[Laser] Cheap 8x11 Lens

[Paul Kelley]

Bob,

Thanks for all the info, that is very helpful. Next time I'm in the "city" 
I'll check the office supply places for lenses. The reason I asked is I did 
find one once that was almost paper thin and wouldn't hold its shape in any 
position.

I'm very interested in playing with reflections of any type and will be out 
there soon despite 30" of snow up here. Your experiences have helped 
encourage me to get at it. I just have a couple of minor things to attend to 
first. 

I need more gain in the receiver. Right now signals that appear to be a few 
dB above noise are barely audible in the headphones. Actually, noise is 
*almost* totally inaudible. It seems quite sensitive, so I think this is an 
easy fix with more gain. We'll see.

I also need to do something about stray reflections/scattering from my beam 
chopper (HeNe laser). I have the laser and receiver on the same mount for 
aiming. The "front" of the receiver is mounted slightly ahead of the beam 
chopper, so there is no direct scattering getting into it... but I get 
reflections of scattered signal from any nearby objects that the receiver 
hears (despite the fact I can't see them!). I'm sure that is avoidable. I 
just need to muck around with the system a bit. An enclosure around the 
chopper should help.

Paul



On Monday 04 February 2002 06:55 am, Bob Williams wrote:
> On Sunday 03 February 2002 16:50, Paul Kelley wrote:
> > Can you tell us how rigid it is? Does it hold its shape well enough to
> > still focus properly when mounted by the edges and perhaps tilted at an
> > angle?
>
> Paul,
>
> Yes, it appears to be plenty rigid when mounted by the apx. 0.25" lip and
> pointed straight up, I don't get any obvious deflection.  It's fairly thick
> plastic.  I imagine that in a good wind, it might, indeed, flex a bit,
> though. The packing, an envelope, will allow you to judge that for
> yourself, if you find one at an office supply store.
>
> > Reflections of what, exactly? From what? Could you please explain
> > further? I have a system ready to go (though using a smaller lens right
> > now), but I'm the only one playing with this stuff here. So I'm curious
> > when anybody says "reflections".
>
> Well, I'm waiting for some warmer weather or good cloud cover to test
> cloudbounce and some bounces off of a not-too-distant tower with some
> dishes on it that make it look interesting.  My reflections were off nearby
> buildings and trees.  At this point nothing technically that interesting
> nor very scientific.  I use my 800 Hz mod. laser pointer, attenuate the
> beam by holding the whole pointer in my hand and letting the beam peek out
> enough to make a barely visible, fuzzy spot.  I then swept the view of the
> receiver through where the spot should be to see if I got a peak, and did. 
> By squeezing my fingers till the spot disappears, I produce a fuzzy spot
> right at or just below visibility.  Not too scientific, but it indicates a
> significant improvement over the set-up without the lens, where I was able
> to detect visible reflections off of a surface painted flat-black.
>
> I've been thinking that if you wanted some quantative measure of
> improvement, neutral density filters such as used in photography should
> produce calibrated signal degradation.  I've also toyed with the idea of
> crossed polarizing lenses as an adjustable attenuater; because the
> attenuation of randomly polarized light should be roughly equiv. to Cos of
> the angle, you could get at least a repeatable figure of merit by measuring
> the angle of crossing. I'm not sure that it would work as well with
> coherent light.  There will probably be some polarization effects.
>
> In the end, I'm going to wait for a bit warmer weather and then just take
> the setup out in the field.  I believe that interesting non-LOS paths
> should be workable with common visibility to water towers, microwave dishes
> and the like. And with the sensitivity I'm seeing now with this very simple
> setup, I don't see why not some over the horizon scatter.

-- 
Paul Kelley
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