To: holsclaw{at}mac.com, atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Hi Dave,

You might find my Couder mask program helpful.  It prints the pattern for a
Couder mask (the mask for Foucault testing) as calculated according to the
analysis done by Nils Olof Carlin.  You can find the program at
http://www.atmsite.org  Look in the subject index under Couder mask.

The Foucault test does take some practice to become reasonably repeatable. 
The shadows do not behave exactly the same at all zone positions.  There
are subtle differences in how the shadows appear and move inside the zones,
even when masking reduces the area visible to a small amount.  The
differences depend both on the zone radius, and on the current shape of the
mirror.  It is not as straightforward as the simple explanation makes it
out.  At each zone I try to move the knife edge to a position clearly
outside the null point and to one clearly inside the null point and then to
try to refine the difference.

You might find it helpful to use zones a little narrower than Nils Olof
suggests.  I have found that with his recommendation for zone width, there
are still some pretty obvious assymetries in shadow movement that
complicate locating the zone null point.  About 2/3's of his widths might
be a good value to try if you are having this problem.

Also, if you wear glasses, you may have the problem of not being able to
get close enough to the knife edge to really see what you need to.  I
haven't tried this myself (my eyes are not too badly off), but you may find
that, even with rather poor uncorrected vision, you don't need your glasses
to view the Foucault test.  When you get right behind the knife edge, the
diameter of the light cone from the mirror is pretty narrow.  Because it
only passes through a small portion of your eye lens, aberrations in the
lens are minimized.  I have slowly become noticibly near sighted in my
right eye, but I still use my right (dominant)eye for Foucault testing.  I
can focus on the mirror quite well despite the fact that it is out of the
usual range of sharp focus for that eye under more normal circumstances.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP