Re: [ATM] Central hole during polishing

[Mike Lockwood <melockwo@uiuc.edu>]

Balaji,

Balaji Ramani wrote:
> I have been working on a 10" f/5 Pyrex glass mirror (1.5 inches thick).  I 
> have been polishing it with a 1/3 W stroke, both MOT and TOT and frequent 
> turnings.  When I put the mirror on the foucault stand, I noticed that there 
> is a huge hole in the center.  The hole is about  4.5" in diameter.  To null 
> out the hole, I had to move the KE towards the mirror about an inch!

An inch is a lot.

If you are switching MOT and TOT and doing a W-stroke, a hole should
not get that deep.  There is probably some other factor at work.

> So, I am wondering how I might have gotten into this state and how to fix it?  
> I would like to get to a good sphere before I start figuring.  Any 
> suggestions, please?

First, look at the mirror.  Is the polish even?  Look for residual
pits with a laser or strong flashlight.  If there are more pits in the
center than elsewhere, chances are that hole was ground in.  If there
are fewer pits there, it was likely polished in.  If there is no
difference, then it is also likely it was polished in.

Second, look at the lap.  (I assume you are using a pitch lap and not
polishing pads.  I also assume you are pressing the lap before use.)
The nastiest holes I have seen were from pitch lap facets that bulged
due to bubbles in the pitch.  The pitch heats up during polishing, the
air in the bubble expands, and that facet rises up and polishes away
more glass than the others.  You might try gently warming the lap and
looking at it from the side, and seeing if there are any facets that
are higher than others.  If you find a bulge, pop the bubble with a
hot soldering pencil.  Then continue, and hope there are no more bubbles!

Another possibility is that the lap has more area in the center, due
to missing outer facets, etc.

	Mike Lockwood



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