[Laser] variable field of view for noise elimination

[TWOSIG at aol.com]

We seem to be straying into some interesting, but off topic areas.  I  will 
start another thread to comment on communication by CCD arrays.
 
A pinhole camera works.  It certainly is not refraction.  I  believe that its 
mechanism can be described by geometry of light traveling in  straight lines, 
what Newton described as a corpuscular theory.  Let  us agree that we 
disagree and turn the discussion to the merits of, or lack  thereof, noise 
elimination as I described earlier.
 
 
I think that we agree that an adjustable aperture at or near the focal  plane 
of an instrument can be used to block the image of some noise sources  while 
admitting the desired signal.  
 
I think that our major disagreement is that to do so is un-necessary  because 
the sensor should be sized so that it is at most a few percent larger  than 
the minimum image size of the source.  To use a larger sensor is to  introduce 
larger noise as the sensor is a noise source proportional to its  size.
 
I would like to state three ideas for your further  consideration.  First, my 
original post was not directed at internal noise,  only to suggest a means of 
addressing optical noise sources for experimenters  who might read the 
posting.  It mighte have been more interesting, and  useful, if there was an optical 
beacon on a satellite and I was suggesting a way  to remove noise from stars. 
 An even better scenario would be a lunar  optical downlink that needed to be 
tracked during the lunar day.  
 
Second, practical light communication systems for experimenters need to be  
made of materials that are readily avaiable and useable by them without  
extraordinary equipment.  Sensor may be as large as 10 mm and I am guessing  seldom 
less than 0.1 mm.  The difference in internal noise is probably not  
measureable compared to the amplifier needed after it.  The equipment  needed to point 
and control a smaller sensor would be difficult.  The  field of view is just 
too restricted.  
 
Third, the quality of affordable optics makes the  sensors impractically 
small if sized as you suggest.   Astronomers, amateur or not, describe better 
telescopes as diffraction  limited.  There is a formula which will calculate the 
blob size.  I  have a 114 mm f/8.  To use round numbers its focal length is  
900  mm.  It is not a great telescope but its blob is probably no more than  
twice the ideal. For that scope at  660 nm (RED) light the  ideal would be about 
8 microradians.  That modest scope would  need a sensor less than 0.015 mm.  
And its field of view would be the size  of a quarter a mile away.  
 
 
 
 
 
I submit that is impractical.  If NASA was trying to receive an  optical data 
link from Mars, we have different priorities and diffferent  budgets.  Much 
of what you suggest as "ideal" may very well be true.   It just does not apply 
for this group.
 
My ideas for optical noise may be useless, but not because I suggest using  
sensor that are too large.
 
 
James
N5GUI