Re: [ATM] polish time for 13 inch

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Mel Bartels wrote:
> Try a one inch oversized lap and see if turned edge magically disappears.
> From a symmetry view, the turned edge, if it's going to occur, is moved off
> the edge of the physical mirror into a larger virtual edge, which doesn't
> exist when the mirror is used to focus light.

As Mel mentioned the magic 6:5 ratio has been written about before, in 
the ATM books, I believe.  I seem to recall it in a chapter about 
Hindle machines, though I could be wrong about the location.

I would like to point out that many of us make mirrors with laps about 
the same size as the mirror, and I personally don't get turned edges 
if the pitch lap is in contact.  The laps I use run from 100% down to 
~25% of the mirror diameter, depending on the mirror.  For large 
mirrors the oversize laps become more of a challenge.

For the softer pitch that I use, cold pressing is a necessity or else 
bad contact leads to unpredicatable results.

The moral of the story is that there are many, many ways to make a mirror.

F/3 is challenging.  The tolerances for measurement are much more 
demanding than with slower mirrors, so it is more difficult to 
determine the true shape of the surface with methods like Foucault 
testing after the error starts to approach ~1/4-1/8th wave.

	Mike Lockwood


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