To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 



>With a f7.8 you can probably go to a 3 zone mask to advantage.

With a little more practice, you'll be making knife edge readings within
about +/- 0.030 or so -- your eye will get more judgmental, but your
readings look exactly like mine the first few times I did foucault.

There's more 'allowable' range to the readings than you expect.  Programs
like Tex or Sixtests will help you by fitting a parabola to the readings
and accounting for the area that each reading represents.  Highly
recommended. As others have mentioned, the central region is less
important, because it is shadowed by the secondary, so don't worry so much
about features there and concentrate on the middle and outer regions.

There are several different ways to think about the meaning of these
numbers. It can get confusing.  What your numbers tell me is the center
region has a much smaller radius of curvature than the outer region than
you'd like.   You want the radii of curvature to be a little closer
together.  That means you want to be more like a sphere and less like an
hyperbola.  So on your way back to a sphere you will come closer and closer
to a parabola.  Ergo, in your position, I would attempt to respherize and
keep measuring, probably after every 10-15 minutes of work or so, and you
should find the measurement differences getting closer and closer.

How to respherize?  If you draw your current figure -- try to draw
something with a tighter radius of curvature in the center than at the
edges -- it'll look somewhat like an hyperbola from your high school math
course -- and draw a somewhat flatter sphere (same radius of curvature
everywhere), below your surface everywhere (since you can only remove
material, not add any).  The closest sphere will be tangent in the center
and at the very edge.  The parabola you want is somewhere in between, but
still tangent at the center and the very edge.  So you need to remove more
material in the mid and outer zones than in the center.

How to do that?  Tool on Top (TOT) helps.  Also shorter strokes.  Why?  The
part of the mirror that gets worn down most is the part that is always in
contact with the lap.  For longer strokes, that is the very center of the
mirror, thus longer strokes wear the center most.  Shorter strokes spread
the wear out over a larger area of the mirror, and somewhat more toward the
outside (though, in theory, not completely all the way -- there's where the
TOT helps, as you tend to transfer a bit more pressure to the edge and
increase the wear there).  W strokes help to bring the surfaces toward a
sphere as well, since it causes the relative high points on each lap/mirror
surface to visit high points on the opposite surface, increasing local
pressure there.

Keep the lap well pressed (but don't let the lap/mirror pair dry out or
they'll stick), wait an hour after polishing before measuring, and keep
your patience up.  It usually takes several attempts to parabolize, heading
back to a sphere in between.

=Matt



>After posting my last attempts at reaching a sphere, several people
>pointed out that I might actually have fouled up and gone straight to
>parabola. I decided that I should at least test that theory. This is a 6
>inch F/7.8 mirror.
>
>I have spent the last week trying to figure out the foucault test. I have
>made masks, pins, everything with little success. Tonight I made a simple
>mask with a one inch hole cut in the middle. I used it to find the point
>where the very center of the mirror nulls evenly. Then I used someone's
>very handy website to generate zones for a two-zone couder mask and cut
>this out.
>
>I used the following:
>
>Zone1 centered at 1.51 inches
>Zone2 centered at 2.6 inches
>
>I did 3 separate tests and came up with the following numbers:
>
>Zone    Test1   Test2   Test3
>1               .092    .104    .025
>2               .250    .235    .154
>
>I expected:
>
>Zone    delta
>1               .012
>2               .036
>
>The difference in the numbers is fairly consistent but am I really off by
>an order of magnitude? Wouldn' this much variation be really obvious in
>the ronchi tests?
>
>BTW, my tester has a light source that moves with the knife-edge.
>
>Did I screw up my calculations? Does this sound like some common problem
>or should I just start looking for a sphere again?
>
>Thanks.
>---
>Dave Holsclaw
>Saint Louis, Missouri
>
>

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