[Laser] Fresnel focus

[Tom Becker]

Folks, I'm building an optical receiver using an 11" Fresnel at the end 
of two stacked plastic buckets.
http://rightime.com/images/WAAS/dscn5246a.jpg
http://rightime.com/images/WAAS/dscn5262a.jpg

I understand that Fresnels don't focus a distant point source sharply, 
so finding the focal point is inexact.  Still, I am puzzled.

http://rightime.com/images/WAAS/dscn5268a.jpg
This is a contrast-enhanced photo of the inside surface of the Fresnel 
when centered on a distant 635nm laser return, taken from the apparent 
point of focus; the center disk of light is that from the laser, while 
the corner illumination is from surrounding daylight background; surface 
scuffs on the lens are also visible..  All is red because the view is 
through a red Rosco gel  mounted in the bottom of the top bucket.

I find that, if I move the sensor (the camera in this case) toward the 
Fresnel from this position, the laser return disk becomes a larger 
circle with a dark hole in the center, not a larger solid disk.  If I 
move away, the disk simply becomes smaller and the surrounding 
background consumes more of the field.  As I write this, I'm thinking 
that the "best" location of the sensor is where the modulated laser 
return (1.5MHz at the moment) is peaked, regardless of what that might 
look like from the sensor's view.  Still, the ideal would be to see an 
entirely-filled disk of laser light from the Fresnel.

If I can't realize that ideal, should the laser return disk be as large 
as possible - including the dark center, or should it be the largest 
solid disk possible with this lens?


Tom
Cape Coral