Re: ATM Focault Tester Setup

["Frank Ward" <thewards@mindspring.com>]

Mike,

>The knife edge is mounted on the left, over and touching against a jumbo
LED
>with the dome cut off, and sanded flat.

Here's the beginning of your problem.

The knife edge must be in the center of the LED at the bottom and
rise above the LED to intercept the returning light beam. The edge
of the knife makes the "slit" on the bottom and the same blade
intercepts it at the top. That way you can always be parallel.

Then both the LED and K.E. move left and right to enter the returning
beam. That's what makes it a moving source tester.

The next motion is forward and backward. That's the amount you are
measuring between the zones in the mask. The total forward and
backward usually isn't more than 1/2" total for a normal mirror.

To find the RoC enter the beam from the right with the knife. When the
shadow enters the mirror from the right you are inside. Now bring the
knife back until entering from the right with the K.E. makes the shadow
enter the face of the mirror from the left. Now you are outside focus.
Start moving forward until the shadow enters from the right and the
left at the exact instant. That is the RoC.

If you want to find the returning beam from the mirror on the K.E.
light a cigarette and blow a puff of smoke at it. You'll see the
returning beam.

That's why we way it's all done with smoke and mirrors.

In friendship,

Frank Ward
Atlanta, GA