[Laser] Fresnel focus

[James Whitfield]

Tom

Really neat pictures.  Thank you.

Please forgive that what I am about to say may be long winded.  I wrote some
time ago about a technique used by telescope users to adjust the focus of
their photographic equipment.  It involves a simple device, often refered to
as a Hartmann Mask.  And it may not work as well for you since you are using
a Fresnel lens, which may have severe differences of focal length across the
face of the lens.  Here is what I suggest:

Take a piece of black construction paper large enough to cover the entire
lens and cut on hole in its center and two holes equal distance from the
center.  Since you have an 11 inch lens, the hole centers should be about
eight inches apart.  I suggest you start with half inch diameter holes, and
enlarge them as necessary.  Temporarily cover the center hole with a piece
of the same construction paper.  The holes will allow some light from a
relatively bright, but distant source.  An isolated street light at least a
quarter mile away should do.  ( The difference between the focal image plane
for a source at infinity will be almost un-measureable from one only a
quarter mile away. )  If you have a focal screen, like a sheet of wax paper,
when the screen is too far or too close to the lens, the source will form
two dots on the screen.  When you get the two dots to merge, you are at the
focal distance.

Now it may get tricky.  Once you set the distance for that configureation,
then turn the mask 90 degrees.  If the two dots are still merged, then you
can move the mask around to see if the rest of the lens is consistent.  If
it is not consistent, you will need to "guess-timate" what is the best
focus.

Once you find how "good" the outer area is, then remove the cover from the
center hole.  Run the test again to see if the center dot falls in the same
place.  If you want to get more "data", you can create a mask with holes at
increments from the center, perhaps at 25, 50, and 75 percent of the radius
of the lens.  A "perfect" lens will have all the dots converge to the same
spot regardless of the orientation of the mask.

If you don't have good convergence, you can make a focusing  mask with a
dozen or so holes in a pattern all over the face, then adjust the focus so
that the cluster of dots forms as tightly as possible.  If that pattern is
larger than your photo sensor, then you can get a good idea of how much of
the signal is getting wasted because your optics cannot collect all of the
light energy into an area that small.

I hope this gives you an idea how to adjust your optics.   You may still
have to fire up the electronics and try to measure the output for maximum
signal.

Best wishes

James
 n5gui



----- Original Message -----
From: "Tom Becker" <GTBecker at RighTime.com>
To: "Free Space LASER Communications" <laser at mailman.qth.net>
Sent: Friday, March 21, 2008 12:38 PM
Subject: [Laser] Fresnel focus


> 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
>
>
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