Wikipedia's Lunar Laser Ranging Experiments; History mentions the first lasers ever bounced off the whole Moon were in 1962, and probably both the US and Soviet groups used Q-switched ruby lasers.

In 1969 the Apollo astronauts placed an array of optical retroreflectors on the moon and carefully oriented it so that the face of the array pointed towards the mean direction of the Earth so that each reflector's contribution would be returned at nearly the same time. See What are these structures on the Lunar Ranging Retro Reflector (LRRR) arrays for? for lots of details.

On the ground almost certainly photomultiplier tubes were used at the focus of the telescope receiving the pulses. I'm guessing that they dealt with reflected sunlight by waiting until the Apollo 11 site was not sunlit, and a combination of a pinhole and filter for the laser wavelength, as well as some nice gated coincidence electronics.

But I have no idea, so I'd like to ask:

Question: How did the earliest measurements of lasers bounced off of Apollo 11 retroreflectors deal with the light from the Moon and pick out single photons?

A photo of the receiving system optics would be great!

Companion question:

For more on the Apollo 11 retroreflectors themselves see


1 Answer 1


The short article in the IEEE Transactions on Nuclear Science (link, DOI: 10.1109/TNS.1972.4326697, S.K. Poultney "Single Photon Detection and Timing in the Lunar Laser Ranging Experiment", IEEE Transaction on Nuclear Science 19(3) 12-17 (1972)) in the answer to your companion question discusses the setup used by the author to make measurements at the McDonald Observatory.

Given the setup at the McDonald Observatory using the 272 cm reflector there, they expected to see 0.2 photoelectrons (signal) per laser firing. The authors have one paragraph labeled "Discrimination Against High Background Noise":

Operation under all background illumination conditions requires a variety of noise discrimination schemes. The extreme case is the detection of a single laser photon against the combined background of bright moon and bright sky. A spatial filter of 6 arc sec and a spectral filter of 0.7 A width still allow about 300k counts/sec of noise.$^{10}$ Another discrimination technique was the setting of a 6 microsecond gate about the expected time of arrival of the lunar return by electronic means. The final technique was the post detection clustering of returns within a few nsec. A number of repeated rangings were, of course, necessary to be certain of a signal return. The RCA C31000F (with ERMA photosurface) which is used as the photodetector exhibited a noise rate of 30k counts/sec at room temperature and several k counts/sec when cooled to about 0°C due partially to its operation at very high gain. The photodetector noise rate is only important when viewing the dark moon at night.

Endnote 10 is

S. K. Poultney, "The Detector Package for the Laser Ranging Experiment at McDonald Observatory: Its Design, Performance, and Operation," Dept. of Phys. and Astron. Tech. Rep. No. 957, University of Maryland, August 1969.

This shows up in the university catalog, and is available as a NASA technical report, along with others detailing the work under contract.

The impression I get is that the experiment was capable of operation regardless of background light ("Operation under all background illumination conditions") using good optical and electronic practices.

  • $\begingroup$ Great answer, thank you! "The impression I get is that the experiment was capable of operation regardless of background light..." Yes, come to think of it, that would be pretty important for better sampling of a significantly elliptical orbit and teasing out the gravitational perturbations of the Sun and other effects and finally determining effects of only a few cm per year. $\endgroup$
    – uhoh
    Nov 1, 2023 at 18:27
  • $\begingroup$ NASA's NTRS server has a copy ntrs.nasa.gov/citations/19700019479 $\endgroup$
    – uhoh
    Nov 1, 2023 at 18:30
  • 1
    $\begingroup$ @uhoh Very cool! That means they likely have the others as well. And they do, searching on the author names. Well, I thought I might get some work done today, but apparently not... $\endgroup$
    – Jon Custer
    Nov 1, 2023 at 18:34

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