From: "mlbrown{at}everstrive.com" 
To: atm{at}shore.net
Reply-To: "mlbrown{at}everstrive.com" 



I just had the same problem.  There appear to be two solutions: 1) wear
down the edge area to match the center 2) wear down the center to match the
turned up edge

#1 would be quicker, since there is less glass to remove, but it's a little
trickier, and you might end up with TDE.  Others are more expert at it than
I am, but the gist seems to be to apply extra pressure to the part of the
lap currently sitting over the edge area.  I had trouble doing this and not
making uneven gouges, etc., so maybe I was doing it wrong.

#2 is slower, but it was the route I chose, and it came out nice.  I just
started parabolizing with progressively longer strokes, noting that the
area of the mirror always in contact with the lap got most of the wear. The
longer the stroke you do, the smaller that area is in the center (and the
higher the differential wear).  Conversely, a shorter stroke wears a wider
diameter region, but slower.


The way I got my TUE was by using short strokes with a lap that was about
7/8 diameter of the mirror.  That meant the outer edge of the mirror got
much less wear than just inside it.  The center dropped, leaving a turned
up edge.  More of the same just made the TUE look worse. Slowly lengthening
my stroke made the TUE region slowly widen and take over the entire mirror,
bringing me to a sphere.  I rode the same train to paraboloidville.


By the way, what have you tried so far and what did it seem to do?

=Matt





Hi !

I am polishing a 8 inch f8 mirror and have a TUE that I have problems geting rid of.
The ROC for the edge is about 3mm shorter than the middle. ( moving source)

What is your advice to get rid of this ?

below are a ronchi picture from inside focus.

http://w1.875.telia.com/~u87509744/ronchi%200407_2%20(Small).JPG

any help is appreciated.

Magnus Arvidsson





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