D2 Optical Telescopes

Topic

Refracting telescopes form an image of the observed object using lenses, whereas reflecting telescopes form that same image using mirrors; it is precisely the use of mirrors that allows reflecting telescopes to eliminate the chromatic aberration to which refracting lenses are prone, while also enabling the construction of apertures far larger than would be possible with lenses.

Catadioptric designs, meanwhile, combine a refractive corrector with a mirror—a combination that achieves wide fields of view within instruments that are far more compact than a pure reflecting telescope.

A telescope's light-gathering power increases with the square of its aperture diameter; consequently, even a modest telescope can capture hundreds of times more light than the naked human eye.

Optical aberrations—such as spherical aberration, coma, astigmatism, and field curvature—degrade image quality as one moves away from the instrument's optical axis; in practice, it is precisely this degradation that limits the telescope's useful field of view.

Next-generation telescopes, constructed from hundreds of hexagonal mirror segments rather than a single monolithic mirror, vastly increase the light-collecting area compared to the largest telescopes currently in operation. Thus, from the image formation discussed at the beginning of this text to the expanded light-collecting area mentioned at the end—and encompassing light-gathering power linked to the square of the aperture and the optical aberrations that limit the field of view—the optical telescope demonstrates that seeing farther and in greater detail depends, above all, on the amount of light the instrument itself can gather.

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