[ATM] how to use the foucault tester

["David Williams" <dwilliams1128@earthlink.net>]

Hello people

I have a foucault tester that I bought from University Optics called the Mirro-Test.  And they didn't include any directions except just say that you should look in the Tex book.  My foucault tester has a little white light, light bulb inside a square cross-sectional axis extruded tube, type chimney.  And this originally had two single edged razor blades with the sharp ends facing each other.  So I loosened the screws and (I heard from a friend of mine that I should use a pinhole test by using a sewing needle and just barely poking a hole in a piece of tin foil) I attached this piece of tin foil that I had made in advance.  That friend's name is Jerry Martinsen and he lives in Stergis MI.  But the thing is that the light source is stable and doesn't move, also there is a micrometer that controls how far in and out the knife edge is (not the same as the ones at the light), the one that cuts the incoming light to your eye from the mirror.  

Some times I have looked at the shadow gram of the 20" mirror with the foucault tester and seen the whole mirror lit up except for a small ring around the outer edge.  Does anyone know the reason for this?  Also I'm testing my mirror in the basement of my parents house.  There the ground is concrete, except for the mat carpet.  I have the back of the mirror leaning against the riser of the bottom of the stair way.  And the area in which I am using the basement is an area that can be partially dark even in the day time.  The 20" mirror of course gives us a radius of 10" so when set vertical like I described, the center of the mirror will be 10" off the floor.  So the foucault tester has a flat base and needs to be propped up so that it is up at the right height to shine correctly on the center of the mirror.  

I don't understand why a mirror would be curved like this:


     ---- \             /------\
  /          \ - - - -/          \
/                                  \


This is the diagram of the hyperboliod, but I don't understand it.  Is it a cross section?  Is it a shadowgram?  How do I interpret this picture?  


David Williams
Thank you.
Bye


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