Re: 1/6 or 1/8 wave
- From: midnightmadness@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (Melweaver)
- Date: 26 Jan 2007 21:17:03 GMT
A few more notes:
The homogeneity properties of AstroSitall and soda-lime window glass
is very good in comparison to Pyrex. Paul said he tested all three
glasses for transmission and Pyrex is not usable. This is an easy way
to determine the homogeneity of any glass.
BTW Just in handling a Pyrex mirror and putting it into a test stand,
you can see the distortion caused from the warmth of your hands. The
distortion will disappear as the temperature stabilizes after a few
minutes.
The statement about Pyrex "keeping" a figure has to do with
the stria in the glass which warps (astigmatism) the mirror as the
temperature goes down and therefore a loss of figure. For example a
70 degree mirror works fine but at 40 degrees could have 1/2 wave of
astigmatism.
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