Re: [ATM] Not tde or tue, important oblate spheroid, ellipsoid

["Jerry" <wa4guu@verizon.net>]

Hello Lutfu

The lap looks much better now. There is some turn down on the edge. The
shape is very much improved. I see your second comment about the hole. It is
a small hole, so shallow that is does not matter. The shape is for all
practical purposes a sphere with a little turned down edge. Much better than
before.

Now you have to work in short sessions. Do not work more than 10 or 15
minutes in one session. You have to test between sessions. You can do more
work in 10 or 15 minutes than is necessary to parabolize. Even if you are
doing something that will take longer to accomplish, you should not work
longer. If everything goes right it will take several sessions to finish. It
may take many more. Sometimes 5 minutes or less might be enough.

Let's see if you can make the edge better without going far from the sphere.


Use the short stroke Center Over Center for 15 minutes. The stroke length
can be anywhere from 1/4 D to 1/3 D Don't do this so long that it goes back
to an oblate curve with ugly zones. Only do it for 15 minutes. Look at it
with the Ronchi test. The edge might get better. It will start to go back
toward oblate. But you won't do it long enough to go far. Don't do long
sessions of the short COC strokes. Even if the edge improves and it hasn't
gotten oblate or developed zones, you will not do the same thing next
session. The next time will be something else.

You want to keep the curve as close to a sphere as possible while you
improve the edge.

Do the short COC stroke for 15 minutes, no more. You can do either MOT or
TOT, and if you wish you can do half of the time MOT and half TOT. Test to
see the results. If the edge is good go on to parabolizing. If the edge is
not yet good do 1/3 diameter narrow W stroke for one session. Make the width
of the W about 1/4 or 1/3 D. Test again. If the edge still needs more work
you can do another session of short COC strokes. 

If the combination of the short COC stroke and the narrow W results in the
mirror going oblate then make the W a little wider. If it goes toward the
parabola, narrow the W. Adjust the stroke so the combination of short COC
and the narrow W holds the mirror close to a sphere while the edge gets
better. 

When you get the edge satisfactory start parabolizing. Now that the lap is
in better shape and the mirror does not have a hill, the best stroke to
start parabolizing with is the one for edge zone correction. 
"The W Edge Deepening Stroke"

http://stellafane.org/atm/atm_mirror_ref/atm_strokes.htm#The%20Normal%20Stro
ke

I like Mike's illustration better.

http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/full_lap/full_lap.h
tml


the one labeled "Modified stroke for more edge correction at start of
parabolizing".

Only do that for a short period before testing. Do 5 minutes of that and
test. You don't have far to go from a sphere to a parabola and that could be
enough. You will want to see how far 5 mninutes takes you before doing more.
You will also want to see how well the correction is going on from center to
edge so that you can adjust the stroke in the next work session... if there
is a next session.

It is not likely that you will make your edge perfect, so you will have to
decide when it is good enough.
 
You do not have to bend the Ronchi bands much from the straight bands of a
sphere to be fully parabolized.

Keep the lap in good condition.
Slow strokes. As slow as you can go keeping the stroke smooth. 
Don't work too long. 15 minutes is a long session. Some might be only a few
minutes. Many short sessions will do much better than many long sessions.

SLOW STROKES.

Jerry


-----Original Message-----
From: lütfü çakmak [mailto:lutfu_cakmak@yahoo.com] 
Sent: Saturday, January 12, 2008 5:03 PM
To: Jerry; atm_list
Subject: RE: [ATM] Not tde or tue, important oblate spheroid, ellipsoid

Hi Jerry,

I agree with you. Already I have worked for about 5
months on this mirror. One time I had to returned to
fine grinding. It is a training mirror for me.
Actually I will make a 10" for planets and a 15" for
deep sky. 15" blank is ready for grinding and I have
waited 10" pyrex blank from a my frend will to send
from USA.

After 30 + 15 min. the big W stroke + 15 min. the big
W stroke using rouge here is ronchi;

http://www.atmlist.net/contrib/lutfu_cakmak-at-yahoo-dot-com/lutfu_1_12_2008
/

I think my mirror returned to shpere, but a litte bit
a hill still remain and there is a little bit TDE on
the edge, for my part.

Well, afterwards what is your suggestions. 

Unfortunately I have not yet a foucault test device.
Surely I will have to make one.

Best regards,

Lutfu




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