To: atm{at}shore.net, scott.e{at}quicksurfer.com
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


>From: "Scott Ewart" 
>
>...I decided to try using a wire instead of a knife
>edge to get less ambiguous readings with the pin stick.

>...Then I tried a single strand of fiberglass .0003" thick....

>...The shadow is still 2" wide....

>Am I missing something?  Why can't I get a narrower shadow?  It's not
>like the diffraction effects are that big.  The illumination across the 2"
>is fairly uniform with a slight increase in the center, then darkest near
>the edge, then quickly to full brightness, then a diffraction band only
>about a half inch wide.

You mention above that you have a diffraction band about 1/2 inch wide
that's at the side of the wire's shadow.  (And in between that diffraction
band and the wire shadow...a section that's full brightness...although you
don't specify how wide that bright section is.)

When I wire tested my mirrors, I would use this band of brightness you
mention for a couple reasons.

A.  It was narrower than the wire shadow itself.

B.  It helped me better evaluate the edge zones and central zones of the mirror.

In other words, I used the edge of the wire shadow, not the center of the
shadow as my measuring point on the mirror's face.  I took all my zonal
readings from the edge of the shadow (where it transitions to full bright).

Nils, others...is this technique conceptually valid?  Will it really help
measure the edge and center zones more accurately...or did I just get a
false and incorrect sense of security from this technique?  (The mirror I
tested was a 16 inch f/6...and it star tests pretty well.  I've always
gotten good agreement on overall correction on bench tests with star
tests.)

Tom Krajci
Tashkent, Uzbekistan

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