A major problem with large spherical mirrors is that
although they do no suffer from chromatic aberration (focusing different colours at different
distances) they do show spherical aberration. This means that rays from points at different
distances form the axis are brought to a focus at different points.
This is serious
problem because it limits the aperture of optical instruments using spherical mirrors – only a
small part of the spherical surface can be used if good focussed images are to be produced.
This is shown simply in Figure
1. Figure 1(a) shows a spherical surface where a large aperture is used. Such a surface would
give serious spherical aberration. Figure 1(b) shows a surface with the same curvature as 1(a)
but where only a small section of the surface is used. Such a surface would suffer far les form
spherical aberration but obviously the aperture (A) is much less. This would be a problem for
instruments such as astronomical telescopes where the maximum possible aperture is needed
to give the best light gathering power.
Parabolic mirrors do not suffer from spherical
aberration (See Figure 2). A parabolic surface brings all rays to the same focus whether or not
they are close to the axis. It is for this reason that they used for astronomical reflecting
telescopes. It is possible to have a large aperture with a relatively short focal length.
Telescopes
like this have a low focal ratio (f/A) and so when they are used for photography they have
shorter exposure times.
The photograph shows a 300mm diameter parabolic mirror in use in the author's telescope.
