Re: ATM Foucault test accuracy

["Tom & Lou Krajci" <krajcit@3lefties.com>]

> From: ToobWiz@aol.com

> I've looked at the old ATM article a few times thinking I might try that
> test, but each time I get to the part about selecting the pin size for a
> given mirror, I just sigh and proceed with the Foucault and Ronchi tests.

I'm not quite sure what you mean by pin size.  If you mean the thickness of
the pin...make it large enough that it's visible to your eye from behind the
knife edge.  (Some people make actual pin sticks...I made strips of
cardboard with narrow pieces of paper sticking up to be the 'pins'...others
draw a magic marker pattern of 'pin dots' directly on the glass before
making the test...all these methods can work well if used carefully.)

If you mean how many pins (zones) to measure...start with a small number for
practice and go as high as you are comfortable with.  Pin locations?  I
simply used the values in Tex to generate the inner/outer radii of a couder
mask...and the pins are in the very middle of the zone...which is the
average of the inner and outer radii for any given zone.

If you are measuring many zones...it may be handy to rig up a laser pointer
(probably filtered to make it fainter) so that you can have it shine on the
mirror just below the particular zone/pin you want to measure.  (Make sure
the laser spot does not reflect off the mirror back to your eye...have the
laser shine onto the mirror from somewhere off to the side to avoid this
problem.)

> I keep thinking that it shouldn't be all that complicated; further, it
> appears that the test would be less prone to "user method differences". A
> tutorial would be a great benefit.

Hasn't someone made a piece of software that shows you what the shadow
appearance for foucalt and wire tests look like at various longitudinal
placements of the knife edge?...Nils?...Jim Burrows?

Tom Krajci