Optical refracting telescopes

A refracting telescope uses lenses to focus the incoming light rays.

Most modern optical telescopes are reflectors rather than refractors, because:

• Lenses suffer from chromatic aberration. Light waves of different wavelengths are bent by different amounts as they pass through the glass, so that each wavelength has a slightly different focus than the others.

• It is harder to make and support a large lens than a large mirror.