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Re: [ATM] Ronchi interpretation and corrective action for highzoneatedge
It looks better. There is some improvement with the edge plus the surface
and curve are smoother. It is a smoother oblate.
The crest of the high zone at the edge is closer to the edge. Just barely.
Look closely at your two Ronchis and compare present to previous. Look just
to the inside of the crest at the edge. Coming from the center, the present
image shows less up slope in the last half inch to that crest. That is why
the crest is closer to the edge. You have worn the inside edge of the crest.
That is pushing the crest to the edge. It might take a while. But you are
going that way.
Since you have that area in the center that is not quite polished, It might
be a good idea to stick to MOT to work the center quicker. But it might be
just as good or better an idea to continue what you are doing and see if the
crest keeps going to the edge. Since it is slow, maybe by the time you have
fully polished it will be pushed off the edge and you will no longer have a
turned edge.
Take a look at the earlier Ronchi. Look at the sweep of one of the first
Ronchi bands left or right of center. So we are on the same page, look at
the first band to the right of center and look at the left side of that
band. You know your edge is turned so if you look at that band and picture
in your mind that the band is the glass and the left edge of the band the
surface. Starting at the edge and working in, you have your turned edge, but
working from the outside it is an upslope to the crest of the raised zone
and then slopes downward as you go toward the center. Then at about two
inches from the edge it changes from a down slope to about level and then
about 4 inches from the edge it looks to be a slope up to a bump. That is
not a bump. You can be misled to believe that the sweep of the band
represents accurately the relative curve right across the mirror. But it
doesn't. I say relative because the idea is to interpret the slope changes
and if you start at the edge with the proper interpretation of the first
slope change (really the first change in the rate of curve change) then when
you get to the last slope change you have to reverse the direction of the
change. If you start in the center and work outward it will be the last
slope change that is reversed in direction. Sounds crazy, doesn't it?
Lets see if that really works.
First lets go over what an oblate curve looks like outside of center of
curve.
() something like that.
And
)( for a curve on the parabola side of the sphere.
Look at the center 1/3 diameter. it looks a little like
)(
That part of the mirror is a hole (center deeper, shorter radius) relative
to the curve just outside it. Keeping that in mind and working from the
edge inward as above you have the upslope to the peak of the raised edge
zone. Then it turns to sloping downward for about an inch then levels and
turns when you get to that center 1/3 diameter and looks to be a bump but we
know it isn't. So reverse that slope to a down slope to the center. Looks
like it worked.
Now let's go center outward. Knowing the center 1/3 of the mirror is on the
parabola side of the sphere,
)(
and that means a deeper center. We can use the same edge of the same band,
except now the glass is the light to the left of the same band and the
mirror faces right. So that center area looks like the hole we know it
is.... So... from the center.. upslope to the rim of that center hole,
levels and turns then to a downslope out to 85-90 % or so. And do you see
how that last slope change would look like a turn up at the edge if you
didn't reverse the slope? So reverse it and it is a turn down.
Son of a gun! ... it worked again.
Now keep in mind, Really it is a change in rate of change of curve. That is
not a typo. It is a Change in the rate of the change of the curve that bends
the band. I am talking about slope changes the bends in the curve.
Which way the change is going.
I don't know if there are shapes where this doesn't work. But I haven't
found one that it doesn't work with. If a curve is bad enough that it
wouldn't work, it wouldn't matter.
Does that make sense to you. I admit, I have a hard time expressing some of
these ideas in print and being clear. It is easier to do than to say...for
me.
Now the center hole might not be as deep as the oblate intermediate zones
are high. So to say the center is raised is not totally wrong. It could be
considered a hole at the top of a wide hill.
Main thing is... what you did worked.
Jerry
Sorry for the long post.
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Polk, Tom
This morning's Ronchigram (both outside focus, 133 lpi):
4-04-06:
http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/04
-04-06/Outside%20Focus%201.jpg
Compared to previous session:
4-02-06:
http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/04
-02-06/outside%20focus%201%20GS%20.jpg
Looks like progress to me (right? ;-).
Are my polishing sessions to long?
Can I go longer between cold presses?
**************************
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