Re: [ATM] TDE (That Darned Edge)

[Mike Lockwood <melockwo@uiuc.edu>]

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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