Re: [ATM] Turned Down Edge

["Jeff Anderson-Lee" <jonah@cs.berkeley.edu>]

Think of hogging out.  If you work the center but not the edge you dig a 
hole in the middle.  The deeper the hole (for the same diameter), the 
shorter the focal length.  The edge however maintains infinitely long 
focal length being flat.  That's a TDE: the outside edge has a longer 
focal length than the center.

Now, instead of starting with a flat blank, start with a sphere.  If you 
only work the center and not the edge the same thing happens: you dig a 
hole in the middle that shortens the focal length there while leaving 
the edge unchanged, and once again, TDE.

Jeff Anderson-Lee

Rich Ball wrote:

>>>This IMHO is why TDE is such a big bugaboo. TUE is easy to fix. You put
>>>      
>>>
>the
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>>>lap on it and it comes down. TDE will not come down if you avoid the
>>>      
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>edge.
>
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>>One can see this demonstrated in mirror making classes very nicely.  The
>>mirrors with edges that lag in polishing almost always have a rolled edge.
>>    
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>>Mel Bartels
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>Please explain this to me.  It seems counter-intuitive.  My notion would be
>that if the edge were not worked (ground or polished or figured) it would be
>rolled up, not down. ???  How does avoiding the edge to anything at all to
>it?
>I'd have thought that to get rid of a TDE you'd work the whole surface of
>the mirror except the edge, leaving the edge higher and eventually within
>the proper figure.
>  
>

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