Diagram of Astronomical Telescope in Normal Adjustment
Disadvantages of a refracting telescope:
Lenses suffer from colour distortion – this means that when white light passes through the lens it is split into the colours of the spectrum. Because violet light refracts more than red light it is brought to a focus closer to the lens than the red light – this makes the image coloured and blurred. This effect is called chromatic aberration.
Advantages of a reflecting telescope:
Mirrors do not suffer from the colour defects of chromatic aberration.
For these reasons all the really large telescopes in the world today are reflectors.
