From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "wayne hilliard" 
>
>
>I might have a dumb question but here it goes..
>
>When doing the foucault test with a slitless moving source tester you
>have your eye right up close to the knife edge.
>
>wouldn't half the mirror be blocked by the knife and only half be visable.
>
>I havn't really had the oppurtunity to try this out but am getting ready
>to and want to have some level of understanding of how it works...
>
>I've read a lot of information on the web and I think I understand the
>theory but it's one of thoses things that until you do it it's hard to
>understand what your going to see.
>
>Thanks
>
>Wayne Hilliard

Hi Wayne
 Hold your finger up close to your eye and look past the
edge of your finger. You'll notice that your finger doesn't make a sharp
edge. This is because it is out of focus. Also, notice that it gets darker
as it gets closer to your finger ( assuming your finger is darker than what
you are looking at ). The reason it gets darker is because more of the
light is being blocked by your finger as compared to the width of your
pupil. If your pupil was a pinhole, your finger would make a sharp edge.
 In the case of the Foucault tester, the light that
passes the knife edge doesn't get darker because of the effect of your
pupil. This is because the light that passes the edge going to either side
of your pupil from the mirror is just a tiny sliver of light ( assuming
that you've nulled the returning light ). Since it has the same width
coming from the right or the left side of the mirror, there is no change in
brightness as your seeing with your finger.
Dwight

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