Fwd: [ATM] how accurate is the foucault test

["Matthew L. Brown" <mlbrown@everstrive.com>]

David,
  It turns out that the absolute radius of curvature isn't as important as 
the differences between the radii of curvature of different parts of the 
mirror.  Notice I write "radius of curvature" rather than "focus."  The 
Foucault test doesn't measure focal length; it measures radius of curvature.

Imagine if you will that your mirror is a perfect sphere.  Put a pinhole or 
slit source of light at the center of that sphere.  The light will go out 
to the mirror and bounce right back.  If you put your eye there the whole 
mirror will look illuminated.

But you don't want a sphere; you want a paraboloid.  It turns out that that 
the radius of curvature of a paraboloid varies smoothly -- the inner part 
has a small radius and the outer part has a large radius.  And it varies in 
a specific way as a function of the zonal radius of the mirror (distance 
along the glass from the center of the mirror).  The Foucault test measures 
this difference.

For the geometrically inclined, see:
http://host67.everstrive.com/~mlbrown/ASTRONOMY/FoucaultGeom.html

Couder masks are useful to help you isolate specific zones on the mirror so 
you can get the knife edge reliably on the intersection of the return beams 
coming from that zone.

=Matt


>Hello
>
>I wondered how accurate the foucault test is because once a person knows 
>that the light or the knife is close to where it should be (inside or 
>outside of focus) how will you know what the focus is.  Won't you have to 
>rely on something crude like a tape measure to roughly measure the focal 
>length?
>
>Can someone talk about the Couder mask.  This sounds like something 
>interesting.



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