The image below shows the design of a refracting telescope.
When light passes through a prism different colours split and dispersion takes place. The same thing happens with a lens but to a much lesser degree. This is called chromatic aberration and causes the different colours of light to focus at different points. To overcome this problem, the reflecting telescope was invented. One design of the reflecting telescope is shown below.
(a) Why is there no chromatic aberration in reflecting telescopes?
Here, light does not pass through a second medium .
There is no refraction, dispersion, or splitting .
It is simply being reflected from mirrors.
Hence there is no chromatic aberration .