(urgent, please) can you edit this

Chromatic aberration occurs in lenses (not in mirrors), as the result of different colors being refracted differently as they pass through a lens. White light will be split up into its component colors, as it is in a prism. Each color from the original white image will then follow a separate path to get to the focal plane. If you are looking for a really sharp, pinpoint image of a star, this can be a problem. What you may get is a little smear of light with blue at one end and red at the other. Focusing won't help, because the distortion is built in to the way that the lens works. It can be corrected - to a great extent - by adding other lenses - at additional cost. Cheap refracting telescopes often suffer from chromatic aberration, and there is little than can be done to solve it in such a situation without buying a better scope.

If you say parenthetically "not in mirrors" you might want to explain briefly why not. If that explanation is irrelevant, then it may be better to leave out "not in mirrors". Even if you choose to leave the phrase in you can still drop the parentheses without loss. ...occurs in lenses, but not in mirrors, ...

You could also drop the dashes with no loss: can be corrected to a great extent by ...

On the whole, it is a good and lucid piece of technical writing provided you are writing for a reasonably technical audience.