Comment by kragen

Yeah, I was just doing some calculations on this. You'd think that with 3.5 meters you could do better than a few kilometers, wouldn't you? Is something wrong with their telescope?

I don't know what wavelength they're using, but at 555nm, 1.22λ/d would be 0.193 microradians, which, unless I'm doing the math wrong, works out to a 74-meter Airy-spot radius at the distance to the moon. At that sort of size, you'd think the majority of the photons in the desired wavelength band would be from their laser rather than stray Earthshine.

I was doing calculations based on λ = 350nm and a 500-mm reflector, and no retroreflector, and getting rather sad estimates of 3 joules of light transmitted per returned photon (per receiver). While that's clearly a feasible commnications system, it's going to be pretty limited in bandwidth. I'm not sure if C-band radio is better?