Optical Echoes from the Moon

From Nature, 194 p.1267 June 30, 1962: “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, 0.5msec duration, at 6934 A was used as the source. The transmitting optical system included a Cassegrainian telescope of 12-in. diameter. The echoes were received on a Cassegrainian telescope of 48-in. diameter, passed through an interference filter of 7 A bandwidth 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 0.5msec interval at the expected time-delay was compared with the counts obtained in 0.5msec intervals where no echoes would be expected and where the only relevant contributions to the count were those due to noise, that is, to Earthlight and scattered light (photoelectric dark current was negligible).

The experiments, conducted at Lexington, Massachusetts, on May 9, 10 and 11 (1962), gave positive results as indicated in Table 1. Column 3 indicates the number of consecutive flashes utilized; column 5 is the average number of photoelectrons due to noise in a 0.5msec interval column; 6 is the standard deviation of the noise count; column 7 is the average count in the 0.5msec 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.”

Hardway (L) and Smullin (R), 1962

L. D. Smullin and G. Fiocco, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Mass.

Later, Apollo 11, 14 and 15 and Lunokhod 1, 2 delivered retroreflectors….