Question

Why doe most telescopes use a primary mirror rather than an objective lens?

Answer 1

There are a few reasons:

• Quality of the glass. Unless the glass in a lens is perfectly homogenic, a lot of blurring will occur. With a (surface) mirror the quality of the material behind the silvering is unimportant.
• Achromatism: A lens will bend light differently according to colour, a mirror will reflect all light the same. There are ways around this by using lenses made out of two (or more) types of glass.
• Support: A mirror can be supported at the whole of the back, a lens can only be supported at the edge. Since glass is a "solid liquid", large pieces of glass tend to sag a bit, ruining definition.
• Absorbtion: With thick lenses absorbtion can become a problem. Also, depending on the type of glass, it will absorb different wavelengths. A mirror doesn't have these problems, at least to far lesser extend.
• Cost: Above a certain diameter, a mirror is easier (=cheaper) to make than a lens (see all of the above).

And more. There are even some advantages to using refractors.