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Here are some advantages of using mirrors rather than lenses for astronomical telescopes.
1. The wider the mirror or lens, the greater the light-gathering power. The mirror in a reflecting telescope can be built up from many small mirrors designed to work together to have the same effect as a very large mirror (search for “segmented mirror”). One cannot do that for a lens that is created from a single piece of glass.
2. A lens is supported only around the perimeter, so mechanical issues limit its ultimate size. In contrast, a large mirror can be supported everywhere behind itself (search for “honeycomb mirror”) and can be much larger in diameter.
3. A lens suffers from chromatic aberration (page 728) because different colors refract at different angles. Light rays reflecting from a mirror all follow the same path no matter what color they are, so reflecting telescopes don’t have that problem.
4. Lenses suffer from spherical aberrations (Figure 25-25), which can be corrected at great expense. Mirrors can be shaped like parabolas to eliminate these aberrations.
5. Making a large lens means casting a large, perfectly clear slab of glass with minimal inclusions, and if one makes a mistake in shaping it, patching it seamlessly is very difficult. In casting a mirror, one can take one’s time polishing and shaping the surface, getting it correct before applying the mirrored coating.
