Re: ATM Caustic testing

["Bob May" <bobmay@nethere.com>]

You're getting wound up in theory without knowing how to.
First off, looking past the KE you see the return from the mirror of the
light source.  Remember that the image is reversed?  Yep, it is and that
makes the single edge of the KE at the light source to be the last part
blocked by the viewing section of the KE.  The dirty little secret that
nobody seems to realize is that a slit source merely blocks off much of the
light from the source when you look at the light for locating the return.
The truth is that only one edge of the slit is actually used in the Foucault
test!  If you don't believe me, open up the slit width and run the test
again  The other edge basically does nothing when it comes to making the
shadows except to reduce the max contrast between the dark and light parts
of the mirror's image.
Thus, even though you move the light source with the KE, that just means
that you have to move the KE only 1/2 of what you do when the light source
doesn't move sideways.  The end result of the movement of the tester ends up
being exactly the same as before if the light moves in and out with the KE
and the motion in and out is 1/2 of what you got before if the light source
was fixed and didn't move when the KE moved about.
Note that this is coming onlist as there's a lot of people out there that
really don't understand how the test actually works.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net