wayne@csri.toronto.edu (Wayne Hayes) (07/20/89)
In article <28736@ames.arc.nasa.gov> yee@trident.arc.nasa.gov (Peter E. Yee) writes: > Once the laser beam hits a reflector, scientists at the >observatories use sensitive filtering and amplification equipment >to detect the return signal. The reflected light is too weak to >be seen with the human eye, but under good conditions, one photon >-- the fundamental particle of light -- will be received every >few seconds. Is this right? I would think that many more than that would get back. Besides, if you only got one back every few seconds, it would be useless for the accuracy of timing needed to calculate the moons distance with "unprecedented accuracy". -- ------------------------------------------------------------------------------ "Open the pod bay doors, HAL." "I'm sorry Dave, I'm afraid I can't do that." Wayne Hayes INTERNET: wayne@csri.toronto.edu CompuServe: 72401,3525
josh@klaatu.rutgers.edu (J Storrs Hall) (07/20/89)
Wayne Hayes writes: > Once the laser beam hits [the Apollo 11 corner reflector] >.... The reflected light is too weak to >be seen with the human eye, but under good conditions, one photon >-- the fundamental particle of light -- will be received every >few seconds. Is this right? I would think that many more than that would get back. Besides, if you only got one back every few seconds, it would be useless for the accuracy of timing needed to calculate the moons distance with "unprecedented accuracy". Hmm. Paul Deitz please check :^), but my blackboard claims: A 100 mW beam (Edmund Scientific league) = 1 million ergs/sec. at one micron wavelength E=hv gives 1.5e26 photons/sec. If the beam is one mile square at the moon and one square foot hits the reflector, divide by 2.8e6 giving 5.4e18, and similarly for the return trip (use a square foot telescope) giving 2e11 photons/sec. since c is 3e8m/s, your return beam is averaging 660 photons/meter, with which you should be able to measure the distance to within an inch. --JoSH