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How do reflecting telescopes avoid chromatic aberration?
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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.
Reflecting telescopes avoid chromatic aberration because they rely on the principle of reflection rather than refraction.1
In a refracting telescope, light passes through a lens, where the refractive index of the glass varies depending on the wavelength of the light. This causes different colors to focus at different points, resulting in color distortion.23
In contrast, a reflecting telescope uses a primary mirror to collect and focus light. Because reflected light does not disperse according to wavelength, all colors are brought to a common focus, thereby eliminating chromatic aberration entirely.1