Question

Why is chromatic Aberration a problem for astronomers and not for thin lens?

Answer 1

Chromatic aberration is a real problem . Chromatic aberration is a problem which lens, or refracting, telescopes suffer from. Light strikes the lens elements and is refracted by them - perhaps you can still remember something about this from physics lessons at school. Refraction is essential for the formation of an image. Blue light is refracted more than, say, red light. This means that the different wavelengths have different focal lengths. The refractive index of blue light is greater than that of red light.

If one imagines the effect of this on the formation of an image of an object, then the blue light will be found at a different location than the red light. This means that the image produced is blurred. But not only that, it also means a difference in the magnification of different colours. In plain language, this means that the different image distances for the respective colours cause different image sizes for them. This means the production of annoying colour fringes in the image