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RE: Kiln Forming Lenses

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Posted by Stephen Campbell on May 27, 2000 09:45:40 UTC

lenses for a refractor telescope are usually flint glass and crown class and other slightly different variations which have different refractive index, a reflector telescope mirror is usually made of pyrex(7740)or other low expansion glass substrate(cervit,fused quartz,u.l.e).
I have considered casting blanks myself,not sure of exact melting temp but i know its around 2300-2400f,the mirror must anneal for a long period of time days,weeks,or months depending on
thickness and diameter,the mold material is alumi
nosilicate fiber,and an electric heating element
array is positioned above the mold,since the heat cant penetrate the sides and bottom of mold anyhow,not sure about this.
I also understand that the mirror blank cannot
be in the mold too long or it will begin to melt
the mold due to the boron oxide reacting with the
aluminosilicate fiber.
I know that the steward mirror lab uses chunks of
high quality annealed pyrex when casting the multi
meter lightweight honycomb structure blanks as well as the spin casting techniques.

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