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


I will describe two methods for understanding your doughnut, one
qualitative and one quantitative.  At your stage of work, an experienced
worker would probably use just the qualitative version, but for a beginner,
or even for an experienced worker who likes to be quantitative about
things, using both is a reasonably good idea.  (Others on this list are
bound to say I am crazy, and put too much dependence on numbers but I think
getting your qualitative eye to agree with the numbers is a good reality
check.)

1.  The qualitative method.  Imagine that your mirror is being illuminated
by a light placed so that the light just grazes the surface, coming from
the same side as your tester's light source.  In other word's imagine that
the sun is just rising over the landscape of your mirror.  The shadows on
the mirror trace out highs and lows in essentially the same pattern as
hills and valleys on a real landscape would.  The crest of a hill, or
bottom of a valley will be marked by a light-dark or dark-light transition.

2.  The quantitative method.  Make an Everest pin stick.  Either: A. Use
the pin stick for testing and  test zones at, and on either side of the
doughnut as well as the more usual zones, or B. Use the pin stick to locate
the doughnut, then cut Couder masks with openings at and on either side of
the doughnut as well as the usual zones.  Then make readings with the
Couder masks.  Plot the results with one of the Foucault data reduction
programs (my favorite is Sixtests).

You will want to be able to make quantitative Foucault readings anyhow
before long, and it is not a skill one picks up right off the bat.  The
practice will be good for you.

Methods 1 and 2 really should tell you the same thing.  If not, something
is amiss and you need to figure it out, because not being able to make the
two agree will likely lead to trouble later.  (It is possible to make good
mirrors using only a partially quantified version of method 1.  (There are
a lot of variations on this theme.)  It is also possible to make good
mirrors using only method 2.  Most atm's use both to some extent.  Some
people have a difficult time with the mental juggling necessary to
"see" the hills and valleys on the surface.  For these folks, the
numbers can be a better choice.  Some folks have a hard time with numeric
approaches.  These folks tend to use the qualitative methods more.  Often,
a really experienced worker will gain a "calibrated" eyeball and
can do at least semiquantitative work without having to make any zonal
measurements.

Mark Holm
mdholm{at}telerama.com

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