Re: [ATM] I think I'm understanding Foucault test

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi David,
You are making this more complicated than necessary.  All you need to
measure is the difference between zones.

For instance, Zone One might measure .027" on the micrometer.
Then Zone Two might measure .054" , (which just happens to be an increase of
.027" over Zone 0ne)
Then, moving the KE back for Zone Three, the reading might be .085"

My point is, the readings are cumulative and the beginning point doesn't
matter at all. You will have to use data reduction software to make the
readings make sense. If you understand the math statement you
started your last e-mail with ie.

 <Delta p' = h^2 / R

<Which means that "The longitudinal aberration of any zone of radius of
incidence h equals the square of <this radius divided by the radius of
curvature R of the mirror.

Then you are ahead of me. However, the math is good if you plan to write
your own software. You will be reinventing the wheel if you do that though.

Set your Foucault tester to null Zone One, and make a note of the micrometer
reading.  At this point make sure the micrometer still has plenty of room
for additional measurements, keeping in mind that the KE is going to be
moving away from the mirror. If you need to reset the micrometer to allow
enough travel, then do so, and make a note of the reading.  Then move the KE
away from the mirror several thousands of an inch and see if you are inside
of, or outside of the null point for Zone Two.  Make whatever move on the KE
you need to, to null Zone Two. Repeat this process for each of the eight
zones you have made masks for (you do have at least eight, right?)
Now that you have made one set of readings, Get a clean sheet of paper and
repeat the process.  Do not refer to the first set of readings. It will
prejudice the second set.

Now that you have two sets of readings. Post them to the list, along with
the following information.

Mirror diameter
Focal Length
Radius of Curvature (This should be twice the Focal Length)
Inside and Outside radius measurements of each Zone
Minor Axis of the secondary mirror.
Light source fixed or moving


Just a few notes on testing.
I am assuming that you know to place your eye no less than 1/2" from the KE
to view the mirror. Closer is better.
When I say 'null' a Zone, I mean the point that you can not tell which
direction the KE shadow is coming from when you are looking at the mirror.
Both openings in the mask will darken at the same time.
I am assuming you know how to read your micrometer.  Mine has dual dial
readings so that cumulative measurements can be taken regardless of whether
the plunger is being depressed more with each reading, or being extended
more with each reading.

I'll be watching for you to post the readings of your mirror.

Clear skies, Jerry

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