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digital heliograph

solar optical transmitter
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This is a high-tech version of a heliograph. (method used to transmit messages using solar rays deflected with mirrors).

The main idea is to use sun rays as a carrier of digital information. The only energy expense is in modulating them.

For this I propose to concentrate sun rays into a point, at this point modulate the ray with a shutter that opens or closes according to the signal to be transmitted. The rays will spread after this point but they are re-collimated via a mirror and reflected into a remote receiver.

A parabolic mirror focuses sunlight at the focus of the parabola. At this point a high speed shutter is used to encode information. A chopper may be added as well to modulate the light and facilitate the detection later. The encoded/modulated light continues after the focus and expands. It reaches a second parabolic mirror, with a focus on the same point as the first one. The axis of the second mirror however is pointed in a different direction, therefore it transmits a collimated beam in a desired direction.

At a distance another parabolic mirror receives the beam and focuses it into a detector. The detector signal is decoded the information, maybe with aid of a lock in amplifier, taking advantage of the optical modulation of the beam.

The system could transmit data at long distances using very little power. The only power needed is for the shutter and the chopper and detector, but not to generate the radiation since it comes from the sun. Since it is collimated it will reach long distances without much attenuation. Since it is modulated it can be detected using a phase locked amplifier even when it's very faint.

A possible disadvantage is the low bandwidth obtained using mechanical shutters. Is there a better way to modulate a high powered solar beam?

atomico, Apr 17 2008

(?) US Air Force planned Space Age Heliograph! http://myweb.cableo...graph_LF_Jerale.JPG
from a 1961 issue of Science and Mechanics' Science Experimenter [baconbrain, Apr 18 2008]

The Chappe Telegraph Systems http://people.seas....history/chappe.html
The mechanical version [Ned_Ludd, Apr 18 2008]

The heliograph http://en.wikipedia.org/wiki/Heliograph
In use even in the 20th century [AbsintheWithoutLeave, Apr 18 2008]

[link]






       <mathem-ooops>the focus of a parabola is *on* the axis.</m-o>
lurch, Apr 17 2008
  

       What? A parabolic mirror requires that light coming in be parallel to the axis, which means the first mirror be pointed at the sun. The second mirror, if parabolic, must be between the first mirror and the sun, pointing away from the sun, which is to say back into the first mirror. The second mirror could have a big hole in the middle, and make all that work, but the light beam is still going to be headed directly away from the sun. Also, the light leaving would still be spreading, as the sun is not a point source of light.   

       A combination of mirrors and shutters could be made to send modulated sunlight in any direction, but two symmetrical parabolic mirrors aren't going to do much good. As described, this is warped optics.   

       This is a heliograph, by the way, not a transponder, and could be in another category, perhaps. See link for a "space age" version of 19th century technology as imagined before lasers and solar cells were common (mirrors F and G are useless, aren't they?).   

       A solar-powered laser would be directional and modulatable. Plus, you'd only have to look for one frequency of light. Hmm, if you alternated two laser frequencies, the phase-locked detector idea would work a treat. [ ]
baconbrain, Apr 18 2008
  

       This sounds like a hi-tech version of the semaphore system that once operated via towers over the length and breadth of France.
Ned_Ludd, Apr 18 2008
  

       Would it allow you to make cheap off-peak calls during the evenings?
AbsintheWithoutLeave, Apr 18 2008
  

       //off-peak calls// in this context are when the mountain-top heliograph operator yells the incoming message down to the valley below.
lurch, Apr 19 2008
  

       // What? A parabolic mirror requires that light coming in be parallel to the axis, which means the first mirror be pointed at the sun. The second mirror, if parabolic, must be between the first mirror and the sun, pointing away from the sun, which is to say back into the first mirror. The second mirror could have a big hole in the middle[…] //   

       A parabolic mirror doesn't have to include the apex of the parabola.
notexactly, Apr 19 2018
  

       Quite so. Reflecting telescopes work quite well.
MaxwellBuchanan, Apr 19 2018
  
      
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