Re: [ATM] TDE (That Darned Edge)

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

Your unlucky day.  I hurt my ankle yesterday so I didn't go to work and have
all day to mess at the computer practicing my typing.

I have been eat up with interest in TDE. You have probably noticed. I feel
like if I can learn to make a good edge at will, there isn't much else I
need to learn. Might be more to learn but won't "need" it. 

First I would say that you should expect to be able to do better than what
you have, and on the first mirror.

The pressure thing you are doing is probably turning your edge. Depending
exactly where you are pressing over the mirror. It is good that you are
trying such a thing. It is difficult for most first time mirror makers to
break the dogmatic based fears and start modulating things to manipulate the
curve. So don't throw the method out. Just be prepared to change your logic
concerning what it will do and where. That is another email, another day. 

If the entire mirror is overcorrected and not just the center then fixing it
is more difficult than if you just mean the center hole area is over
corrected. Yes, I see it but the Outside ROC Ronchi does not show the center
well. Some Foucault readings would help. That F ratio will not have much
bend to the Ronchi bands. If you are doing that pressure very near the edge
it is dangerous for the edge if you do not place it exactly in the right
place. If it is in the right place, it is easily overdone in minutes. It
could have done the right thing early on and not being checked by test just
went on by to more bad.

How long are you working before testing? When you say 5 or 6 hours work,
that is a looooonnnnnggg time for figuring if you mean between testing. It
is not a long time for figuring a first mirror with no skilled person
helping you. But it really only takes 15 minutes to turn a sphere of that
size into a parabola. Some of the 15 minutes would be thinking about it. You
know... important things like "should I go to the bathroom before I start?"
And things like that.

I would not suggest a stroke or placement of the stroke until you have
shortened the work sessions to nearly nothing. The reason follows.

Recently, too explain to someone how little glass was removed in
parabolizing (12.5" in that case) I went through a little rough
guestimation. Lets do it with this 8". The mathematically endowed may
elaborate if they wish and add precision to my guestimate. I am guessing but
with some knowledge of a past similar mirror I worked. I welcome criticism
on this, I don't like spreading terribly wrong info.  Maybe I'm mistaken.

If you put the sphere of your mirrors radius on the parabola with that
radius in the center, so they touched in the center, there will be a total
of about 15 to 20 millionths inch difference at the edge. To be conservative
lets say 20 millionths.  That is about 1 wave of green light by the way.
The best fit parabola that would touch that sphere at the 71% zone would
have a max error of 10 millionths at the center and the edge. Of course
there would be 0 error at the 71%zone. Therefore the average thickness to be
removed from the spherical mirrors surface to make the sphere a parabola
would be 5 millionths inch.  The area of the mirror, pi, round to 3 X r^2 or
16 =48 and we under did pi by .14, so lets say 50 sq inches / .000005" =
.00025 cu. Inches of glass to remove from the sphere to make a parabola.

A typical piece of paper for your printer is .004" thick.  Cut a 1 inch
square out of a piece of printer paper. Cut the 1" square half. Take one of
the halves and cut it in half. Take one of those halves and cut it in half
and cut one of those in half, keep one and discard all of the rest of the
paper. You now have a 1/16" to the side square .004" thick. Hold that in
some tweezers and inspect it carefully under a bright blue high resolution
light. You are looking at the same quantity of paper as the quantity of
glass you need to remove from the sphere to make it a parabola. You may now
discard that piece of paper. However you may want to put it by your figuring
work position as a reminder of what you are trying to do.

If I am wrong on this there are 100 people who are going to jump me. And
they should.

Short time in figuring sessions can do very much work compared to the work
necessary.
 

As for how hard to work on the edge.... That is why I am so interested. I
haven't made a lot of mirrors but I made each many times trying to make the
edge perfect.  Haven't yet.  Haven't done as good as I think I should be
able to.  When I have had a great edge, I could not say that I could
reproduce the results with confidence. Most any other aspect of hand
figuring I can reproduce at will and rarely go wrong.


I still stand by my usual advice to first mirror makers to work on the edge
a little but don't get stuck.  Learn to figure the rest of the mirror and
then you can spend the rest of your life working on the edge.  Whenever you
think you have done all you can stand you put the best parabola you can on
there, it will be easy when you know how to figure, and you put it in a
telescope and use it.  Then make the second one perfect.

Still working on the second one...... The one with the perfect edge.
Jerry




-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Meeks Robert-PT1784
Sent: Wednesday, April 26, 2006 12:31 PM
To: atm@atmlist.net
Subject: [ATM] TDE (That Darned Edge)

Hello - I've uploaded a not-so-good picture of a Ronchi pattern.


After about 5-6 hours, 

the center hole is mostly gone 

I've taken out a fair amount of glass to reduce the rest of the surface
around the hole,


edge has not budged.

I used mostly TOT, slightly offset, and with a little pressure on the inside
edge of the tool (not the part that was overhanging) to focus in on the area
between the center and edge.
 
This is my first mirror

I've heard several of you indicate that *most* mirrors have some amount of
edge defect.

I'm inclined to finish the figuring (slightly over-corrected at this point),
mask off the edge and move on.  But I have no basis of comparison to be able
to say whether this is a "bad" TDE or a "little" TDE.  It's hard to tell how
much is bad edge and how much is diffraction.
 
I know this is subjective, but if this was your mirror would you invest
the time to work on the edge or is this amount of TDE "acceptable"
(assuming I masked it off)?  
 
Here are the link and relevant facts:
 
http://www.atmlist.net/contrib/bob-dot-meeks-at-motorola-dot-com/IMG_310
9.jpg
Diameter:  8 inches
ROC:  122 inches
This picture was taken .5 inches outside ROC
 
Thanks for your help,
Bob
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