FW: [ATM] I need more help

["mlbrown@everstrive.com" <mlbrown@everstrive.com>]

Yup, same thing happened to me on my first mirror -- I followed the long,
wide W stroke advice too much and dug a hole in the middle.

What's happening here is that most of the glass is removed where both
mirror and lap are in contact most.  So if you do a long stroke, the only
area of the mirror that is always in contact with the lap is a small zone
in the center.  So that gets worn down and you end up with a hole in the
middle.

Of course progressively less material is removed tapering to the outer
edge.  If you do a short stroke, then material is removed from a wider area
of the center.

The next trick is to realize how this all appears in the Foucault test,
which gives you the radii of curvature of the various zones.  The best
technique I know how to understand all this is to attempt to draw the
profile of the mirror from what the Foucault test is telling you.  Then
figure out where you want material removed.  

OK, so how does this help you in your situation?  A TDE is just an edge
with a longer radius than the adjacent zone.  Draw the profile and you
realize that you want to remove material *just adjacent* to the TDE.  And
the way to do that is a short stroke.

One note: shorter strokes remove material from a broader area.  Less
thickness is removed from the larger area.  So it takes longer to make
corrections with a shorter stroke.  (A longer stroke will dig a hole quick,
as you've no doubt discovered).  So be patient.  It will look like you
aren't making any progress for awhile.  You'll know you are there when the
diffraction ring goes all the way around the mirror.

=Matt


Original Message:
-----------------
From: Rich Ball ShooterBall@hotmail.com
Date: Fri, 11 Jun 2004 23:10:49 -0500
To: atm@atmlist.net
Subject: [ATM] I need more help


The parabola was too deep on my 8" f8.  I have had some success in taking it
back to a spherical configuration. I''m not there yet but I am getting
closer.  But I still have some TDE.

Will my short stroke technique eventually take care of the TDE as it seems
to be doing with the bad figure?

Thanks to all who answered my initial request for help.

Rich Ball
Oak Park, Illlinois
_______________________________________________
ATM mailing list http://www.atmlist.net/


--------------------------------------------------------------------
mail2web - Check your email from the web at
http://mail2web.com/ .


_______________________________________________
ATM mailing list http://www.atmlist.net/