[ATM] Fixing oblate sphere/TDE (was shop practices)

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Mark Holm wrote:
> I'm not so sure of the short stroking cure for TDE for the same reason. 
>  My experience with TDE is neither extensive, nor terribly successful, 
> but I can honestly say I haven't seen MOT short stroking do a whole lot. 
>  With a full sized, or nearly so, lap, I have had better luck going TOT 
> and using a tangential stroke or a fairly wide W to get action over the 
> outer zones and the edge.

Just thought I'd comment on how TDE usually manifests itself for me, 
since I'm seeing REPEATED examples of it in my shop right now, and in 
pictures posted by list members.

A friend just sent me a mirror that I'm going to refigure for him. 
Also, I'm helping another friend in town finish an 8" F/6 that he has 
ground from the start, and it now trying to fix the edge on before 
figuring.  Both mirrors have the same problem.  It is the problem 
pictured in the first "Foucault image" posted by "RML" near Montreal 
(see the thread "12.5 inches BVC mirror polishing and figuring 
difficulties" from about a week ago.  His pictures are at

   http://www.axeinformatique.com/temp/Mirror/BVC_Mirror.html

See the second row of pictures, first photo on the left.  It shows 
(from the point where the knife edge is positioned) a classic oblate 
spheroid (assuming the Ronchi line enters from the right, which I 
assume based on his comment about a central bump).  I have seen this 
many, many times.

The central bump is the least of the worries!  The edge is a mess, but 
the fix is not difficult.  This is NOT a bad case of TDE, since the 
outer zones are high also.  Knock down the outer zones, and the TDE 
will be eliminated in the process.  The TDE is DUE TO THE OBLATE 
SPHERE FORM OF THE MIRROR.

What I believe is happening in the polishing process in all these 
cases is that the front of the lap is digging in on the forward 
stroke, and the back of the lap on the rearwards stroke.  This digs 
out glass inside the outer zones, causing the TU outer zones (as are 
characteristic of an oblate sphere).  Once the lap goes over the edge, 
it is riding on the last bit of the raised outer zone (the edge), 
which is polished away with a vengeance due to the high local 
pressure.  That causes the TDE.

The cure for me is to work TOT with good contact and slow strokes.  A 
wider W and longer strokes will often help.  Most importantly, one 
must avoid edge plowing.  I do this by moving the point that I apply 
downward force to the mirror away from the leading edge of the lap. 
So, when I move the lap forward, I apply more pressure with the heel 
of my hands.  When I move it back, I generally reduce the pressure and 
try to apply it with my finger tips.  This stops the plowing.

Mixing in a little MOT work with longer strokes will help deepen the 
center, and can help match the shorter radius outer zones with the 
longer radius inner zones more quickly.

Texereau's cure for TUE can also be used here, but it requires some 
practice.

Once you learn to "feel" how the lap is working, you can often tell 
just how bad the oblate sphere is by how the lap friction changes 
during the stroke, so long as the lap is in contact.

	Mike Lockwood




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