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Why are modern large optical telescopes usually reflectors?
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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.
Modern large optical telescopes are predominantly reflectors rather than refractors for several key technical and economic reasons:
- Elimination of Chromatic Aberration: Refracting telescopes use lenses that can suffer from chromatic aberration, where different colors of light fail to focus at the same point. Reflecting telescopes avoid this issue entirely because reflected light does not disperse according to wavelength.12
- Structural and Economic Efficiency: Reflectors have shorter tubes than refractors of the same diameter, which significantly reduces the cost of the telescope tube and the dome required to house it. Furthermore, because the back of a mirror serves only as physical support, it does not need to meet the stringent optical quality standards required for a lens.1
- Scalability: Constructing large single-mirror telescopes is more cost-effective than building massive lenses, as the cost of large single-mirror instruments increases approximately with the cube of the aperture diameter. Modern designs, such as multimirror telescopes, further allow for the creation of massive apertures that would be impossible to achieve with traditional refractive technology.3