Re: ATM Foucault tester

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

You don't need that much tape and really don't need any at all.  The purpose
of the diffuser it to attempt to get an even illumination of the mirror as
it looks past the source KE.  I run with just a  regular green LED and don't
have any problems when using a video camera to look at the results of the
test.
The light comes from the light source (a small area source as it is beyond
the source KE by a little so it is going to be using different parts to
illuminate the various points of the mirror from that point beyond the
source KE - simple geometry here) and proceeds past the exact edge of the
source KE to the mirror and back to the testing edge of the KE where the
image is reversed and the part of the image coming back of importance is
that edge caused by the source KE.  That particular point is where the light
takes a sudden dip to 0 illumination and, as you cut it off with the testing
edge of the KE, you will be going through that rapid slope and thus
producing your apparent shadows on the mirror's surface.
If you have the diffuser in/near focus, you can get the image of the fine
detail of the diffuser in the returned light (remember that there will be
some change of the source of the light due to the geometry of the test - the
left side of the mirror isn't being illuminated by the same exact spot of
the source as the right side) and that isn't a nice thing as it will bias
the results of the shadows.
You will only find one spot where the mirror (assuming a perfect sphere)
will gray out evenly and that point is at the average of the two dimensions
of the source to the mirror and the KE to the mirror and since you are using
a slitless tester, those dimensions will be equal and thus you will be
seeing the results of being at the ROC of the mirror.  Please note that if
you don't have a perfect sphere, differing parts of the mirror will be
showing differing locations of the ROC of the various parts of the mirror -
this being what we measure when we go and parabolize the Newtonian mirror.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net