Re: [ATM] Turned Down Edge

["Jonathan Bishop" <jonathan_bishop@sbcglobal.net>]

Hi,

Interesting. I read onetime that it was possible to avoid TDE by using an
oversized lap - which would support what you are saying. But I would
hesitate to use an oversized lap as I wouldn't know how to do a cold press!

On the other hand many people recommend trimming the lap back from the edge
to avoid TDE. Maybe that is to avoid forming a ridge when you cold press.
This ridge would easily create TDE.

So maybe some useful ideas are...

1) Do not use a too soft lap.
2) When polishing make sure to polish the edge equally as well as the rest
of the mirror.
3) When figuring it is necessary to polish everywhere, including the edge.
Of course in figuring we want to selectively polish some zones more than
others, but it is not OK to not polish the edge at all (oops double
negative).

What do you think?

Jon

----- Original Message -----
From: "Scott Ewart" <scott.e@quicksurfer.com>
To: "ATM List" <atm@atmlist.net>
Sent: Thursday, July 15, 2004 10:41 PM
Subject: Re: [ATM] Turned Down Edge


> Jon,
>     It does sound like your problem was indeed caused by soft pitch.  But
> you might be surprised how steep and narrow an edge can get the other way,
> even when the stroke takes the lap beyond the edge.  I suppose this is
more
> common with under-size tools.  But the two can be hard to tell apart.
> That's one of the things that make them so sinister.
>
> Scott
>
>
> ----- Original Message -----
> From: "Jonathan Bishop" <jonathan_bishop@sbcglobal.net>
> To: "Rich Ball" <ShooterBall@hotmail.com>; "ATM List" <atm@atmlist.net>
> Sent: Friday, July 16, 2004 1:22 AM
> Subject: Re: [ATM] Turned Down Edge
>
>
> Hi,
>
> I think we are talking at cross purposes here. When I think of TDE I am
> thinking of a very narrow zone, say 1/4", at the edge of the mirror which
> has a dramatically longer radius of curvature than the zone next to it.
>
> I suspect the most likely cause of this is the mirror digging into a too
> soft lap. That is what I believe to have caused the problem in my case. So
> the "cure" was to switch to a harder lap and do short COC MOT strokes.
> Without getting a harder lap I just could not fix this TDE no matter what
> stroke I tried.
>
> What I hear being discussed in this thread is a zonal defect caused by
> selective polishing. For instance deepening the center and leaving the
edge
> alone. I think that in this is better referred to as a zonal defect.
>
> I think it would be difficult to produce the narrow severe TDE I did by
> polishing the mirror everywhere but the outer 1/4"!
>
> Although I think we have some on this list who are capable of doing even
> that :-).
>
> Jon Bishop
>
>
> .net/
>
> _______________________________________________
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