Why is concave optical glass lens of 13 feet diameter so hard to make?

Question

why is Bush an idiot, why did president of USA start a war when it knew good and well that the prices of the standard of living will rise

Answer 1

Because it's so heavy.  The strain on the middle part makes it very susceptible to breakage.  Plus, getting two optically smooth uniformly curved surfaces on an area of glass that large is next to impossible.