NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Ball Bearing in a dish as a reference
From: Brad Morris
Date: 2018 Jun 10, 10:14 -0400
From: Brad Morris
Date: 2018 Jun 10, 10:14 -0400
Having utilized many a laser in a professional sense, I can assure you the you cannot see the laser beam off axis, unless there is something in the laser path, like dust motes. The laser beam hits those particles and the light scatters due to reflection. So the light you see in that circumstance is no longer coherent.
If, on the other hand, you placed your mark one eyeball directly on axis, in the path of the laser beam, you would see it, ever so briefly. Then, with your remaining eye, you could stumble back to the Lunar Lander. Never, ever, put your eyes directly in the path of coherent light.
I am unsure of the strength of the beam after traveling the distance to the moon, but recognize that it has to travel back to earth AND transit the atmosphere twice. I strongly doubt any test pilot (as those astronauts were) would risk their eyes in a quest to see the beam of light from earth.
On Sun, Jun 10, 2018, 1:52 AM Paul Hirose <NoReply_Hirose@fer3.com> wrote:
On 2018-06-09 0:45, Peter Monta wrote: > Was this at visible wavelengths? Would the astronauts > have been able to see anything if they were told when to look? Of course > they had a lot to do besides gawk, but on the other hand that's an > opportunity that doesn't come along every day, to see an artificial light > source from Earth with the naked eye at lunar distance. Apparently no reflection was truly detected at Lick Observatory until August 1, though it was announced days earlier as the moonwalk came to a close. I have a DVD with the whole moonwalk, and the announcement from the "voice of Mission Control" is on the soundtrack. I don't know why it was premature. A 1969 article co-written by the chief scientist for the experiment is online, but Science Magazine wants $30 to view it. I'm not that interested! The abstract does have some useful information: "On 1 August between 10:15 and 12:50 Universal Time, with the Lick Observatory 120-inch (304-cm) telescope and a laser operating at 6943 angstroms, return signals from an optical retro-reflector array placed on the moon by the Apollo 11 astronauts were successfully detected. After the return signal was first detected it continued to appear with the expected time delay for the remainder of the night. The observed range is in excellent agreement with the predicted ephemeris. Transmitting between 7 and 8 joules per pulse, we found that each return signal averaged more than one photoelectron. This is in good agreement with calculations of the expected signal strength." http://science.sciencemag.org/content/166/3901/99 I think the stated wavelength is a deep red color. The spot size on the Moon has been variously described as about 1 mile or 7 km. The pulses were on the order of a nanosecond even back in the day, so I doubt they would have been perceptible. "Apollo 11 anniversary: Lick Observatory scientists recall landmark experiment 40 years ago" https://news.ucsc.edu/2009/07/3111.html Unfortunately, I have not been able to locate the "first-person account of the project" mentioned at the bottom of the above article. I did find a 1971 Bendix Corporation familiarization manual for the laser ranging retro-reflector. It contains a description and instructions for setup on the Moon. On the last page is a diagram of the Sun compass and level that was used to orient the device within 1 degree. https://www.lpi.usra.edu/lunar/ALSEP/pdf/ALSEP%20%2323%20-%20LRRR%20Familiarization%20Manual_RevA_050171.pdf This article shows Aldrin standing beside the seismic experiment (the one with the troublesome ball bearing level). One of the NASA mission review documents I saw online mentioned an ineffective leveling device which needed investigation. https://www.nist.gov/nist-time-capsule/any-object-any-need-call-nist/moon-and-back-25-seconds
