And What are some reasons for using mirrors rather than lenses in telescopes?
Well, …, The larger the aperture the more you can see. They the following quick and easy experiment:
Close your left eye, form a very small "OK" with your right hand and look through the center hole will your right eye. Take a mental note of everything you can see. Now open the "OK" and look again. You’ll immediately be able to see more. The ability to gather light, NOT MAGNIFICATION, enables you to see more. Thus a 6" refractor can see much much more at lower power than an 80mm (3.1 in) refractor.
We use mirrors more commonly because:
1) Large mirrors are much easier to make
2) There’s a limit to the about of glass that can go into a lens before light can no longer pass through the lens of the lens becomes so large that it breaks from its own weight.
3) It’s much easier to remove chromatic distortion from a mirror than a lense.
4) Mirrors are cheaper to make.