1. Chromatic Aberration
An image of the planet Jupiter showing strong chromatic aberration

2. An example of chromatic aberration in an image produced by a telescope with a simple converging lens

3. 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

4. Stong chromatic aberration shown around the broght blue stars in the foreground of the image

5. Dispersion of white light produced by a prism
The converging lens can be thought of as a collection of prisms

6. 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.

7. 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

8. 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

9. A spherical lens
radius of curvature

10. 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.

11. 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

12. 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”.

13. Bernhart Schmitt (1930,s) introduced a thin correcting lens to correct the image in a spherical mirror.
Many large telescopes are Scmitt reflectors