[ATM] telescope mirror - testing for temper or strain.

[Curt Diggs]

If you have a laptop or LCD computer display this can be used as a polarized light source. Place the blank in front of a white background on the display and use the aforementioned Polaroid sunglasses to view it. Just rotate your head (or the display) to get the filtering effect. The mirror should lighten and darken uniformly as you change the angle of the glasses. Any strain will show up in the glass as distinctively light and dark areas, usually bars or bands. The most common strain shows up as a "cross" in the mirror.  If I remember correctly what I've read, most porthole glass is generally free of strain but it's wise to check.
   
  Curt Diggs
   
  >------------------------------
  >Message: 12
>Date: Tue, 19 Dec 2006 08:58:50 +1000
>From: "Thomas Janstrom" 
>Subject: Re: [ATM] telescope mirror - testing for temper or strain.
>To: "'horace r davis'" 
>Cc: atm at atmlist.net
>Message-ID: <010a01c722f8$17cea180$0b01a8c0 at Mylaptop>
>Content-Type: text/plain; charset="windows-1250"
  >To test for stain (how much stress is in the glass, tempering is the locking
>in of large (some would say huge) amounts of stress into the glass) you need
>polarised light (a flat piece of glass viewed at @57 degrees will reflect
>polarised light) and you interpose the glass to be tested between this light
>source and a polarising filter (Polaroid sunglasses work really well), you
>rotate this filter until you get the two polarizing axis' to cross, you will
>know when this happens as things will get dark suddenly.
  >Now when you do this with the porthole, what you want to see is a slight
>uniform darkening of the whole surface, what you don't want to see is a
>cross (although this isn't really bad), or lines, of squares or most
>definitely colours (this would be really bad), these are in order of
>severity, you can live with a cross, but anything more is not really usable.
>Squares or colours is down right dangerous, I have had a tempered piece
>explode in my hands, there is sharp cracking sound and your left holding
>small bits of glass, and there is glass everywhere!
  >
  >Cheers, Thomas.
   
   

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