Why are the largest research telescopes in the world almost exclusively built using mirrors instead of large lenses?

Answer

Mirrors can be supported from the back, preventing sagging under their own weight

The physical limitations related to gravity and material science heavily influence the size attainable for primary optical elements. Lenses, especially very large ones, can only be supported around their edges. If a lens becomes too large, the force of gravity causes it to sag or deform under its own weight, destroying the precise parabolic or spherical shape required for focusing light accurately. Mirrors, conversely, can be supported uniformly across their entire back surface. This structural advantage allows engineers to construct reflecting telescopes with vastly larger apertures than is feasible for lens-based refractors, thus maximizing light-gathering power necessary for cutting-edge astronomical research.

Why are the largest research telescopes in the world almost exclusively built using mirrors instead of large lenses?

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