Reflecting Telescopes:

The problems that plague refracting telescopes, especially lens sag, can be largely overcome by using mirrors rather than lenses as objectives. The first surface mirror, with the silvering on the outside therefore the light never passes via glass, can be ground therefore it brings light to a focus which does not differ with wavelength. The mirrors can be supported from behind; therefore it is possible to make them in diameters numerous times larger than lenses without meeting the sag problem.