[Laser] Re:: heliograph vs. laser indeed!

[F1AVYopto at aol.com]

>thanks for doing the math Yves!
>wow, i didn't expect a problem  with a collimator being too
>small..  so it looks like a 5 milliwatt  laser can equal the
>brightness of a 1 kw heliograph at 200km, IF the  laser can
>be collimated to 20 microradians (3.8 meter spot). But,  it
>would require a collimator lens or mirror diameter more
>than 1  meter to accomplish. So the heliograph wins! The
>math always seems to  bring up interesting and sometimes
>un-expected results... 

>I  was wondering how you find the maximum collimation
>capability for a given  diameter lens or mirror. That can
>give an idea of the maximum results we  can expect from our
>optics. It would also be interesting to see the  figures for
>fresnels to compare with glass lenses.


It is like  for the radio and the antennas directivity.
The parabola size determines the  beam angle because the refraction effects.
With a perfect parabola the -3dB  beam angle is about = 70 x (wavelength / 
Diameter) 
A 1 meter perfect  parabolic mirror gives with a 1 micron infrared laser :  
= 70 (10^-6 /  1)
= 7 . 10^-5
= 0,000070
70 microradians ! 

>That's  interesting about the 10 watt array of Luxeons on
>the moon collimated to  cover one whole hemisphere of earth
>(2 degrees), it would be PGP  detectable with 30cm (12 inch)
>telescopes? awesome! 

The signal  will be PGP detectable but for the beacon message the baud rate 
will be  extremely low.
May be about 1 character / minute versus the FFTDSP spectral  resolution used 
by the decoding program.

>At any rate, all this goes a  long way toward understanding
>the minimum power densities required for  detection with a
>given sized lens or telescope, and transmit power /  beam
>divergence etc.. maybe math isn't so bad after all 8)

Math  often describes ideal properties but, as explain Charles Pooley KD6HKU 
in his  last mail, the laser diodes are not perfect sources and have many  
aberrations.
For example a powerful infrared laser diode has its active  output area like 
a thin rectangle 1 micron x 200 microns. 
The spot on a  target is a rectangle with astigmatism and focusing problems.
As explain  Charles theses problems can be reduced by a cylindrical lens or 
worst by a slot  diaphragm that gives enormous losses.

>"Another way is to use the VESCEL (vertical cavity) lasers, except they  are 
not made in visible wavelength"

The best are the Yag lasers but it is an other story.
Charles, I find  many info about your "microlaunchers" but can you give us th
e link to go  directly to your small optical space data link using 7 cm 
transmitter optics and  a 30 mw 650 nm laser.
Besides the optical mouse IC to track the earth  relative movement is a 
fantastic idea !
73 to all.
Yves F1AVY