To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 15:18 2003-05-09 -0400, Jerry wrote:

>         When doing the Foucault test you do not focus on the image of the
>slit. Not with the eye or, in the robo fashion, with the camera. The object
>of the test is to determine the point on the axis that the beam from
>opposite sides of the mirror zone cross the axis. That it is not in focused
>at that crossover point does not prevent the knife on axis from blocking
>half of the light from both sides of the mirror zone that the knife is set
>for.

A slight change to this makes it closer to reality and easier to understand
what is going on in the typical Foucault test.  Think of the case of a mask
with only two holes (the Ritchey mask) at a particular zone radius on the
right and left side of the mirror.  These small mirror patches can be
considered planar (the tangent planes to the mirror surface) simply
reflecting the source.  When the KE is at the point where the two patches
gray, the KE is at the point where these two rays cross, and it doesn't
have to be on the optical axis.  In fact, if the source isn't on the
optical axis, or the mirror has astigmatism, the point won't be on the
optical axis.  The error committed by assuming the crossing is on the
optical axis when reducing the data is tiny, a "cosine
correction" as the engineers are fond of saying.

Finally, it is possible to work Foucault on the image plane - I've done it.
 You replace the KE with a frosted glass slide and move the slide
longitudinally until the returns from the two mask holes merge.

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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