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How does an optical telescope collect and focus light?
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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.
Optical telescopes collect and focus light using either lenses or mirrors.
Refracting telescopes use an objective lens to bend light rays, bringing them to a focal point where an eyepiece magnifies the inverted image. The diameter of this objective lens is called the aperture, which determines the telescope's light-gathering power.12
Reflecting telescopes, on the other hand, use a concave primary mirror to reflect light to a focal point. This method avoids chromatic aberration, a distortion common in refracting telescopes where different colors of light don't focus at the same point. Mirrors in reflecting telescopes are typically coated with aluminum for high reflectivity.3