How telescopes "see"
Almost all telescopes use mirrors to gather and focus the light from objects in the night sky. But mirrors reverse the image. So why use mirrors instead of lenses to focus the light?
The most important reason is that the optics (that is, the mirrors or lenses that focus light) of a good telescope must be nearly perfect. The mirror or lens must be just the right shape and the surface must be perfectly smooth and polished. In the case of a lens, even the inside must be perfect. Otherwise, the image appears distorted.
Lens—not so good
The larger the light-gathering surface (that is, the lens or mirror), the brighter the image. The brighter the image, the fainter the stars that can be seen. A lens (like in a pair of eyeglasses or a magnifying glass) must be very large and very thick if it is to be very powerful. Unfortunately, a big lens is a heavy lens.
Not only must the front and back surfaces be perfect, but the inside must also be perfect, because the light passes through the whole lens. Any flaws in the lens will ruin the image. It would be like looking through some kinds of shower doors or warped windows.
Mirror—much better
A mirror, on the other hand, can be very thin, even if it is large. It just has to have the right curved shape. And there is only one surface to clean and polish. It is much easier to make a large, near-perfect mirror than to make a large, near-perfect lens.
Mirrors, by the way, since they are much lighter weight than lenses, are a lot easier to boost into orbit, too, whether Earth orbit or solar (Sun) orbit.
