Re: [ATM] Fixing Tom's oblate sphere

[Mike Lockwood <melockwo@uiuc.edu>]

Hi

Polk, Tom wrote:
> There are three corrective methods I'm considering:

I assume you mean correction for the oblate sphere?

> 1. MOT; long strokes (~ 5"), 4" wide. The mirror edge may be a lost
> cause.

That's the parabolizing stroke.  It's not time for that yet.

The edge is NOT a lost cause, and I recommend you bring it back with 
the method I have outlined.  I have seen it work for 10-20 of my own 
mirrors, and for several mirror-making students.  It is a proven 
technique, and a valuable one, in my opinion.

I looked at your older messages, and realized you are working a 12.5", 
0.75"-thick mirror, which is bigger and thinner than I thought it was. 
  Therefore, you may need to put one hand closer to the trailing edge 
of the lap and add pressure there during the stroke rather than just 
shifting the pressure to the heel of your hand.

> 2. TOT; long strokes amplitude = ? 5", width = ? 4". Should this stroke
> be longer? Might save the outer 3/8" (I hope it gets this small).

This can help turn the edge.

> 3. Use a 1/2 D tool. Still working on this but considering a "normal
> parabolization" stroke with minimal edge work or a "W" with a tighter
> center stroke with minimal edge work. Smooth out with full sized lap.
> I've got one made up with a new lap, #64. The weather here will be
> warmer so I can increase room temp to about 73-74^F for a softer temper.
> 
> Does method #3 sound any good?

If you have not worked with a subdiameter lap before, it is not 
recommended.  However, it might be useful later on when you are 
correcting the mirror (to a parabola).

I know you have made two mirrors before this one, so it might be time 
for you to start learning this technique.  But don't do it until the 
edge is fixed!

	Mike Lockwood

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