Re: [ATM] Ultrathin glass and mirrors

[Ken Hunter <atm_ken_hunter@yahoo.com>]

David...
   
  You are using the term "astigmatism" incorrectly. Astigmatism manifests itself in different focal lengths at different orientations of the glass. You are describing zonal irregularities and calling them astigmatism. This can cause confusion and waste many hours of questions and replies trying to deal with your glass's "astigmatism".
   
  A poorly matched lap should be warm pressed, then allowed to cool in contact with the glass. There is no "quick trick" to making a lap behave. By heating then immediately polishing, you are setting yourself up for problems especially on the thin glass you are trying to work. Even full thickness PYREX will mis-behave when treated in this manner. But irt won't be astigmatic.
   
  Ken Hunter
   


vorblesnak@peak.org wrote:  
A trick I have used on a poorly matched lap, one that should be OK, that
has already been pressed but for some reason skips or jumps a bit, is to
warm it a bit in water and then polish. That quick dip softens the pitch
just a bit and smooths that last bit of unmatched surface. That tiny bit
of warmth will distort the glass on contact and start an astigmatism. 
Small laps warm the glass where they are, but no where else, and cause
astigmatism. Figuring has become a challenge.


		
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