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Chromatic Aberration
An image of the planet Jupiter showing
strong chromatic aberration
An example of chromatic aberration in an image
produced by a telescope with a simple converging lens
The image on the left shows strong chromatic aberration
compared to the image of the same star field at the same
magnification on the right
Stong chromatic aberration shown around the broght
blue stars in the foreground of the image
Dispersion of white light produced by a prism
The converging lens can be thought of as a
collection of prisms
Fe
The converging lens can be thought of as a series of prisms. A prism
disperses light of different frequencies
The objective lens brings each image to a slightly different focus.
The eyepiece lens has to be 1 focal length from the principal focus
of the objective lens. Choosing this distance for green light leaves
the blue and red out of focus.
After much investigation it
was found that by adding a
second lens of a different
glass composition this
effect could be corrected
crown
glass
flint
glass
Spherical Aberration
Spherical aberration in lenses is an effect which produces an
imperfection in the image.
It happens when light being refracted from the edge of a lens is brought
to a focus slightly before light from the centre of the lens.
This is due to the fact that the surfaces of simple converging lenses are
lenses are spherical. (That is they form part of the surface of a sphere)
circle of confusion
A spherical lens though simple to make is not the ideal shape for a lens
A spherical lens
radius of
curvature
Spherical Aberration In Lenses
The effect is minimised with telescopes of long focal legnth
In lens systems, the effect can be minimized using special combinations
of convex and concave lenses, as well as using asymmetric lenses
With different radii of curvature at each side
The amount of spherical abberation is proportional to 1/f3 and is therefore
greatly reduced with objective lenses of long focal lengths.
Other Problems With Refracting
Telescopes
• Faint light must pass through the lens so it must be
totally free of optical defects such as tiny bubbles.
Therefore lenses are very expensive.
• Glass is opaque to certain frequencies of light.
Especially UV.
• Even visible light is substantially absorbed as it passes
through the lens.
• It is impossible to produce a large lens free of chromatic
aberration
• It is difficult to support large lenses without blocking a
great deal of light.
Therefore mirrors are preferred
Spherical Aberration In Mirrors
spherical mirror
Spherical mirrors are easier to grind
and polish but have an appreciable
degree of spherical aberration
parabolic mirrors do not
suffere from spherical
aberration but do not have a
wide field of view like
spherical mirrors. (they suffer
from “coma”.
Bernhart Schmitt (1930,s) introduced a thin correcting lens to
correct the image in a spherical mirror.
Many large telescopes are Scmitt reflectors