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Why can mirrors in reflecting telescopes be supported more easily than large lenses?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

In reflecting telescopes, mirrors are easier to support than large lenses because of how they are constructed and mounted. A lens must be supported only by its edges, as light must pass through the entire transparent material to form an image. If a large lens were supported from the back, the support structure would obstruct the light path and interfere with the image.1

In contrast, a mirror reflects light from its front surface, meaning the entire back of the mirror is available for structural support. This allows engineers to use various mechanical systems to support the mirror across its entire rear surface, which is essential for preventing the mirror from distorting under its own weight—a critical requirement, as even a few microns of distortion can cause significant image blurring.23

Furthermore, because the back of a mirror does not need to meet the high optical quality standards required for a lens, it can be designed specifically for physical stability. While maintaining this support is still a complex engineering challenge, the ability to reinforce the mirror from behind provides a distinct advantage over the edge-only support required for refractive lenses.23