Re: [ATM] TDE (That Darned Edge)

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

Hello Mike...
Let me explain my comments....

Concerning the stroke with pressure TOT.  It has much to do with the width
of the zone pressure stroked with the edge of the lap and how far inside (or
outside) the edge. Robert was unclear perhaps, saying "slightly offset", but
then saying to "focus on the area between the center and edge". If he wants
to work that area he should stoke a pressured W across it inward to the rim
of the hole, not only just a narrow area just inside the edge. If it is
overall overcorrection he should be pushing the crest of the doughnut down.
This may explain long duration work to see results in the center. Pressure
near the edge will take a long time to make change to defects in the center.


For example...Results of pressure as I view it...

Lets take the outer 3 inches of the mirror and say that the inner inch of
the 3 to the middle inch is overcorrected slightly. And the outer zone is
under corrected a little from the middle zone.  Working the pressure across
the middle zone will decrease the correction from the inner to the middle
zone. It will increase correction from the middle to the outer zone. Two
birds with one stone. Hence easier to fix an overcorrected inner zone if
some outer zone is under corrected. Inward from the inner 1" wide zone of
those 3 zones would not change much. The work goes right on the middle of
the 3 and changes its relation to the zones on either side of it and not
much else.  

If the stroke was applied across both the inner and the middle of the 3 one
inch zones, from the inner to the middle would not change much and the
middle to outer zone correction would increase. There would be a decrease
from inside those zones to the innermost of the 3 outer zones.

That is my reasoning on that.


On the degree of TDE.... Just like Robert, I do not have a reasonable basis
to know how good an edge is reasonable to expect. I know what a good edge
looks like. I know what is reasonable for ME to expect. Something less than
perfect. I get diffraction rings but they are not balanced. I think there is
a chance that pride biases available data on just how good other peoples
edges are.  If I had a dollar for every case I have seen of people posting
pictures of bad edges who after much advice and work said that they fixed
their edge but did not post a picture of the "good edge" I would have the
money for the big dream telescope now. Those people who make perfect edges
would be the ones most likely to post images.  I can't find many perfect
edges posted by amateurs. I think that says something.

I don't see many mirrors on the test stand that aren't either mine or made
by someone following my advice. Theirs look just like mine. How well do
other people do? Individually and on average, how well do they do?  I know I
want to do better, but I could be doing better than most and not know. I
could be doing worse.

On the speed of fixing a badly turned edge... A badly turned edge can be
made into a not so bad turn quickly when you get the work on the proper
location. Getting the not so bad edge great is the issue at hand.


Being the one who has not made a perfect edge, my comments should be given
appropriate weight by the readers of this.

I am finding more on the subject now, so I think I'm on my way.

Thanks Mike for your comments and help.

Jerry


-----Original Message-----
From: Mike Lockwood [mailto:melockwo@uiuc.edu] 
Sent: Wednesday, April 26, 2006 3:30 PM
To: ATM list
Cc: Jerry
Subject: Re: [ATM] TDE (That Darned Edge)

Hi,

Jerry wrote:
> 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.

Clearly it is something in your figuring techique or lap management,
according to your previous post in which you say you have a good edge
after polishing.  It can be something as simple as having a light too 
close to the work, which dries the lap/mirror at the edge, and helps 
turn the edge.

In my experience, if you don't do radical figuring strokes near the 
edge, TDE is usually caused by lap contact problems.  Think about that.

> The pressure thing you are doing is probably turning your edge.
> Depending exactly where you are pressing over the mirror.

I disagree - Robert said:
   "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."

What he did was apply pressure on the edge of the lap that was NOT
overhanging, and this should have the intended effect - helping
alleviate the central hole.  Good work Robert.  One discovers that a 
lap used on top with accented pressure will have its "maximum wear 
area" somewhat inside the edge of the lap.  What you have to learn, as 
you progress, is just HOW far inside the lap edge that is.  It depends 
on the pitch hardness, your technique, and the asphericity of 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.

Agreed.  Intelligent (and documented - take good notes!) 
experimentation is a good thing for learning.

> 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.

I have to disagree.  Overcorrection over the whole mirror is easily 
fixed with a full-size lap and TOT or MOT normal strokes.  It has been 
noted before that overcorrecting a mirror and then bringing it back to 
a parabola with normal strokes can help smooth the mirror.

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

A severely turned edge will not come around quickly, so shortening 
work sessions may lengthen the agony....

> 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.

Jerry, it is time to pinpoint the cause of your TDE, I think.

In another post you ask how good the edge should be - the best you can 
do is to have the diffraction ring around the mirror be the same 
brightness on both sides of the mirror.  Then the mirror should show 
no TDE in the star test, in my experience.

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html

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