Re: ATM Foucault trouble

[kevin@aquilagroup.com (Kevin Ferguson)]

Hi Glen & list,

Robert Duvall wrote:
>What I have found makes this difficult for folks is knowing just HOW to
>look- the focus of eye or whatever. 

True...The following seems to have worked for a couple of
guys I coached through this via email...

As you are reading this, close one eye, and hold your finger just 
in front of the open eye. (or glasses in my case)  You will find 
that you can still read through a fuzzy band at the edge of your 
finger. If you are in a bright room, only one or two words will
fall into the fuzz.

In foucault testing, the whole mirror must fall in just such a fuzzy
band at the edge of the knife.  The closer your eye is to the
knife edge, the wider this fuzzy band will be, and the less
critical the exact positioning of your head will be.
Also, the width of this band depends on the size of your pupil,
so it is wider in a dimly lit room than under strong lighting.
So it is important to test under subdued lighting even if
you have a very bright source. Use of a small telescope, per Berry,
gives you an artificialy enlarged pupil, which could be
handy for making the fuzz band wide enough to cover a 
very fast mirror.

The problem I had when I first tried testing was that I would move
my head in order to cause the knife to cross mirror.  This is
wrong. Just keep keep looking at mirror through the fuzz at edge of
knife as you move knife back and forth. The distance that the knife
must be moved is so small that it should not be necissary to move
your head at all as the mirror goes from full bright to black.
(unless you are WAY off on the 2f axial distance.) 

In Ronchi testing, there is a band of fuzz produced by each line of 
the grating, and they all overlap, so there is no need to position
your eye carefully...as long as you are looking at mirror through
the grating, it will be in the fuzz band of some of the lines.

Even in bright light, the grating pitch is very fine compared to
pupil size, so given a bright source, ronchi testing still works.

Above all, stick with it.  Noticing the fuzz-band worked for me,
but someone else's suggestion may be the one that works for you.
Once you get it it will seem really simple.

-Kevin

P.S.  Sorry for the use of highly technical "fuzz band" terminology.
I guess it is a dumbed-down way of talking about vigneting....but
it is not neccssary to understand why the fuzz band exists in order
to exploit it.