|MadSci Network: Astronomy|
Telescope lenses are traditionally made out of glass which is ground to a particular shape. These telescopes are called refractors because the light enters the telescope and gets bent (refracted) by the lens to a focus (see the diagram below).
Most large telescopes do not use lenses. Instead they use mirrors. This is because a lens, which lets the light pass through it, gets very thick and heavy for very large telescopes. It actually begins to deform under its own weight when the telescope changes position, resulting in a poor quality image. Also, a large lens requires a lot of structural support which is very heavy and expensive. A mirror reflects the light (hence telescopes with mirrors in them are called "reflectors") and can be supported from behind (see the diagram below). That means you can make the mirrors much thinner than you can a lens.
Some observatories are starting to use adaptive optics where they try to account for atmospheric blurring by flexing a deformable mirror very quickly (the Keck Observatory is one of these, see http://www2.keck.hawaii.edu:3636/realpublic/gen_info/kiosk/index.html ).
That cannot be done with a lens.
Try the links in the MadSci Library for more information on Astronomy.