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 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.

For more on that see my companion question:

But here I'd like to ask:

Question: What laser technology/ies were first used to bounce off of the Apollo 11 retroreflectors to accurately measure distance to the Moon?

Was it still Q-switched, ruby lasers? They would presumably be looking for nanosecond(s) scale timing rather than the milliseconds for the full Moon work in 1962.

A photo of the laser would be great!

These days I believe either pulsed Ti-sapphire or Nd:YAG lasers (infrared, frequency doubled to green) are used. (cf. this answer to When they shoot lasers at the Moon for ranging, what is the shape of the beam?.

Assuming they started with ruby lasers, I will probably ask when they switched from ruby to this technology in a separate question, but I will wait to find out if ruby was the first since Nd:YAG lasers date back to 1964.

  • 1
    $\begingroup$ I noticed there's no [instrument] or [equipment] tags, if I'm missing any please feel free to add. $\endgroup$
    – uhoh
    Nov 1, 2023 at 8:40

1 Answer 1


A 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)) discusses the setup used by the author to make measurements at the McDonald Observatory. The paper references S.K. Poultney and D.G. Currie in the IAU Telegram Circular 2164, August 1969, as the first announcement of such a measurement. I cannot find that one online (and my Morse code is rusty anyway, but, seriously, the Central Bureau for Astronomical Telegrams at Harvard does not have it up).

Descriptions of laser parameters are scattered through the paper, since the focus is on the timing system. It apparently is a 4-ns, few-Joule Q-switched ruby laser at 6943 angstroms firing at 20 pulses per minute (round trip travel time is 2.5 seconds, so pretty good utilization). They used the 272 cm reflector at the observatory to collimate the laser to a 4 km spot on the moon. With a quantum counting efficiency of the photomultiplier tube of 8% (!) they get 0.2 photoelectrons detected per laser pulse (out of ~10$^{19}$ photons sent up).

In the Future Plans section they state that a second US station is being built in Hawaii that would use a 0.2 ns Nd:YAlO$_{3}$ (note YAP, not YAG) in the green (so doubled, but that is not noted in the paper).

A later paper is in Science (C.O. Alley et al., Science 167 (3917), the 23 January 1970 issue. There they specify that a

Space Rays custom-built, two-stage Q-switched ruby laser (7 joules, 20-nsec pulse length, $2.4 \times 10^{-8}$ radians full beam divergence from 0.75-inch rod, one pulse every 6 seconds

was used for the measurements. (Space Rays as a laser company can be found in a few hits on Google, so seems to have been a real company. Great name for this particular application.) I have no explanation for the discrepancy in pulse length between the various accounts.

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    $\begingroup$ chock-full of interesting details, thank you for your answer! $\endgroup$
    – uhoh
    Nov 1, 2023 at 18:21

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