Why mirrors beat lenses in the quest to see the cosmos

Space · Published 21 October 2026 Swipe more like this →

In short: To see the furthest reaches of the universe, astronomers need to capture every possible photon. While early telescopes relied on glass lenses, modern giants favor mirrors. This shift in technology wasn't just a preference; it was a physical necessity driven by the weight of glass and the demands of space travel.

The fundamental goal of any telescope is to gather and focus light. The size of the primary optic—whether a lens or a mirror—directly determines the telescope's power; larger optics collect more light, allowing scientists to detect fainter, more distant objects. However, the physics of light creates a significant engineering hurdle for large-scale observation. Refracting telescopes use lenses to bend light, similar to eyeglasses. While effective for smaller instruments, large lenses become incredibly heavy and thick. As they increase in size, they also begin to block more of the light passing through them, making them impractical for massive arrays or space missions.

Reflecting telescopes solved these issues by using mirrors to bounce light rather than pass it through a medium. Mirrors can be manufactured much thinner and lighter than lenses of the same diameter, making them the ideal choice for space exploration. Most of these mirrors are shaped as paraboloids—3D versions of a parabola. By using specific parts of this shape, engineers can tune a telescope to different wavelengths. For example, the James Webb Space Telescope uses a golden honeycomb mirror to reflect infrared light, while the Hubble Space Telescope uses a similar shape to collect ultraviolet, visible, and infrared light.

The complexity of telescope design increases when observing high-energy radiation like X-rays. Because X-rays can pass through most solid materials, they cannot reflect off the center of a standard mirror. Instead, engineers use 'grazing incidence' mirrors, which are tilted so X-rays skip off the surface. These are often arranged in nested, onion-like shells to capture the radiation. From the first lens prototypes in 1608 to these sophisticated nested shells, the evolution of telescope optics has been a constant battle to balance weight, light collection, and the specific properties of the electromagnetic spectrum.

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