Telescopes' Aberration (Chromatic and
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
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.