[Laser] mechanical rotating shutter

[TWOSIG at aol.com]

I wanted to comment on the post by Yves from the thread  :

Re: [Laser] Sky illumination experiment

But I wanted to break to a new  title.
 
I have had contact with a scout organization that has an annual event  
throughout the western United States where the scouts go out to various  locations 
with mirrors and heliographs to send messages by Morse code and relay  them to 
another station.  They use amateur radio, when possible, to provide  liaison 
and message confirmation.  They have confirmed messages on 176  mile hops.  
Anyway, they would be interested in other message systems than  eyeball read 
Morse (though they seem reluctant to extend the event to nighttime  
communications).
 
Anyway, a mechanical shutter could be adapted to almost any light  source, 
even reflected solar rays.  Good thinking, Yves!   One  system that can be 
adapted to laser, LED, IR, gas discharge, florescent,  arclight, limelight, candle, 
flashlight..........
 
For the scout event, an electronic reception and decoding system would have  
to cope with the signal to noise of daylight, but the power of the solar  
reflections is limited only by the size of the mirrors used.  A mechanical  system 
would be slow by radio standards, but the competition is hand sent Morse  by 
kids, most of whom are not even HAMs.  ( If there is not the potential  of 
getting these guys interested in laser communications and other forms of  
information transfer by lightwave, then we are doing something wrong! )
 
I can imagine a ton of problems:   
 
If a rotating shutter is size limited and the desired range requires a  much 
bigger solar aperture, then is it worth trying to concentrate the solar  rays 
into a smaller beam, or attempt to rock the large mirror on and off the  
target like a British design for heliograph, or maybe a louvered shutter  system?   
What about a bank of rotating silvered prisms, like those  billboard signs 
with three advertisements that change every few seconds. 
 
The data encoding system, should it use a frequency shift, say 20Hz for a  
Mark and 40Hz for a space, or would it be better to use a fixed 30Hz and vary  
the duty cycle from 25 to 75 percent?  (Please don't suggest a red filter  for 
Mark and Green for Space, unless of course it will improve the S/N by  15dB.)
 
 
The motor to drive the shutter, should it be an AC synchronous motor that  is 
driven to accurately match the encoded driver frequency, or should it be a DC 
 slot car motor that is driven by amplitude modulation.  (Hmmmmm.   If  the 
speed is increasing, that is a Mark, decreasing  Space........NRZ......)

 
For a 100Km path, which is a better investment pound for pound (after all,  
these kid have to carry it to the top of a mountain) a larger heliograph or a  
larger telescope?   ( Hmmmm.  You HAMs out there, where do you  put your 
money: receivers or linears?   Me?  I like  antennas.  Wire antennas. )
 
Anyway, thank you Yves.  
 
James
N5GUI



Hi  all 
May be  a simplest system could be used.  
Why not  a simple mechanical rotating shutter associates with a  powerful 
halogen  lamp? 
A  synchronic motor powered by a  simple audio amplifier with an up 
transformer  should be adapted.  
One can  find 240 V small synchronic motor in all the low cost AC  automatic  
timers. 
A PC  with LASERSCATTER or JASON in the 70  to 90 Hz range should drive the  
motor. 
73 de  Yves