Telescopes' Aberration (Chromatic and spherical)

Chromatic aberration

All refractor telescopes suffer from an effect called chromatic aberration (or color deviation or distortion) that can produce a rainbow of colors around the image. As light passes through the lens, the longer wavelength (dict.) corresponding to redder colors is bent less than the shorter wavelength light (bluer colors).

There a couple of ways to reduce chromatic aberration. One way uses multiple lenses to compensate the aberration. The other way is to use the longest possible focal length (distance between the focus and the objective) to minimize the effect. This is why the early refracting telescopes were made very long.



Spherical aberration

Both reflector and refractor telescopes can suffer from a defect called spherical aberation. In this case, if the mirror is not well curved or the lens is badly shaped, not all light is focussed to the same point.

To compensate this problem corrective optics can be used to intercept and correct the light beams from the secondary mirror before they reach the cameras and spectrographs.
The images obtained with a telescope that suffers from spherical aberration can also be computer-enhanced to produce sharper images.

 

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