 above.

Column 3 indicates the number of consecutive flashes utilized; column 5 is the average number of photoelectrons due to noise in a 0.5 msec. interval; column 6 is the standard deviation of the noise count; column 7 is the average count in the 0.5 msec. interval where an echo was expected.

The expected time delay of the signal was computed from ephemeris data and the position of the illuminated area on the Moon. Column 4 gives the number of range-intervals per trace used to obtain the data in columns 5 and 6.

I thank J. Daley, jun., of Lincoln Laboratory, for help in setting up and operating the telescopes, and G. Hardway and S. Kass, of the Raytheon Co., for help in the use of the maser. The work was supported in part by the U.S. Army Signal Corps, the Air Force Office of Scientific Research, and the Office of Naval Research.

Louis D. Smullin
Giorgio Fiocco

Research Laboratory of Electronics,
Massachusetts Institute of Technology,
Cambridge, Mass.

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