[Laser] Riflescope, Telescope, Eyepiece, One Pixel Camera

[TWOSIG at aol.com]

To add to my previous comments about using a telescope in a light 
communication system, I would like to bring to your attention the technique of eyepiece 
projection.  It is commonly used to observe sunspots, by moving the eyepiece 
further away from the objective lens than would be used when people look through 
the telescope.

I stated before that when the eyepiece is adjusted for normal viewing, the 
image is focused at infinity.  If you move the eyepiece further away from the 
virtual image from the telescope objective, then the real image from the 
eyepiece moves closer.

If you are using the technique to observe sunspots, the size of the image of 
the sun (and its spots) is bigger the further away the image is.  It is also 
less bright.  There is the additional problem that on a telescope that is not 
clock driven to synchronize its motion with the Sun the image will move out of 
the field of view much faster.

The adjustments that provide a large image when observing sunspots translates 
into narrow field of view on a communication system.  Narrow field of view 
means better signal to noise (same signal, less noise) but more difficulty 
performing alignment.  Brightness is not an issue.  As long as your light source is 
small enough to be considered a point source, it will still result in a point 
image, with all the light energy in that point.  However, atmospheric 
disturbance to the path may cause the point image to miss a small detector.  This 
would probably seem like severe scintilation (like the twinkling of stars).   
De-focus so the "image" of the point source is larger than the detector might 
help image jump caused by atmospherics, but it will not affect scintilation, and 
it comes at the price of less light energy on the detector.  If you have 
excess signal to noise, it may be worth the trade.


James
N5GUI