[Laser] Lunar downlink

[J. Forster]

Glenn Thomas wrote:

> [snip]
> I'm not sure that the tale of the observatory location being off by
> 100m is credible. If the beam is already 2 km on the moon and 18 km
> on the return, a translational error of 0.1 km seems insignificant.

I'm inclined to agree. What matters is the angle between the incident and
reflected beams from the retros on the Moon. I vaguely remember seeing some
rejects as surplus and they were <0.5 arc seconds as I remember.

> Likewise a return signal measured in single photons seems
> questionable when you consider the effect of noise due to thermal
> radiation from the atmosphere and even the telescope itself.

I think this is not correct. I remember seeing some video of the experiment. I
think a cooled PMT was used (typical dark count under 1 per second) and the thing
was range gated. Every time a photon was detected in the right time slot it'd ring
a bell. The dings were roughly 1 in 3 pulses. The laser was pretty powerful and
the optics pretty large. There may well have been subsequent experiments, of
course.

> Perhaps
> someone with a better grounding in physical optics than I can
> comment. Not having worked through the math, I might believe 100's or
> 1000's of photons!

I don't think it was anything like 100 or 1000 photons per received  pulse.-John

> 73 de Glenn wb6w