What primary engineering difficulty drives the use of segmented mirror designs for modern ultra-large telescopes?

Answer

Constructing and polishing a single monolithic mirror larger than about 8 meters is extraordinarily difficult and expensive.

The extreme difficulty and expense associated with manufacturing and maintaining a single, solid piece of glass exceeding approximately 8 meters in diameter necessitates the use of segmented mirror technology for achieving larger scales.

What primary engineering difficulty drives the use of segmented mirror designs for modern ultra-large telescopes?
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