tech99 said:
The scientist Hanbury-Brown did work on the light from binary stars and its fluctuation. He used a 931A photomultiplier tube at the focus of a wartime searchlight. This tube can detect individual photons. Maybe a searchlight itself would be a suitable light source - it is an arc light in a 1 metre reflector. I also understand that the Moon makes continuous small movements which disturb the incoming signal. We know that bandwidth and power can be traded, so I wonder if a very fast transmitted packet could get through unscathed, as its duration is shorter than the mechanical movements of the Moon. Of course, this requires a lot of bandwidth and hence a lot of power.
Not looking for wide bandwidth. Not (deliberately) using the three arrays of corner-cube retroreflectors. The largest array (A15) has an effective aperture of less than 0.5 m
2, useless for communication purposes, but small enough to define its location for, say, Apache Point Observatory Lunar Laser-ranging Operations (APOLLO) in New Mexico. You should read the paper this group released describing the operation in detail. Not interested in measuring the distance to the Moon. Not interested in transmitting picosecond pulses twenty times per second with an average power on the order of one hundred to three hundred watts. Not interested in frequency doubling a Q-switched Nd:YAG laser to create a green probe beam. Cannot afford even a surplus Hollywood search light, with or without arc source. This is a problem in small-signal recovery, not brute force.
Hams are allowed up to 1500 watts transmitter power to their antennas, which in my case is a cheap "beginners" model 102mm refractor. Zapping its lens with 1500 watts is a non-starter. Also doesn't mean that is necessary for optical-EME comms. FCC also says hams must use the
minimum amount of power necessary to communicate. I estimate the one watt is doable, but it will take perhaps ten minutes tracking a small (two kilometer diameter) illumination spot on the Moon to collect enough photons. This is the size of the illumination area for ANY diameter laser beam aimed at the Moon.
The Earth is surrounded by a hollow spherical concave-convex variable density atmospheric lens. This lens causes about one arc-second of divergence of the collimated beam as it leaves the atmosphere. From the POV of the Moon, every collimated source is a point-source because of the vast distance. The geometry is simple: calculate the spread of a collimated beam of light on its journey to the Moon if it spreads one arc-second on its way to getting there. Compare this area to the effective area of the A15 array. That's how much the laser power is attenuated when it is bounced off A15 to measure the distance to the Moon. Not interested in doing that. For optical-EME comms, photons coming from the entire illumination area must be detected and counted. This requires "tagging" the illumination photons to distinguish them from the regular photons reflected from other light sources: earthshine, starshine, sunshine. Lots of other shine to reject. A narrow-band optical filter will help suppress most of this backgo
So, your suggestion to use a search light lens is spot-on. But totally impractical for amateur radio. Hams are notorious penny pinchers. I grew up that way in the 1950s as I tried to pursue a budding electronics hobby. I raided the dumpsters behind radio and TV repair shops for discarded chassis and "slightly used" electron vacuum tubes. I can "pinch" and Indian Head penny so hard that his feathers fall off.
Today, in retirement, I have inherited a Celestron 102GT Computerized Telescope from my younger brother who died in Sarasota FL ten years ago. The "seeing" here in Venice FL is not quite as bad as the seeing in Sarasota (fewer bright lights) but it is generally quite awful. I thought I would give it a try anyway. The arrays of corner-cube retroreflectors were my inspiration, but after doing due-diligence research I realized that hams don't need them, and probably cannot use them, for communication purposes.
Hams have been using the Moon as a passive reflector of radio frequencies (VHF, UHF, and microwave) since the 1950s. Weak-signal digital signal processing is the
de facto method used today to modulate and demodulate their signals. There is a group on the Internet devoted to exploring and extending this technology. They are all standing on the shoulders of Claude Shannon, as do I.
The quantum photon detector of choice is an avalanche photo-diode detector (APD). A ham friend in Australia (Rex VK7MO) uses a largeish one (US$ 500) for his line-of-sight and "cloud bounce" optical comms receiver. He collects light transmitted from an array of LEDs with a single large Fresnel lens focused on his expensive APD. I want to do the same type of "receiver" using an array of Fresnel lenses to collect photons and focus them onto the photo-cathode of a 1P21 PMT. This PMT is "new old-stock" or NOS that I acquired many years ago, still in its original box. I only have two of them, but am willing to share with another ham who is seriously interested in optical-EME.
Baluncore said:
A laser is really not that good a source of light once it has passed through a few hundred metres of atmosphere. If the light source is from a bank of LEDs, each with a Fresnel lens, all that fear of lasers, and the bureaucratic regulations, can be dismissed, and you get a better S/N.
Rex VK7MO in Australia ran across this problem when he was doing line-of-sight and so-called "cloud bounce" comms with a large, red, high-power, LED array. Folks would see the red glow on the horizon and "alert" the authorities to the "forest fire". He solved the problem by switching to near-infrared LEDs.
