Understanding Optical Defects

Some readers may wish a deeper understanding of some of the concepts I’ve been covering. In particular, the following text deals with more detailed explanations of the three most common optical short-comings: spherical aberration, coma, and astigmatism.

Figure 10: A spherical mirror focusing starlight.

In order for an objective mirror to focus properly, its reflective surface must have a parabolic curve. Unfortunately, when the curve of a mirror is being created through grinding and polishing, it must be forced to become parabolic because the curve’s natural tendency is to become spherical. By spherical I mean that the curve of the mirror wants to be a section of a sphere as shown in Figure 10. For a telescope to function properly, ALL of the light must converge at the focal point. Notice how light from the outer edges of the spherical mirror is focused behind what should be the mirror’s focal point. This defect was what originally plagued the Hubble Space Telescope. For obvious reasons it is called spherical aberration.

For the sake of accuracy, I wish to say that I exaggerated some of the reflection angles in Figure 10 in order to make the false focal points easy to see. A lens may exhibit this defect as well if it is curved spherically.

copyright 2004 Singularity Scientific

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