[Laser] Cheap 8x11 Lens

[Bob Williams]

Yesterday, I picked up an 8 X 11 "Magnifying Sheet" at Staples office supply 
store for $5.50.

The focal length is about 12.75 inches.  So I was able to mount it on a 
stationary box lined with black paper to minimize reflection of stray light.  
I used another smaller box to bring the plane of the detector to the focal 
point.  Now I can definitely "hear" reflections that I can barely detect 
visually, even in my very light-polluted suburban neighborhood.

If you decide to try doing the same, determine your focal length by using the 
lens to project an image of a DISTANT bright scene, like a neighbor's house 
or the rising moon, at a wall and focussing that image, then measure the 
distance from the wall to the lens.   That's the distance your detector 
should be from the center of the lens. 

Remember, that focal length will only produce a good image at distances 
approaching infinity.  When sources get in closer, they will get fuzzier.  So 
if you do this, then don't be surprised when a spot reflected off the ceiling 
or a nearby wall doesn't produce a sharp image.  It shouldn't.  But at those 
close-in distances, you probably won't need a sharp focus anyway; the lens 
still casts quite a bit of light on the detector.

The magnifying sheet is great fun, and cheap for the amount of light 
gathering surface you get. 

Bob AA1ZC
Gales Ferry, CT FN31xk
trillium@snet.net