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Has Mr. Porter tried interposing it between the sun and an eye. It could be a
solar telescope filter. OTOH, I'd expect such a filter to be rather light -- a
gold film on thin optical glass. Mine uses Kr and less expensive ones are
coated mylar. I go with the IR mirror guess -- first surface with a thick
pyrex backing for high Q and rigidity. -- 1/10 th wave length flat and rigid.
bc who has a whatz this -- and requests the loan of a web p. to display.If you e-mail the picture to me, along with any text you'd like to include