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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….

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Light Modulation With The Supersonic Cell

The Jeffree cell, ~85 years ago, an early acousto-optic modulator that could be constructed in the home workshop using water or kerosene.

From US Patent 2,155,659 (Filed Feb. 27, 1935), Light Modulating Device, “This invention relates to light modulating devices such as may be used in television, sound recording and for other purposes. The present invention makes use of the principle that when high frequency mechanical waves are caused to traverse a body forming part of an optical system, they bring about general retardations and accelerations of the wave front of a beam of light passing through or reflected from the body, these retardations and accelerations corresponding to the regions of compression and rarefaction or of varying displacement of the body produced by the waves. Owing to the regular spacing of these regions, corresponding to the wavelength employed, there can be produced optical effects similar to those caused by a diffraction grating.”

“I Claim: 1. Apparatus for controlling the intensity of a beam of light in accordance with a plurality of mechanical carrier waves of different frequencies, said apparatus comprising a body capable of transmitting mechanical waves to produce diffraction grating effects, means for applying to said body a complex mechanical vibration comprising a mixture of carrier waves to cause said complex mechanical vibration to traverse said body, means for acting selectively on carrier waves of different frequencies in said mixture traversing said body to cause each of said carrier waves to follow a path in said body which is spatially separated from the paths of the others of said carrier waves and means for directing light of a beam upon the carrier waves in each of said paths to control simultaneously and independently of one another the intensity of spatially separated parts of said light beam.”

This early acousto-optic modulator technology was used in the Scophony system of mechanical television.

Just add a proper transducer and driver to the cell and begin scanning.

References:

[1] Editor, “Light modulation with the supersonic cell: a simple explanation,” Television and Short-Wave World Magazine, No. 126, Vol. 11, p. 461, August 1938.

[2] J.H. Jeffree, “The Scophony Light Control,” Television and Short-Wave World Magazine, No. 99, Vol. 9, p. 260, May 1936; British Patent 439,236 (1934).