Re: [ATM] Determine the glass type of a primary mirror

["Bob May" <bobmay@nethere.com>]

Besides the color tests, there is also the inspection of the shape of the
glass.  Pyrex is usually molded (but not always and in addition, the sides
and back may have been processed from that molded shape) while plate glass
is usually polished on the backside with ground sides of some kind or
another.  If the sides are polished, they will probably be done with a flame
polishing system rather than by mechanical polishing.
The problem with looking at the color is that the glass could also be some
optical glass which is also quite transparent.  You can often tell the
difference between optical glass and pyrex by the striations inside the
glass which the cheaper pieces of pyrex usually has to some degree or
another.
All you really can do without fancy tests for the index of refraction is to
guess and pray that you guessed right.  Then again, if you are working with
unknown glass, you're working on the cheap side of life and what you get is
what you get.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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