AN ASTRONOMICAL TELESCOPE HAS A LARGE APERATURE TO

Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:

    * In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
    * Light of different colors travels through a medium other than vacuum at different speeds. This causes chromatic aberration.
    * Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
    * There are technical difficulties involved in manufacturing and manipulating large-diameter lenses. One of them is that all real materials sag in gravity. A lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.